Prema 6048 teardown - Page 1
Forum section: electronics / general

author: branadic
author_rank: Super Contributor
timestamp: June 10, 2020, 09:05:32 pm
content: Hi everyone,it just so happened that I bought a Prema 6048, which arrived today, so I now have a small collection of 8.5 digit meters (R6581D, S7081).Since noone ever posted teardown pictures, this thread is addressing exactly that and also shows the hardware differences to Prema 6047, that Dave already torn down:https://www.eevblog.com/forum/blog/eevblog-613-prema-6047-multimeter-teardown/The meter arrived with a lot of Fluke stickers all over the place. Looks like it was calibrated in 08/2009?Before powering the meter I visually inspected the meter and caps and took a few pictures, so this is just a first teaser, not a complete teardown yet.The first difference is already known, Prema 6048 has a LTZ1000ACH reference, compared to LMx99 in Prema 6047. The reference board pretty much looks like how you're doin' and is a piggyback solution. Most likely from preaging and selection. Interestingly, they have choosen LT1078 instead of LT1013, while the circuit in the manual indicates LT1013 too. Maybe someone else can open his unit? Otherwise no fancy resistor magic going on here.The reference is connected to where the LMx99 is populated on analog board in 6047. It pretty much looks like an afterthought. Some hermetically sealed RHK02 (now part of Vishay) 5k, 7k and 70k resistors can be found on the analog board.But the most interesting part is, what is unpopulated in 6047. While we can find a PR BK7 on the analog board in both models, there is a second PR BK7 on the digital board. I was told that MickleT. revealed that this additional ADC is for temperature correction on the meter.Edit: here is a link to MickleT.s findings:https://www.eevblog.com/forum/blog/eevblog-613-prema-6047-multimeter-teardown/msg441667/#msg441667That's it for now, more pictures and measurement results on this meter coming soon.-branadic-

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 10, 2020, 09:26:53 pm
content: Congrats.The MM2020 took place a few days too early, obviously.Would've liked to play with that famous instrument, and of course to check its calibration. (I'm still missing my 2ppm)Please shoot some pictures of the instrument, while measuring something. Beautiful.THXFrank

author: essele
author_rank: Frequent Contributor
timestamp: June 11, 2020, 12:45:07 pm
content: Interesting that yours looks like it's been hacked in ... mine is actually a standard part of the board...More complete board photo...

author: branadic
author_rank: Super Contributor
timestamp: June 11, 2020, 01:13:37 pm
content: Thanks for sharing. Looks like your analog board is a later version (6048-1-5), compared to the one that is in my unit (6048-1-2 and SN 1027).Meter is still warming up with inputs shorted and offset is already down to -2.4µV starting at about 200µV on first power on. Maybe the unit was switched off for quite some time and need to stabilize first.-branadic-

author: z01z
author_rank: Regular Contributor
timestamp: June 12, 2020, 10:59:32 am
content: Do you know what those round traces are? Is there any component on the other side of the board, held by the screw?

author: branadic
author_rank: Super Contributor
timestamp: June 13, 2020, 06:30:53 am
content: They are just "board design", having equal distances to the screws. Somebody had fun making the layout for that board and signal traces are surrounded by guard traces almost everywhere to prevent leakage.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: June 18, 2020, 07:01:15 am
content: I made some progress on my unit.First, I noticed some intermittent reset issue on the meter, indicating some power supply problem. So within the last days I fully recapped the meter by good quality Rubycon caps and one single axial Vishay cap on the digital board, renewed some bodge wires and gave the unit a careful cleaning and let it warm up overnight.Seems like the reset issue is now gone, but I do have a slight zero offset of some -39µV now.I also received some OTPs and a new zeropower SRAM module. Once my EPROM programmer arrives I will save the original EPROM and SRAM data and flash the latest firmware with the factory calibration block to the meter.Finally the meter will need some adjustment I guess. More pictures of the unit in the next days.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: June 20, 2020, 10:08:26 am
content: So meter is running for some days now and I couldn't notice any further reset. So replacing electrolytic caps fixed that issue.Still waiting for the EPROM programmer to arrive.Attached are some more pictures.As my unit is missing the original feets I will need to design some replacements and get them printed. Thanks to essele I have an idea of their dimensions.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: June 27, 2020, 10:14:40 pm
content: Feets printed and mounted (data attached in prior post), latest firmware updated with factory calibration block copied into an Atmel AT27C256-45PU OTP and M48Z12-70PC1 zeropower SRAM module replaced by a new one, thanks to TL866 II+ that arrived meanwhile.Attached is the measurement with the input shorted. First 1.5h in 8.5digit mode (20s integration time per sample), second 1.5h in 7.5digit mode (4s integration time per sample). I would say, the meter is pretty quiet.ToDo: Measure linearity and calibrate the meter.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: June 28, 2020, 01:45:10 pm
content: And a measurement on one of my LTZ references in 8.5digit and 7.5 digit mode.What I can conclude by now is, P6048 (20s) is faster than S7081 (51.2s) in 8.5digit mode, low noise, easy to repair and 7segment display will last much longer than any (custom) VFD or gas discharge display. It seems to be stable without any drift correction or ACAL functionality.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: July 05, 2020, 05:00:29 pm
content: I measured INL of P6048. I'm not too happy with the result. Maybe someone else with such unit wants to share his result?To me it looks like some buck of the result calculation?-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: July 05, 2020, 06:42:48 pm
content: The error near zero looks like some kind of extra jump at or near zero.Without that jump the INL curve looks actually quite good.From the circuit it looks like there are relays used to switch the polarity. I have not found a detailed description, but it looks a little like they may use this between positive and negative side. A near 1 ppm jump would be some 10 µV (or 20µV ?), quite a lot !There may be a calibration constant to correct the jump.

author: branadic
author_rank: Super Contributor
timestamp: July 05, 2020, 07:52:43 pm
content: Well, calibration is done by offset calibration of the ranges first and by suppling either a positive or a negative voltage in the individual ranges next.What I could do is recall the factory settings, to see if that changes something. If INL is different, then meter needs a full recalibration. Luckily I have already saved the SRAM, that stores the calibration constants, so nothing to loose here.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: July 05, 2020, 09:56:20 pm
content: Performed a quick INL test with factory calibration data. Result have somewhat improved, but are still not amazing.-branadic-

author: TUMEMBER
author_rank: Regular Contributor
timestamp: July 06, 2020, 10:18:54 am
content: Put trend lines on the chart, it will be clearer.You can now see a large number of colored dots.It is not convenient for a human to evaluate a signal.Describe the colors because you "forgot" - what is what.Sorry, it's just a google translate.

author: branadic
author_rank: Super Contributor
timestamp: February 01, 2021, 08:04:52 am
content: Hi,I currently have the reference board of the unit in my thermal chamber and t.c. looks rather large (positive and linear), so does noise (numbers and diagrams later this day), even though the reference is currently powered by 3x 6V leaded battery plus LT1763 to provide 12V.The reference is powered by 12V inside the meter. It uses LT1078 in a low grade socket, BC547 and R80 (nominal 400k) is missing, temperature setpoint resistors R77/R78 (12k1 + 1k : 1k) are like R79 (100R + 20R), they don't appear of highest quality.I was planing to replace the socket and LT1078 by LT1413. Furthermore I have a 13k:1k VHD divider and a 120R VHP202 resistor on its way. At least the footprint for R80 is present, so it would be possible to trimm the circuit for low t.c.On the other hand Prema 6048 uses some temperature compensation. Is there someone with knowledge about it? Will I do anything bad to the meter, if I improve the reference, since temperature compensation is already accounting for this effects?Edit: Attached a picture of the reference board. Seems like initially Prema wanted to use LMx99 and later decided it wasn't good enough for P6048. You can see, that the reference board is an afterthought and connected to LMx99 footprint. This is good, as I'm able to remove the reference board to characterize it. Newer board revision do have the LTZ reference circuit directly assembled to the mainboard.Measurements of internal temperature showed ~13K above ambient temperature. The meter is fanless, though I think it would help to cover the reference board in some way.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: February 01, 2021, 08:38:04 am
content: The chip is a LTZ1000A and thus normally does not need the 400 K resistor for compensation of residual TC. It may still help a little in some cases, but likely with a much higher value.Using the simple LM199 footprint with just 4 wires may add some effect of wire resistance, as there is no proper Kelvin sensing close to the chip. This could explain some of the problems.In this respect the low power LT1078 may not be so bad.A cover for the reference is definitely a good idea and the lack of a cover may explain the higher than normal noise.However there are rare case of LTZ1000 with extra popcorn noise - so it could be just bad luck.The resistors in deed don't look great. On the other side VHP202 is kind of overkill already for a LTZ1000 ref board with a proper connector. There is little need for a board with more mediocre interface. So the old resistors may be good enough in this context.

author: dietert1
author_rank: Super Contributor
timestamp: February 01, 2021, 09:31:13 am
content: Can't you wire in one of your other LTZ1000 references to find out about TC calibration? I mean before starting the revision.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: February 01, 2021, 10:30:52 am
content: @ Kleinsteinthe reference board only uses the power supply pins of the LMx99 footprint, while all other connections are done at proper points on the main board.Refering to the datasheet the resistor divider (here 13k:1k) and the 120R resistor are the ones with tighter requirements. 120R is nothing you find in stock at lower grade. Since I was able to get both of them from someone without waiting weeks/months I took what I was able to pick up, even if it might be overkill.I don't see popcorn noise and the reference board in the temperature chamber is connected in a Kelvin scheme fashion. So the interface is okayish and doesn't need modification. As I said, data later this day, as temperature profile is still running.@ dieter1good point, but unfortunately all of my references have the 10V boost, are trimmed and t.c. is compensated for the overall circuit including the 10V boost. So I currently have no zener output only boards at hand.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: February 01, 2021, 04:47:43 pm
content: So here are the results, 0.2ppm/K, which is not too bad, though I have seen better than that, but it's quite noisy.Curious, the reference is missing the 1meg/100nF at the heater opamp.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: February 01, 2021, 05:10:01 pm
content: There are 3 capacitors - so may it is just the resistor missing.Not having the 1 meg and 100 nF to limit the higher frequency gain at the temperature control coud in theory lead to a less stable regulation or even some oscillation. They are part of the frequency compensation to make the regulator a PI regulator as opposed to a nearly I regulator only. The exact values are likely not critical. The A version at relatively low temperature would have power and thus less loop gain could better have a slightly larger resistance and smaller cap to get similar overall loop parameters.Even if not oscillating, a regulator close to oscillation can show more noise.

author: branadic
author_rank: Super Contributor
timestamp: February 01, 2021, 07:12:44 pm
content: I managed to reverse engineer the board and found that the 1meg is populated with 100R. There is also a footprint for the unpopulated trimm resistor. So the board is pretty much standard, just some minor differences, such as LT1078.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: February 01, 2021, 08:02:10 pm
content: A 100 Ohms resistor in the compensation of the heater makes not much sense. I would change it to the more usual 1 M. With only 100 ohms the heater control could be very slow and chances are the higher noise is from temperature variations, especially without a cover.

author: Andreas
author_rank: Super Contributor
timestamp: February 01, 2021, 09:05:41 pm
content: Mhm,what is actually populated on the "original" board? (100R or 1 Meg?)https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg448574/#msg448574with best regardsAndreasEdit: Ah found it:https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg449750/#msg449750so its more the 100R as the 1 Meg.

author: branadic
author_rank: Super Contributor
timestamp: February 01, 2021, 09:15:40 pm
content: Andreas, someone populated 100R instead of 1meg, must have picked up the wrong resistor I guess???Unfortunately I ran out of 1meg myself, so can't populate the proper value today. However, I already removed the socket for the opamp and since I had it laying around, populated LT1013 instead of LT1078.-branadic-

author: Andreas
author_rank: Super Contributor
timestamp: February 01, 2021, 10:27:43 pm
content: Quote from: branadic on February 01, 2021, 09:15:40 pmMhm,not shure if it is a mistake.in the early 1986 databook the resistor wasnt there at all.https://www.eevblog.com/forum/metrology/vintage-ltz1000-from-1986-nib-!/msg1827785/#msg1827785I think the 100nF * 10K (input resistor) = 1 ms time constant is enough for a heater.The 1 Meg resistor adds some measurable noise visible with the naked eye/scope at the heater pin.At least I do not want a "noise generator" near my reference. So I have added additionally 10nF between output and input of OP.My guess is that someone "improved" (really?) the cirquit at the time when the A-version was introduced.with best regardsAndreas

author: dietert1
author_rank: Super Contributor
timestamp: February 01, 2021, 11:08:27 pm
content: In general a thermostat without proportional term is near unstable due to the delay between heater and sensor. I am writing delay because it isn't a time constant, but a higher order low pass. Probably somebody tried how large the proportional term can be made for the LTZ1000. And the observed noise may not be noise but real: the thermostat working against air drafts.The LTZ1000 heater is very fast. Without any bandwidth limiting in the regulator i once saw it oscillate at about 50 Hz, so the delay time from heater to sensor is about 10 msec.Regards, Dieter

author: Andreas
author_rank: Super Contributor
timestamp: February 03, 2021, 07:59:21 am
content: Quote from: dietert1 on February 01, 2021, 11:08:27 pmHello,I think you mean the LTZ1000A?So the best way to check this would be either to stimulate the Setpoint by a small amount and see the reaction.Alternatively examining the power up / power down sequence like in my LTZ1000 measurements e.g. here:https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg832030/#msg832030If there is any instability on the heater output which is not related to the setpoint you should see it on power up/down.with best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: February 03, 2021, 08:37:03 am
content: I can not see the heater delay from the trun on transients shown in the link (have not checked the whole long threadthough). It seems the supply is coming up too slow to get an instant temperature reading.One can estimate the delay for the heater from the dimensions: from the die picture the distance from the heater (outer ring) to the sensor is about 1 mm. The thermal diffusivity for silicon is at 87 mm²/s - so expect a little over 10 ms as a dead time before power from the heater to reaches the sensor.The PI adjustment rules (e.g. Chien, Hrones und Reswick) want the time constant in the PI regulator to be something like 2.3 - 4 times the delay for good reaction to disturbance.From my experiance shorter can lead to instability, while longer slows down the response a little. The thermal layout external to the chip would not effect the delay very much, It can effect the best gain factor for the regulator. Even there is mainly changes the heat loss and not so much the heat capacity, which is the main factor effecting the permissible loop gain.So one should expect a suitable time constant somewhere in the 50 ms range. Shorter may be problematic, a little longer is more on the safe side.The LTZ1000 standard circuit calls for 100 nF and 1 M and thus 100ms , which sounds like a good, safe start and about right.

author: branadic
author_rank: Super Contributor
timestamp: February 03, 2021, 08:25:41 pm
content: Replacing the 100R by 1meg (and the LT1078 by LT1013) didn't change anything. Next step is to wait for the resistors to arrive and replace the 120R first, repeat the measurement and second place the ratio divider.For the measurement in the thermal chamber the board was and is wrapped into some non-conductive white foam, so airdrafts are already prevented.-branadic-

author: Andreas
author_rank: Super Contributor
timestamp: February 03, 2021, 10:03:10 pm
content: Quote from: branadic on February 03, 2021, 08:25:41 pmI did not really expect any large change by this.OK the LT1078 is a bit slower and has a little less open loop amplification and current output but should be good enough.I would check the 1/f noise next.Perhaps we have a "little jumper" (popcorn noise) or a too low zener current because the 120R is not conductive. (you could measure it across the diode).with best regardsAndreas

author: essele
author_rank: Frequent Contributor
timestamp: February 08, 2021, 01:42:54 pm
content: Drifting slightly off the current discussion, just wanted to report that my Prema 6048 has just suffered a SRAM failure (Error 8 ), I had just ordered a replacement M48Z12-70PC1 when I realised it was the same pinout as the CAL ram in the 3458A.So I can now report that it appears to all work nicely with an FRAM adapter and an FM16W08 FRAM (which is partially annoying as it's too late to cancel the other order!)I do need to run through a home cal again as I've obviously lost the prior cal (again!), and I will confirm that everything is properly stored, but I'm certainly not getting the Error 8 anymore.I not sure how to read the date code from my M48Z12. Branadic's one from earlier in this forum has the form H99XXYYZZ, where YYZZ is year/week. Mine is missing that, but that does suggest that W1728X is NOT a date code, therefore my next best guess is 1999.Details of the adapters I used are here, with the zip file of KiCad files a couple of posts down...https://www.eevblog.com/forum/metrology/3458a-upgrade-questions/msg2951062/#msg2951062Cheers,Lee.

author: branadic
author_rank: Super Contributor
timestamp: February 08, 2021, 05:41:19 pm
content: I had mine replaced right after the unit arrived. Battery was still okay, so that I was able to read the content. However, good thing is, that you don't loose factory calibration.Today I received my resistor kit, thanks to the help of some board members here. Resistors have now been replaced, reference is warming up and will then be monitored and measured over temperature.-branadic-

author: meandeev
author_rank: Regular Contributor
timestamp: February 20, 2021, 08:01:27 pm
content: Hello,can somebody take some detailed pictures around the power-supply at the preamplifier board? I have a "jumping" 6048 on the bench, which starts 1..5Volts too high when cold and after 2hours warm-up it is within 50-60ppm to a reference, but also jumping 10ppm up and down.So I started as usual with the power-supply. The +27V rail is real +32V and I saw a serial connection of a zener (with 30V!) and a normal diode for CR6 (on the schematics it is 27V). But it seems genuine, the soldering is like other original one.I think, that saw at the pictures of branadic also something like that, but the resolution is too bad.

author: branadic
author_rank: Super Contributor
timestamp: February 20, 2021, 08:25:09 pm
content: So you bought that 6048 recently off ebay?The construction is indeed original, no need to worry, though not documented in the manual.-branadic-

author: meandeev
author_rank: Regular Contributor
timestamp: February 20, 2021, 09:06:19 pm
content: Hello Branadic,yes I bought it. Meters with this precision come not up in ebay-europe very often. It was not a bargain (especially with these errors), but I couldn´t resist and it should be fixable.The +32v on the +27v-rail made me nervous. But if you say its ok, then I´m fine.Thanks!

author: branadic
author_rank: Super Contributor
timestamp: February 20, 2021, 09:37:18 pm
content: That was the second 6048 within a few month on ebay:https://www.ebay.de/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313&_nkw=prema+6048&_sacat=0&LH_TitleDesc=0&_fsrp=1&_odkw=prima+6048&_osacat=0&LH_Complete=1&rt=nc&LH_Sold=1Give it the normal electrolytic replacement and check for solder joint cracks, which was the fault on my unit - at the bridge rectifier, that caused some intermittent restart.Can you please report if you have a separate reference board or the LTZ already installed on the ADC board? I'm asking as my unit is serial number 1027 and yours is 1035, so they are pretty close and I wonder at what point they had the new board revision installed.-branadic-

author: meandeev
author_rank: Regular Contributor
timestamp: February 21, 2021, 01:21:48 pm
content: I have ser-no 1085 and it has the "new" (?) PCB with the LTZ onboard. One can find IC´s dated from 8706 to 9011.I saw the other auction too, but it was too risky to me because of the description: "does something" - whatever that means.This one seems a litte bit better because of the photo of the resistance-measurement - so it must have passed the selftest.But there was definitely somebody in - look at the not cleaned soldering and some white residue and the lifted pad on the integrator PCB . It was repaired by Prema in 2015 - but I doubt that this bad soldering was done by them.

author: branadic
author_rank: Super Contributor
timestamp: February 21, 2021, 01:45:04 pm
content: Interesting, it looked like 1035 on the back.Someone really messed inside, that is not looking genuine. The white residue propably is IPA. Good luck on repair attempt.-branadic-

author: meandeev
author_rank: Regular Contributor
timestamp: March 05, 2021, 04:46:37 pm
content: one more question to the 6048 owners:which AC voltage do you measure at the input of the rectifier CR1? Iset the device to 240V mains and measure 38.4V at CR1. This comes down to 21.7V DC over C1 and is the input for U1 (7815 voltage regulator). I ask, because this regulator is hot after half an hour (ca. 60-70°C, I cannot touch it with my fingers for a long time).So is this normal? I think I have to open a separate repair thread...Thanks!

author: branadic
author_rank: Super Contributor
timestamp: January 11, 2023, 08:41:39 am
content: It seems that the feets for Prema multimeters are available at Farnell, at least they look identical:https://de.farnell.com/okw-enclosures/a9257209/klappfuss-unterbau-schwarz-pk4/dp/775484https://de.farnell.com/metcase/a9257207/klappfuss-unterbau-grau-weiss/dp/775472Just in case someone is in the need for them.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: January 16, 2023, 05:43:58 pm
content: Hi folks,I'm in the process of characterizing (INL), calibrating + adjusting a Prema 6048, but am somewhat stuck at the moment.The Prema 6048 is somewhat special, as it detects polarity at its input and sets the input relays accordingly, but also switches to a third position if the input is zero.During calibration only zero and only one input value (something between 5% and 100%, preferably between 50% and 100%) is asked for to adjust a range. There seems to be no gain correction, which is kind of weird and leads to a large INL error, obviously similar for positive and negative polarity, but with a totally different value for zero. To reflect that issue I performed a quick 1 V step INL measurement with the factory constants.Has someone experience with adusting a Prema 6048 and could give me a hint?-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: January 16, 2023, 07:26:22 pm
content: The error shown does not look like a gain error, but more like an offset problem. The slope is the same for the positive and negative sign. Somehow the zero seems to be OK in the middle - I don't see how the 3rd mode for near zero readings is done. It looks like there is a tiny bit of offset and both modes should be able to read zero, though not much room for hum and the like.The polarity reversal with the switches also looks like it should not cause any change in the gain. The same gain constant for both signs makes sense. There is however a possibility for offset errors, for both the input part (amplifier) and the ADC part.Some 12 µV is not that much for the ADC and could come up over the years.Chances are a zero offset correction (for the part behind the reversal, so it could be the ADC itself) step in the calibration could correct much of the shown INL error.

author: branadic
author_rank: Super Contributor
timestamp: January 16, 2023, 07:45:17 pm
content: The meter was shorted/ zero'd before taking this measurement and it was powered up for quite a few weeks too. So no, that's not the answer to the observation.I already adjusted the meter at 10 V (50% of range) and confirmed the same INL error afterwards.I then adjusted the meter again, first by adjusting the 10 V range at 1 V (5% of range), then checked at 10 V, adjusted there and verified the 1 V point again, were I could the observe a shift to the prior set value for 1 V. Whatever I do, this behavior is reproducable.I would totally understand if 5% and 50% to 100% were used to adjust one range, the INL curve would then be flat, but that is not how the meter behaves. Hence why I ask here for someone with experience. It seems like something is missing in the manual.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: January 16, 2023, 09:29:39 pm
content: The descritption of the calibration is rather short and split over 2 parts (7.11 and 9) and there may be something missing. In the description in chapter 9 I am missing a zero adjustment. Under 7.11 it calls for zero steps. These may be needed, even if the meter already ready zero.I would expect to have a seperate zero constant for the ADC and than one for each range for the input amplifier. It is not that clear when the extra zero for the ADC is done - this would need something like a zero reading with both polarities, and ideally switch the polarity relay during a zero phase. It may be just 1 ADC zero for all ranges - a partial calibration may miss this.Some drift for the ADC would not be that surprising. So I would expect this to be part of the normal CAL procedure or maybe even a part repeated with a normal Zero command without the switch in CAL mode.

author: branadic
author_rank: Super Contributor
timestamp: January 17, 2023, 09:24:47 pm
content: Within the last days I have tried several different things, like zeroring as part of the adjustment as well as zeroing priopr to adjustment. However, the INL curve is always the same. So the manual is missing something important.An email to Prema was yet without any answer. I will try to give them a call next, otherwhise I'm running out of ideas.-branadic-

author: Mickle T.
author_rank: Frequent Contributor
timestamp: January 18, 2023, 06:46:23 am
content: Prema Instruments has used its patented unipolar ADC type in almost all of its products. Perhaps the user manuals for other models have more detailed information about zero calibration? For example, Prema 6030/6040.

author: branadic
author_rank: Super Contributor
timestamp: January 18, 2023, 10:21:33 am
content: Thanks Mickle T.I had a phone call with Prema and their DAKKS cal lab today, so we are currently in the information exchange process. Question, where did you get this 0.1 ppm INL number from?-branadic-

author: Mickle T.
author_rank: Frequent Contributor
timestamp: January 18, 2023, 10:52:12 am
content: Quote from: branadic on January 18, 2023, 10:21:33 amThis is a very old table and contains several inaccuracies. But as far as I remember, I took the value of non-linearity from the datasheet on the PRI5610 ADC and publications from the bbs.38hot community. I understand that this is not official data, but I did not have a better estimate at that time.Is the non-linearity problem described above similar to the one already discussed?https://www.eevblog.com/forum/metrology/possible-problem-with-prema-6047-or-6048-meters/

author: alm
author_rank: Super Contributor
timestamp: January 18, 2023, 10:59:21 am
content: Quote from: Mickle T. on January 18, 2023, 06:46:23 amI checked the Prema P6031A manual (an older, late-eighties 6.5 digit DMM), and it's pretty similar to the P6048 manual. It sounds like zero calibration is updated every time you zero the meter, and not part of factory calibration. They don't specify a zero calibration as part of the adjustment. I guess they relay on auto-zero to null any offsets, and consider linearity inherent in the ADC design. I didn't find anything about linearity in their theory of operations, except that how their lack of switching produces better linearity compared to other designs that switch during integration (cf. HPAK Multislope I/II/III).The P6031A manual is a bit more detailed in its description of the polarity detection, in that it mentions the turnover is within 1 LSD. But of course 1 LSD at 6.5 digits is a bit more lenient than at 8.5 digits.

author: branadic
author_rank: Super Contributor
timestamp: January 18, 2023, 09:16:34 pm
content: Thanks Mickle T., I've quickly recreated the measurement of David Partridge aka perdrix:https://www.febo.com/pipermail/volt-nuts/2018-January/005593.htmland performed that test with the units set back to its factory constants. Indeed it looks identical, but on the other hand we have never heard if that firmware update fixed that issue.Seems like I have to dump and reprogramm the EPROM with "new" firmware but original factory calibration constants and check INL afterwards again, first with factory settings and with adjusted range.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: January 21, 2023, 07:16:22 am
content: I've flashed an OTP with the latest firmware and the factory cal data and performed the quick INL test in the 20 V range on the same unit again. While the offset issue was solved, the INL picture hasn't changed at all.From my quick phone call with Systems Engineering Kalibrierlaboratorium GmbH & Co. KG, the cal lab that performed the DAKKS cal for Prema back in the old days, I've learned that this INL is to be expected. I'm still waiting for a response to the INL plot I've showed them, but I'm not expecting surprises.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: January 21, 2023, 10:32:13 am
content: The INL curve really looks like an offset (some 11 µV) for the ADC itself that is not included in the right way. The actual offset can be larger, the 11 µV is the part not handeld right.The ADC offset may drift a little over time and it would thus make sense to include correction of the ADC offset with the zero function (with external short). Is the zero function switching the polarity relay to measure the zero for both polarities ?The only strange point is the zero readung in the INL curve. I would have expected a different zero readings depending on the polarity relay setting. A zero may come up by averaging both cases (coming from pos or neg values).For the linearity clams on some meters there could be a confusion with the turn over error. Some meters note the turn over error as a measure of linearity, though this is only 1 point - maybe the easiest to measure.

author: branadic
author_rank: Super Contributor
timestamp: January 21, 2023, 10:52:57 am
content: Maybe I'm getting you wrong or I simply don't explain everything detailed enough.1. Performed zeroing in 20 V range2. Adjusted 20 V range with known 10 V source3. Performed INL measurement in 1 V steps4. Calculated deviation from straight line (first order line fit) taking all measurement points between -10 V and + 10 V into accountLooking at the result the zero point is almost near zero, but the +/-10 V point are 10 µV too high/low. Wouldn't it make more sense to use the end-point INL instead (0 V and 10 V points), as these two were adusted before?The result would be, that 0 V is at zero deviation, 10 V is at zero deviation too and in between we see some negative deviation for the positive part and a positive deviation for the negative part, while overall INL would be way lower.We also have to remember that 10 µV is 0.5 ppm of range for this instrument.-branadic-

author: alm
author_rank: Super Contributor
timestamp: January 21, 2023, 11:30:36 am
content: Quote from: branadic on January 21, 2023, 07:16:22 amWith the adjustment procedure that only adjusts a single point there can't be a linearity adjustment. So unless the meter had an autocal-like mechanism for linearity adjustment, I wouldn't expect INL to change at all after adjustment. There may or may not be a factory adjustment for linearity, but I don't think we've seen any evidence for that.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 21, 2023, 01:42:15 pm
content: The INL curve shows a jump of some 22 µV in the INL error from -1 V to 1 V, just in the area of polarity reversal. The 0 V reading is a bit odd - it could still be the average of reading in the positive and negative range. No knowing the details I leave that point out for now.Except for the jump at the polarity reversal the rest of the curve is pretty linear. So if one would correct the data with an -11 µV shift for positive and + 11 µV for the negative relay setting one could get a much better INL curve. The residual linear part would be removed from new line fit or end point curve.There is only a single external voltage for the zero adjustment, but there should be still 2 zero readings done for both settings of the polarity relay. The difference of the two gives the offset of the ADC and the average gives the offset of the input amplifier. So a correct procedure for the zero could correct for the ADC offset and the amplifier offset and thus just for the type of problem seen here.There could of cause also be a linearity problem at the ADC, for readings very close to 0 V, so that the zero adjustment does not work correctly and the error gets visible for the larger voltages.It may help to have another look at the INL curve for a few more points between -1 and 1 V , making sure to keep the data for both relay settings separate.Just in case: it might make a difference if the zero adjust is done in auto mode for all ranges or just for one range with manual ranging (this could accidently limit the zero adjust also to 1 polarity - which would be bad).

author: branadic
author_rank: Super Contributor
timestamp: January 21, 2023, 03:04:12 pm
content: The zero adjustment was performed only for the current range of interest, I didn't use zeroing in auto ranging mode.During adjustment of the 20 V range I can confirm the INL error already. I have a well known 1 V / 10 V reference. If I use the 1 V (5% of the 20 V range) to adjust the 20 V range, then apply 10 V to it, I can observe a deviation of the 10 V reading. If I use the 10 V reference for adjusting the range and apply the 1 V afterwards I can see the very same amount of deviation too.Looking at the datasheet of ADC5601 it claims 0.0001% INL so 1ppm. It further mentions "Gain and offset adjustments are normally made by the microprocessor which controls the analog to digital converter...". Assuming a similar behavior of the BK7 ADC I can only draw the conclusion, I can only draw the conclusion, that there is a bug in the firmware, leading to the observed results.-branadic-

author: Mickle T.
author_rank: Frequent Contributor
timestamp: January 21, 2023, 03:19:30 pm
content: Quote from: branadic on January 21, 2023, 03:04:12 pmI think BK7 == PRI5610. The last have up to 0.08 ppm INL (PRI5610E).

author: Kleinstein
author_rank: Super Contributor
timestamp: January 21, 2023, 05:27:40 pm
content: I bug in the firmware is possible explaination. However I wonder why such a rather visible problem (quite some discontinuity when the polarity relay switches and likely 0 V not reading as 0 in one of the settings) would not have been recognized before. This would be a rather gross fail, not just cosmetics. Of cause the extend of the error can depend on the individual hardware and with new meters the error could have been smaller or a hidden factory cal values to take care of it in the initial state. Getting the same INL curve with a new ROM and thus likely different factory cal values sugests there is no such value however.The 0 V point would be rather close to one end of the ADC range and that can be a bit more prone to trouble (e.g. supply ripple or hum reaching to saturation at times). With internal offsets the ADC may not work well all the way to 0.One could try to intentionally change the ADC offset and this way see, if there is at least some compensation for it. Allowing for a little more overlap may also reduce the chance for reaching the limits too early.I do not fully understand the ADC circuit, but it looks like the part around U11,R34,R35,R36 should add a little offset to the input signal to the ADC to get some overlap and zero in both ranges. A resistor (e.g. 100 K range) in parallel to R36 would increase that shift. Ideally the zero adjustment should take care of such a shift - if not it would cause a huge INL error / discontinuity (e.g. some 100 x the current).A large error would kind of proof that it is a software side (may still be cal constant) problem. If it is a range limit problem of the ADC things may get a little better, though possibly not yet perfect with a relatively small shift in the 5 mV range.

author: branadic
author_rank: Super Contributor
timestamp: January 22, 2023, 09:10:16 am
content: Mickle T., the PRI5610 could be a version of the BK7, yes, but both have different package with different pin quantity and we don't know for sure as there is no datasheet for the BK7. The patents don't show more details either.I guess you mix up a few points Kleinstein.1. The latest firmware I flashed into the OTP has the original factory cal constants copied to it that I restored from the original EPROM before.2. There could be a small offset at 0 V coming from the source (Analogics AN3200) that has to be ignored and is not the problem as I compare the Prema 6048 against the Solatron 7081, not against the set value at the source.3. When I short the input the Prema 6048 reads zero, so again, don't worry about the offset.4. The large INL at 1 V, falling of towards 10 V, is real and not just a coincidence, but I'm performing a smaller step INL measurement right about now, will see how that turns out. I also increased soak time and take a few more samples at each voltage.-brandic-

author: perdrix
author_rank: Frequent Contributor
timestamp: January 23, 2023, 12:52:17 am
content: Maybe the attached firmware that Prema sent me will help somewhatDavid

author: branadic
author_rank: Super Contributor
timestamp: January 23, 2023, 07:09:52 am
content: That is the firmware that I'm currently running my INL tests with, thanks.Attached is a measurement with more details between -1 V / +1 V. As can be seen it's not an offset and how it's handled internally, but is a real INL effect of the meter.-branadic-

author: Mickle T.
author_rank: Frequent Contributor
timestamp: January 23, 2023, 08:54:50 am
content: Wow! There are over 800 points on this graph. This would require over 11 hours of measurements on the Solartron 7081.branadic, can you share CSV data?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 23, 2023, 10:42:34 am
content: The INL curve shows a ratgher sudden jump in the INL error around 0 V. The other ranges, when looked at separately do not look that bad or nonlinear. Depending on the assumed gain curve (end point, least square fit, nominal) the INL curve would get a straight line added or subtraceted. The problem in the INL is the jump near 0 V, not that about straight sections for the negative and positive sides. The residual slope is a gain error, that would go away when the jump near zero would be gone.The problem is not the offset that is visible to the outside or the offset of the source for the INL test. The point is than internaly with 6048 the firmware has to handle 2 separate offset values for the input amplifier and the ADC itself. One could also see this as seprate offsets for the positive and negative polarity settings. If that is not done correct one gets twice the error in the ADC offset as a jump when the polarity relay switches. Chances are the firmware already does some correction for the ADC offset, as the error is only 10 µV. Beside a software problem this could also be an issue with the ADC itself, so a HW point. The 0 V reading is quite close to the edge of the range of an unipolar ADC and there is a chance for an extra INL error of the ADC when very close to the edge. In this case it could help to add more offset to the ADC (the R34-R36 part) to get more overlap and shift the weak spot of the ADC outside of the used range.If it is a software problem, I would expect an externally visble offset for one of the relay settings. So the 0 V input one may get 2 different readings depending on the polarity relay setting.

author: branadic
author_rank: Super Contributor
timestamp: January 23, 2023, 04:56:14 pm
content: I have added the file to the previous post.I will start a similar measurement with all three Prema 6048 in parallel soon to proove that it's a systematic issue, not a unit specific issue.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: January 23, 2023, 09:12:23 pm
content: I am wondering what this meter will do with near zero DC yet with some low frequency AC superimposed. Will it try to operate the polarity switch following the AC?Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: January 23, 2023, 09:22:11 pm
content: Meanwhile a fast INL sweep on three units in parallel was performed. No, they are not all mine, one is here for service - a white unit (red dots) - and one for repair (green dots), which showed some additional issues that I try to fix by comparing it to one of the other units. However, the unit that needed repair shows the worse INL with oposite direction, while my unit though the oldest among them all is slightly better to the white unit, but not by much. Overall they are both identical. So this proves a rather systematic behavior.If I find the difference between the units we can probably find a way to improve INL.QuoteI can't see how that adds an offset to the ADC, can you elaborate on that?The schematic is barely readable, so the connections are hard to spot. However, R34-R36 are wirewound 1% resistors, R31 is hermatically sealed. It would be possible is to use a 10k resistor network such as TDP16031002 to create R31 (5k), R35 (10k) and R36 (40k) from the very same device. R34 has a value of 750k, nothing too common as precision resistor.-branadic-

author: alm
author_rank: Super Contributor
timestamp: January 23, 2023, 10:10:49 pm
content: I think the new sweeps are really interesting. Based on that, I'd say that:This behavior is clearly indicating a design issue (software or hardware).Based on the green series the readings near 0 V can be pretty much anywhere. They don't have to be in between the -500mV and +500mV values. Was the green unit zeroed before the sweep?The slope (gain error?) can have either sign. This is consistent with the theory that this is an effect that is partially but not fully corrected.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 23, 2023, 10:40:03 pm
content: The U11 and R35/R36 part is creating a negative reference of 1/4 the main reference.This voltage is than used to add some negative current via R34 to the summing node where the input signal via R29 and the switchable refrence current via R31 meet. From the sign it is opposite of the reference and thus allow getting a little beyound zero.The part with U10, the 1M resistors and DG308 CMOS switch chip acts as a precission switch with sense from the output side.So there is either current flow of some 1.5 mA or not.I don't any other connection of the reference to the ADC chip - so chances are high the DG308 would be the main ref. switching of the ADC.The gain setting resistors to set the ADC gain in relation to R31 should be R31/R32/R33 from the amplfier PCB/part. They somehow use the same resistor lables there, which can be really confusing.I don't think it would make sense to replace R31 as it has to be matched to the 3 other ones. One would need a matched set of 10 K , 2*5 K and 500 ohm.Because of the rather small contribution R34,R35,R36 are much less critical - they could still have drifted over time and caused the problem (a factory cal value not longer good). Ideally I would keep the 750 K and if required change the R35/R36 pair for the voltage gain. Matching from R31 to R34 would be nice, but no need to also match the R35/36 pair. The the start as a test it would likely be soemthing in parallel to R36 to see if there is change at all.The current added through R34 (750 K) is rather small: 1/600 the reference or 1/60 of the 20 V (+ overrange) range. So this should be only some 0.3-0.5 V in the 20 V range and quite a bit of this would be lost to a minimum time for the ref. Switch to be on. To me it already looks on the low side to start with, with very little overlap.A combination of low DC and AC could be a problem for the DMM. Too much hum (in the signal) or maybe the supply could than cause linearity problems for a voltage near zero.

author: branadic
author_rank: Super Contributor
timestamp: January 25, 2023, 07:10:26 am
content: I have used the P6048b which showed the opposite INL with respect to the other two and attached a 5M resistor to R36. As can be seen the INL curve has flipped and is slightly smaller in max value.I already prepared a TDP1603 network yesterday to fit instead of R35/R36 to see if that makes any difference.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: January 25, 2023, 10:41:11 am
content: The change in the jump is from -35 µV to +24 µV or some 59 µV of change.The shift in ADC input zero should be ref Voltage(~7V) * 10K/5M * 50K/750K = 930 µV for the 20 V range. If not corrected for this shift should contribute twice to the jump near zero.The 50 K /750K factor is R33(amplifier board 20 V range) /R34.So the shift in the ADC zero point is corrected to some degree (some 98.5%), but for some reason not perfectly. Somewhat doubt it, but it could be a serious INL problem of the ADC for low voltages (like < 100 mV), so that the zero correction is that far off.Another possible cause could be a software problem, like missing a calibration factor (or using a wrong one) for the ADC gain in the zero term. Some 1.5% off from nominal gain is plausible (e.g. for the 7 V ref., maybe a resistor ratio)I don't think one would need a resistor network for the R34/R35 couple. I see no reason why an accurate 10:40 ratio should solve the problem.The 5 M parallel to 40 K is a bit less than 1% change, so quite a lot compared to resistor stability. So there is no need for extra high stabilty, but more for just the right ratio.Trimming the R34/R35 ratio is not ideal, but could be a relatively simple fix for the INL. It would still take quite some time for 2 x INL measurement with different settings and use of the ouput of U11 as an intermediate reading, assuming that the jump is linear depending on that voltage.

author: branadic
author_rank: Super Contributor
timestamp: January 25, 2023, 05:13:00 pm
content: I used what I had at hand, above 820k the choise in my stock is pretty small. So I've used 5M||40k.The reason for my idea to use a network for the R35=10k and R36=40k is good thermal matching, but also the ease of adjustment by adjusting individual 10k elements with lower parallel resistors instead of something in the mega ohm range. In this configuration 4x 10k elements can be used to form the 10k and the other 4x elements to form the 40k, with the elements in the network used in an interdigital fashion.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: January 28, 2023, 12:25:30 pm
content: While I can flip the sign of the INL curve with a parallel resistor to R36 as shown before, adjusting it's value doesn't change anything and INL stays the same.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: January 28, 2023, 02:38:55 pm
content: Quote from: branadic on January 28, 2023, 12:25:30 pmSo not a simple factor that is wrong with handling the ADC offset.If the error stays the same or just 2 possible scenarios (signs of the INL error) a possible point could be having the Zero adjustment done only with 1 relay setting (as a failure in the Zero procedure) and the initial offset determines which setting is used. Is the polarity relay actually switching during the zeroing call ?A point worth trying could be doing the zero procedure when in autoscale mode and thus doing it for all ranges together. Ideally it should not make a difference, but one does not know until one tries.The full adjustment may do things a little different (maybe just waiting time) than doing the zero for only 1 range at a time.A possible source of error could also be leakage between the 2 supplies for the ADC and amplifier part. Especially the current source for the resistance could be an issue as the PM relays may show some leakage - not sure if K13/K14 get a clock/drive all the time, or only when a resistance measurement is active. Ideally only the pins 9,10 and 11 of J4 should provide a link. K13/K14 should switch alternating with enough pause in between so that not much current flows there. With meter off and the connector J4 removed one could check the isolation across K13+K14. These should be pretty low leakage (e.g. 200 pA to cause a 10 µV error).

author: branadic
author_rank: Super Contributor
timestamp: January 28, 2023, 03:32:29 pm
content: Checked that. In manual range it seems to run through the same nulling twice without switching the relay, while in auto range it changes polarity in each range for nulling. So some odd behavior indicating a firmware bug?-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: January 28, 2023, 03:55:23 pm
content: Not changing the polarity for the null would be a firmware bug, as both polarities can have a different offsets as a combination of ADC and amplifier offset. With just 1 setting one can not measure both.It may be worth doing at least a fast INL check with the null in autoscale mode. There is a chance that this could be a relatively easy work around to solve the problem.

author: Mickle T.
author_rank: Frequent Contributor
timestamp: January 28, 2023, 04:37:02 pm
content: Interestingly, the 7.5-digit Prema 6040S has built-in periodic ADC zero correction in both polarities. To do this, all 3 MOSFETs (switching 500, 5k and 50kOhms gain resistors) are opened, and only the leakage current actually flows to the input of the ADC polarity reversal unit.

author: branadic
author_rank: Super Contributor
timestamp: January 29, 2023, 02:11:56 pm
content: Removed the parallel resistor to R36, shuffeled of few IC's around, gave the unit a good warmup, then performed nulling on all three units in auto ranging, adjusted 10 V range to the same 10V reference and performed INL test again.For some odd reason the INL of the P6048b is now smaller than the others, but still visible, while on the other two units it hasn't changed at all.So I guess the only thing left we can do is to analyze, understand and improve firmware for the 65C02 processor. Martin already started with that.-branadic-

author: Mickle T.
author_rank: Frequent Contributor
timestamp: January 29, 2023, 03:59:59 pm
content: If you look closely at the nonlinearity graph, you can see that in addition to the discontinuity (the cause of which may indeed lie in the firmware plane), there is a quadratic shape of the curve near zero. My research into the firmware showed no sign of special handling of this case. Therefore, I assume that the reason for the non-linearity is in the hardware.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 29, 2023, 04:21:31 pm
content: I had a more detailed look at the fine INL data from this posthttps://www.eevblog.com/forum/metrology/prema-6048-teardown/msg4655983/#msg4655983The attached graph shows the difference from the P6048 to Solatron 7081 (corrected). Vertical is difference in µV and horizontal the voltage in V.The data seem to be recorded from positive to negative. So very likely the polarity changes at some -45 mV at the point with the jump.However the data on the positive side of this still show quite some nonlinear effect. This indicates that there is not just a problem with the offset, but also a more gradual problem with linearity for voltages in the +-100 mV range. This part looks more like a hardware problem, not a firmware problem.I would expect a similar shape for the part going from nagetive to positive and thus reading the small voltages with the negative relay setting. Form that it looks like the offset handling is not that bad at all.With the nonilnearity at low voltage it only looks like the step is caused by an offest problem.If the problem is with the ADC itself, a possible fix may be adding more offset (e.g. another 100-150 mV and thus some 50-30 K in parallel to R36) this way not use the ADC in the very low voltage range. Depending on how the polarity change is controlled (ADC reading or voltage after the offset is removed) one may still have to force it back to the same voltage and not use the full larger overlap.Another possible cause of INL may be leakage from the reference transfer for the resistance ranges. One may be able to test if there is significant leakage there and for a test maybe disable &/ remove the link (e.g. remove R37/R38 or K13/K14 of the ADC board and add a link for a dummy 0 ref instead).

author: branadic
author_rank: Super Contributor
timestamp: January 29, 2023, 04:23:38 pm
content: I probably wouldn't interprete too much into that as also the S7081, which the measurements are compared to, has an INL error that is worse than a 3458A, something like +/-0.2 ppm in the 10V range. So everything below a certain level could possibly come from the device we are comparing it to.Edit: Ah, now I see were this arguement is pointing to...-branadic-

author: branadic
author_rank: Super Contributor
timestamp: February 03, 2023, 06:05:15 pm
content: QuoteSome 30 - 50k in parallel to R36=40k would be quite something, resulting in 17.1k ... 22.2k and the t.c. of the parallel resistor would contribute quite a lot. So in theory it would be better to replace it by a low t.c. 20k instead, wouldn't it?-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: February 03, 2023, 08:10:56 pm
content: The R35/R36 part would still have a limited weight. It adds an offset through 750 K while the 20 V range signal comes via 50 K. So a factor of 15 reduced sensitivty.The parallel resistor would be for testing first. If this turns out to really help with the INL one could look for a better pair of resistors to replace R35 and R36. There is a good chance that it would imporve on the INL, but not sure.A possible reason for not so good INL with low voltages could be that in this case the reference is switched on for only a very short time. The OP-amp at the integrator needs some time to settle after the reference is turned on. If turned off before settled this leads to an INL error. Onece settled it would only be a fixed contribution to the offset.The OP77 is an obsolete pretty good BJT based OP-amp (could well be selected ones for the 6048), so replacing it is a somewhat risky step. With the OPA202 there would be a possible upgrade with higher speed (about 2x) and also lower 1/ f noise. With possibly some additional gain inside the Prema chip it is a bit tricky to replace the integrator OP-amp and expecially go much faster. A faster OP-amp is also not necessary faster settling. Even the parasitic capacitance between the amplifier PCB/supply and the ADC PCB can effect (in a good or bad way) the settling as some of the capacitance is at the integrator input. Chances are R29 is there to make it more a good way. Because of this measuring the settling may not work, as the scope (also just the ground) would effect the result. The settling part in question is also tiny and not sure one would see it: Something like 1 mV for some 2 µs would be enough to cause an 1 ppm range INL error. So a measurement would only be to get an idea on how short the pulsed get.

author: branadic
author_rank: Super Contributor
timestamp: February 06, 2023, 06:36:58 pm
content: I can report that 47k in parallel to R36=40k leads to 100mV offset. Nulling however leads to Error4, which means "Error during offset measurement. The offset which is present at the input sockets is too large."So this approach is no approriate way to get down INL error.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: February 06, 2023, 08:12:15 pm
content: Not allowing a much higher offset / shift is unfortunate. It would have been a relatively easy fix.As a last resort, one could try it in combination with a modified ROM, that accepts the higher offset. Still extra effort on understanding and changing the code (probably just a constant).I don't see a principle reason why the ADC should not have enough headroom - actually surpring that the 2 V range is only 2 V and not 5 V.Replacing the OP77 may or may not work. To judge the chances a bit better, it would help to know the frequency of the reference switching and thus how short the pulses get, when at low voltage.It could be pretty hard to see the settling part of the integrator with a scope: If there is no additional gain in the prema chip it should be around 0,8 mV (dI / (2*pi*GBW*C_int) ) steps and possibly a bit tricky triggering so that the averaging mode in the scope can be tricky. The extra capacitance from the cope would also not help.From the photos it looks like the OP-amps are mostly in sockets. So a replacement may not be such a scary thing.The open loop response of the op77 does not look special - so not extra much phase reserve. The OPA277 and OPA202 as 2 possible substitutes look like they both have more phase reserve. So there is a good chance for faster settling. The OPA277 may even be available as a DIP version for a direct drop in, but with more LF noise (may be a relevant part of the overall noise).

author: Rax
author_rank: Super Contributor
timestamp: March 30, 2023, 05:09:05 pm
content: Hey, branadic and all, great thread and work on here!I may have just gotten one of these - currently in transit and I really have no idea yet of its condition (other than "it turns on..." argh). When I clicked the BIN button, I felt like I'm signing on my voltnut certificate!It looks like this exploration is still ongoing (I'm still getting up to date with it all). branadic' last post is almost two months old - I am aware of at least one other project he's been on, as that's also of high interest to me - so maybe there was just not enough time for progress. Is an update in order?I do not really right now know if/what I can contribute more data from my sample, but I'll definitely be happy to.

author: branadic
author_rank: Super Contributor
timestamp: March 30, 2023, 05:38:31 pm
content: No progress on my side and I'm not tempted to spend more time on it at the moment. Unfortunately, essele left the community and is selling hisunit, the only one who could have contributed. So yeah, I gave up on it I guess.-branadic-

author: Rax
author_rank: Super Contributor
timestamp: March 30, 2023, 05:52:40 pm
content: Quote from: branadic on March 30, 2023, 05:38:31 pmThe obvious question is - who are they selling their unit to, and is the receiver interested in furthering this?Disregarding me for a second here. Not looking for an answer to this, just to set a tone here of looking at the future.Birdseye view on this, this was a hair over 10uV offset at the 0V crossing point, is it not? Which may be as close to .5ppm (+/- of range) to be borderline compliant.

author: Rax
author_rank: Super Contributor
timestamp: April 02, 2023, 03:24:40 pm
content: I wonder of notes #2 and #3 (p.2/1) in the manual gives an indication on this observed behavior ("values in each case +/- 1 digit and after offset correction. [...] +/-0.0005% m. rdg. must be added during 100ms after signal change"). Maybe this also relates to Kleinstein's point on settling time.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 02, 2023, 05:00:45 pm
content: The 100 ms settling time are for the settling of the input amplifier - though this looks a bit excessive. It may include a little DA effect that can be relatively slow. Esepcially small changes in the input voltage should get away with less waiting. AFAIK the observed INL error was with enough waiting times and thus not extra error from settling.The settling in the ADC that could effect the INL is settling of the integrator in response the the reference switching as part of the SD ADC function. At low voltage (or slightly negative, before switching the polarity) this could include some pretty short pulses down to about 1-2 µs. The question is if the relatively slow OP77 at the integrator is fast enough to get reasonable settling in this time.The way the ref. switching is done, the input impedance of the integrator changes and incomplete settling would effect the result in a nonlinear way. How fast the Integrator settles depends on the details of the OP-amp and the part inside the custom chip (e.g. capacitive loading) - so hard to tell.

author: Rax
author_rank: Super Contributor
timestamp: April 02, 2023, 11:23:23 pm
content: Quote from: Kleinstein on April 02, 2023, 05:00:45 pmThank you, Kleinstein. I am adding a couple of OPA277s to my next parts order. As it seems all these are on sockets, it seems a fairly convenient experiment.It seems there's a couple of flavors at my typical supplier, depending on how tight the specs are on the input. Would I want the tighter spec? (P vs. PA suffix). Not sure how critical this aspect would be.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 03, 2023, 06:02:40 am
content: The main difference for the grades of the OPA277 would be the offset voltage (does not really matter as long as it is stable) and the amount of testing. So I see no real need for the higher grade version, as the other parameters are not that critical.For the settling part (hopefully fixing much of the INL problem) the OPA277 could be an easy test. For low noise an OPA202 with an adaper board from SO8 or SOT23-5 to DIP could be better (less 1/f noise).

author: Rax
author_rank: Super Contributor
timestamp: April 03, 2023, 06:33:26 pm
content: Does anyone have some insight on if/how R56 is supposed to be adjusted?The resistor on pin 11 (per 74HC4046 datasheet) seems to set the frequency range of the VCO, but I couldn't find any indication in the manual as to how and when is it supposed to be adjusted.

author: dietert1
author_rank: Super Contributor
timestamp: April 03, 2023, 06:54:59 pm
content: The 4046 doesn't require any precision adjustment. When the PLL locks, this is the adjustment. If the VCO input is stable in the 0.9 V to Vcc-0.9 V range, it is locked. When unlocked the VCO control voltage will be outside this range or oscillating.Regards, Dieter

author: Rax
author_rank: Super Contributor
timestamp: April 03, 2023, 08:27:02 pm
content: Quote from: dietert1 on April 03, 2023, 06:54:59 pmThank you, Dieter, but that doesn't explain why the control and how to adjust it (or I don't see it), or when necessary. There's no trace of direction on it in the SM.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 03, 2023, 08:39:48 pm
content: The trimmer abjusts the capture range of the PLL. An adjustment may be needed when switching from 50 Hz to 60 Hz mains frequency.The trimmer will effect the PLL test signal / test point, that needs to be stable and well inside the valid range (e.g. 1-4 V).To test if the trimmer setting is good one may have to also check a new start up, as the catch range can be smaller than the hold range.The exact trimmer setting should still not be that critcal.

author: Rax
author_rank: Super Contributor
timestamp: April 06, 2023, 02:29:20 am
content: I got my unit today. The best news about it is - it's not nearly as ugly as I thought it'd be, judging by the pics online. Score!!But more seriously, it's actually in better shape than I thought, given what I paid for it. It seems to be measuring in the ballpark in most ranges - still running through the gamut - except current (which doesn't seem to work, though a big grain of salt on that as I've not had enough time to assess).I enclose pics measuring various sources I have, including the FX which is still in my custody (about to be shipped back).Inside, the unit seems extremely clean, no work prior or butchery detected, which is great.(running into some issues with pics - how do you guys handle this, usually? I've seen tons of high res pics attached to posts!?)

author: bdunham7
author_rank: Super Contributor
timestamp: April 06, 2023, 02:38:38 am
content: Quote from: Rax on April 06, 2023, 02:29:20 amYou need to use a photo-editing program to get them down to 500k in size (for 10 photos/post), or up to 4M if you only post one photo. You still should be able to keep a pretty good resolution and quality if you use .jpg or .png. and just the right settings. Then getting them to appear as full-sized rather than a thumbnail, that's an additional step.

author: Rax
author_rank: Super Contributor
timestamp: April 06, 2023, 02:46:30 am
content: Pics.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 06, 2023, 06:16:53 am
content: Quote from: Rax on April 06, 2023, 02:29:20 amThe 6048 has very few resistors in the gain path: the 200 mV/2 V and 20 V mainly have 1 pair of canned resistors and the LTZ1000 reference to produce drift. So I would expect it to hold the calibration very good, even without ACAL. The 2 V range may be the more stable range, as here it is the ratio of 2 equal value resistors.For the current ranges there is the obvious fuse that can be bad and also relay contacts could be an issue. The terminals for current are separate, so a bit different than with most other meters. If there should be an issue this should be relatively easy to fix, though getting a relacement relay or shunt can be tricky (or ugly with a different form factor).

author: Rax
author_rank: Super Contributor
timestamp: April 06, 2023, 12:58:41 pm
content: The FX value, for anyone looking to see the deviation of this Prema from it.

author: Rax
author_rank: Super Contributor
timestamp: April 06, 2023, 08:18:42 pm
content: A question for branadic, or maybe others - does it look like the calibration procedure for this meter saves independently constants for each range and mode?I'm considering doing a quick calibration with the FX I have at hand in the 20V range, and use my KVD to divide that 10V down for the 2V and 200mV calibrations. I could possibly use my DP8200 for other ranges and DCA (or just leave alone the old cal for those ranges), but only provided recalibrating the ranges above would not discard the constants for the ranges I'd (intend to) leave undisturbed.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 06, 2023, 08:50:47 pm
content: The calibration should have constants for each range, maybe only a divider ratio for the 200 V and 1000 V range. Due to the relay way of changing the polarity, the gain should not change. The offset seems to be adjusted once before use. So proper warm up before the zro adjust can be important.The 20 V range seens to show some 5 ppm discrepancy with the FX reference. Not clear how much of this is due to the reference and maybe settling of meter internal reference after a longer time of no using it.It depends on the quality of the last adjustment though. Chances are that the meter still has the factory cal data and no later adjustments done.Chances are the 2 V range could be about as accurate. So I doubt a calibration with a KVD or to the DP8200 would improve things. An adjustment would likely only make things worse. Chances are the 6048 is still way more accurate than the DP8200 even though the calibration is 9.5 years old and the last adjustment even longer. So unless there is a damage / lost cal data or one has a really good reference one should not do an adjustment. Even many cal labs could adjust to the full factory specs.The KVD and DP8200 would only be a check. I would more consider it a check of the DP8200 and KVD if they give plausible votlages.

author: Rax
author_rank: Super Contributor
timestamp: April 06, 2023, 09:07:44 pm
content: Quote from: Kleinstein on April 06, 2023, 08:50:47 pmThank you, Kleinstein. My plan is not to adjust this with the KVD + DP8200 (at least not what my question is about, apologies if I wasn't clear).The plan is:Adjust the 20V range with the FX reference directlyAdjust the 2V range with the FX divided down with the KVD. For instance, a .1000000 ratio from the KVD would output .999996194V out and into the DMM which is at roughly 50% of the range, so well within a recommended stimulus for setting that rangeAdjust the 200mV range with the FX divided down with the KVD. For instance, a .0100000 ratio from the KVD would output .0999996194V out and into the DMM which is at roughly 50% of the range, so well within a recommended stimulus for setting that rangeI'd probably not touch any other range. At least not at this time. My statement on the DP8200 was just to mention I could do other ranges with the DP8200 (no KVD involved, at least I don't see the benefit there), but maybe later if at all.The huge benefit with the steps above would be standard transfer from the FX. I don't think I'd find another opportunity for an 8.5 meter adjustment/calibration (that's also affordable - but this far all my nearby cal labs have declined adjustment for this, and most have actually declined cal altogether).Even after the adjustments above, I could still restore the factory cal data from the other memory with the procedure in the manual. The only thing I have now which I risk losing with the steps above is the calibration from 2003 (which, you're right, may have just been plain cal, no adjustment involved).

author: bdunham7
author_rank: Super Contributor
timestamp: April 06, 2023, 09:18:26 pm
content: Quote from: Rax on April 06, 2023, 08:18:42 pmI would not change the calibration constants at this point. The meter appears to be within the 1-year spec @10V. You haven't checked it for mid-term stability, it has just been shipped and may have been dormant for some time, and also you don't have it in a temperature controlled environment. How much of that 4-5ppm apparent deviation is due to tempco is unknown at this point--on one hand they claim a 'typical' tempco of 0.05ppm with the advertised compensation scheme,but the specs list a maximum tempco(outside the +/-5C window) of 1.0ppm.Edit: that was the 6047 spec...

author: branadic
author_rank: Super Contributor
timestamp: April 07, 2023, 07:18:31 am
content: The 6048 has factory cal constants stored in its EPROM, that can be restored at any time with the unit turned off, setting the switch at the back and turning the unit on. This writes the factory cal constants from the EPROM into the NVRAM.The current calibration constants are stored in the NVRAM. If adjustmentshave been made in the past you will find them here.So if you consider to adjust some of the ranges I suggest to backup the NVRAM with the As Is statefirst, so you can restore them at any time into the NVRAM.Second, restore the factory cal constants and again backup the NVRAM, so you now can compare both and see if adjustment have been made in the past.Third, if you then adjust some of the ranges, e.g. based on the cal constants from the first backup written back into the NVRAM, you can afterwards make another backup of the NVRAM content and intercompare the three to find the constants that have changed.I also suggest to check the age of the NVRAM, it's maybe worth replacing it with a new one or with FRAM such as FM16W08.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: April 07, 2023, 08:10:52 am
content: Unless there is another change to the hardware, I don't see a need to do an adjustment. It is not sure that the FX reference is more accurate than the meter. For the KVD this may also not be so clear. It could still be well worth it to check the 2 V range against the KVD and FX reference. The DP8200 may be OK to check for the linearity in the low range - not all units may have the same low voltage problem. The INL error observed by brandic looks so obvious that they should have noticed (and ideally fixed - e.g. with a little more shift to neg. side) in developement.The 6048 stability is sensitive to the offset drift of the OP77 and LT1007 in the ADC. So when exchanging the OP77, one would have to readjust. The offset of the OP-amps is effecting the ref. voltage seen by the ADC and would thus be relative to 7 V. This should have the same effect on the scale factor of all ranges (V, amps, ohm).A backup of the NVRAM and maybe an exchange is indeed a good idea.

author: branadic
author_rank: Super Contributor
timestamp: April 07, 2023, 04:26:43 pm
content: Let's discuss based on facts, not on assumptions or expectations.1. There is a bug in the firmware, that is, with fixed voltage range zeroing is not working as one would obviously expect, the unit is not changing polarity. That only happens if the meter is set to auto range mode.2. Three different units show identical bad INL behaviour. Anyone can come up with a good explaination why that is.3. During adjustment of the 20 V range I can observe on all units that if I adjust with 5% of range and check at 50% of range, still being in adjustment mode, the 10 V is already way off.What we make out of that?-branadic-

author: bdunham7
author_rank: Super Contributor
timestamp: April 07, 2023, 04:54:55 pm
content: Quote from: branadic on April 07, 2023, 04:26:43 pmI'd make out that it's not good, but can you quantify those errors? For bootstrapping backyard-Bob type calibration such as I do, being able to transfer a single reference up and down by relying on things like the meter accurately reading 1V on a 10V range is key. If that doesn't work, I'm out of luck. So how much discrepancy is there between a 1V and 10V reading on the 6048?Edit: Ok, so I read back through the thread and I see your data. I'd just say that this meter joins the Fluke 8500A series in that INL makes up a good portion of the error tolerances. I couldn't find any express statement of guaranteed INL in Prema's specs or manual. So yeah, 10:1 or 1:10 transfers are not going to be good.I'd suggestRaxset up his KVD with the FX reference and his KVD and see what the readings are for ratios 0.1, 0.2 and so on up to 1.0.

author: Rax
author_rank: Super Contributor
timestamp: April 07, 2023, 07:12:27 pm
content: Quote from: bdunham7 on April 07, 2023, 04:54:55 pmI can do that, but I do have an ask - I hope we'll be taking this as anadditionalpiece of information on the Prema 6048 saga. None of the instruments I have here and would use for this are infallible (just like research relies on peer review) - so perfect linearity would be a great sign, but imperfect linearity may be due to any one of the components involved. All useful/additional information though.

author: bdunham7
author_rank: Super Contributor
timestamp: April 07, 2023, 07:32:15 pm
content: Quote from: Rax on April 07, 2023, 07:12:27 pmYes, either very good results or large enough errors will answer some questions, but intermediate size (small but not small enough to ignore) errors are inconclusive and just raise more questions. But that's what we're here for, right?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 07, 2023, 07:32:38 pm
content: The shown INL curves very much point to a problem of at low voltages, like below some 100 mV in the 20 V range. As a rough description this leads to all readings higher than 100 mV to be some 10 µV shifted up. So for 1 V in the 20 V range this would be some 10 ppm, for a 10 V ref. only an 1 ppm effect. The INL curve would let us expect that the adjustment at 1 V would be about 9 ppm off (to low a reading at 10 V). What was the amout of error observed ?The INL problem at the low voltages is just at the very edge of the ADCs range. This is a range where the reference is active only for very short times ( e.g. 0.15 % PWM ratio for a 120 mV shift of the zero point). I don't know the modulation frequency, but it should not be much below 1 kHz and thus a pulse length of some 1.5 µs or less. This is in a range where the OP77 at the intergrator may not yet be fully settled. Residual voltage at the integrator input can lead to an INL error. Something like 1 mV for 1 µs would be enough to explain a 10 µV error.There may be a way to make the DMM accept the larger zero shift: the manual gives a limit of 0.2% of FS and thus 40 mV for the offset correction in the 20 V range. A shift in the hardware of a little less than 40 mV (e.g. ~ 150 K or 220 K parallel to R36) has a good chance to be accepted by the firmware. From the INL curve the error would be already down to about half from this smaller shift.There is a small chance that this 40 mV limit is relative to the last setting. So one may be able to make it accept a larger shift in steps. One could test this by a simpe repeated Zero with something like 30 mV , 60 mV , 90 mV at the input. If this works, chances are it could also be used to make it accept a larger shift in the hardware.

author: branadic
author_rank: Super Contributor
timestamp: April 07, 2023, 09:08:17 pm
content: QuoteThe answer to that is already given in the data package I attached to this post:https://www.eevblog.com/forum/metrology/prema-6048-teardown/msg4652476/#msg4652476Attached a quick difference to S7081 plot of that files for 20 V range with factory constants, 20 V range adjusted at 1 V (5% of range), 20 V range adjusted at 10 V (50% of range) and 20 V range with factory constants again.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: April 07, 2023, 09:29:45 pm
content: The difference between the 1 V cal and 10 V cal curve is around 12 ppm. So that is still just in the range that could be explained by the INL error seen at low voltage. So I would not consider this a much different problem.

author: Rax
author_rank: Super Contributor
timestamp: April 07, 2023, 10:49:02 pm
content: Quote from: branadic on April 07, 2023, 09:08:17 pmCan you please detail both the exact measurement and the units on both axes of the graph? Secondly, I assume the "InitCal1" and "InitCal2" are factory cal constants during two subsequent restorations? Thank you very much in advance.

author: Rax
author_rank: Super Contributor
timestamp: April 08, 2023, 12:38:52 am
content: Here are some measurements with the FX and an ESI RV722 KVD.I used an integration time of 80 seconds - which made it very tedious, but I feel it'd return the most reliable results - and have not attempted to zero anything (which I don't think this meter is very peculiar with). Connections from KVD to P6048 are low-emf and shielded and separately guarded (AB-Precision cables), those from the FX to the KVD are just regular banana (all copper mating surfaces). Ambient temperature has been around 23.5C.A verification short with one of these banana copper cables returned a tiny fluctuation around .0000000V (last digit fluctuating up to maybe 5 on both sides of zero).9.99996194V .............. 10.0000395V (a little further delta from expected than we've seen before... fyi, it's been running since I got it).1 ratio on KVD ........... .99998300V (2V range, auto).2 ratio on KVD ........... 1.99996792V (2V range, auto).1 ratio on KVD ........... 1.0000690V (20V range, manual).2 ratio on KVD ........... 2.0000616V (20V range, manual).3 ratio on KVD ........... 3.0000561V (20V range, auto).4 ratio on KVD ........... 4.0000533V (20V range, auto).5 ratio on KVD ........... 5.0000479V (20V range, auto).6 ratio on KVD ........... 6.0000449V (20V range, auto).7 ratio on KVD ........... 7.0000412V (20V range, auto).8 ratio on KVD ........... 8.0000414V (20V range, auto).9 ratio on KVD ........... 9.0000365V (20V range, auto)Looking forward to the surgical analysis....

author: bdunham7
author_rank: Super Contributor
timestamp: April 08, 2023, 12:56:43 am
content: It looks at first glance like yours has the exact issueBranadicdescribes, if I'm understanding things. Also your 2V and 20V scales don't match, but I think that is secondary for now. Could you do another set of readings on the 20V range using ratios of 0.01 to 0.10, and the same with reversed polarity? You could probably shorten up that integration time quite a bit since there's no point in wringing out every last digit at this point.

author: Rax
author_rank: Super Contributor
timestamp: April 08, 2023, 02:45:35 am
content: Quote from: bdunham7 on April 08, 2023, 12:56:43 am4s integration on the 20V range (manual) at 26C, positive readings:.01 ratio on KVD ............. .100098V.02 ratio on KVD ............. .200099V.03 ratio on KVD ............. .300098V.04 ratio on KVD ............. .400098V.05 ratio on KVD ............. .500097V.06 ratio on KVD ............. .600097V.07 ratio on KVD ............. .700096V.08 ratio on KVD ............. .800096V.09 ratio on KVD ............. .900095VNegative (leads reversed):.01 ratio on KVD ............. -.100088V.02 ratio on KVD ............. -.200090V.03 ratio on KVD ............. -.300088V.04 ratio on KVD ............. -.400087V.05 ratio on KVD ............. -.500087V.06 ratio on KVD ............. -.600086V.07 ratio on KVD ............. -.700086V.08 ratio on KVD ............. -.800085V.09 ratio on KVD ............. -.900084V

author: bdunham7
author_rank: Super Contributor
timestamp: April 08, 2023, 03:06:08 am
content: As you can see you have a ~.00009V constant offset that reverses itself with polarity. Just like the 3 unitsBranadicmeasured.What does it measure with a shorted input? If this is a design issue, I don't see how it could ever meet its specifications of 5ppm + 1ppm/range (20uV).

author: Rax
author_rank: Super Contributor
timestamp: April 08, 2023, 04:52:34 am
content: Just for fun, I did the .2V range too. 4sec integration, auto-ranging.positive readings:.01 ratio on KVD ............. .09999710V.02 ratio on KVD ............. .19999321V.03 ratio on KVD ............. .3000058V (jumped range).04 ratio on KVD ............. .4000043V.05 ratio on KVD ............. .5000032V.06 ratio on KVD ............. .6000018V.07 ratio on KVD ............. .7000000V.08 ratio on KVD ............. .7999989V.09 ratio on KVD ............. .8999969VNegative (leads reversed):.01 ratio on KVD ............. -.09999920V.02 ratio on KVD ............. -.19999524V.03 ratio on KVD ............. -.3000072V.04 ratio on KVD ............. -.4000056V.05 ratio on KVD ............. -.5000040V.06 ratio on KVD ............. -.6000022V.07 ratio on KVD ............. -.7000009V.08 ratio on KVD ............. -.7999998V.09 ratio on KVD ............. -.8999978V

author: Rax
author_rank: Super Contributor
timestamp: April 08, 2023, 05:04:23 am
content: Quote from: bdunham7 on April 08, 2023, 03:06:08 amAs far as I know, these things were supported until recently, at least in Germany (branadic may have more info on this). My own unit says "MFR SPECS" (what that means exactly, is anyone's guess...).The obviously question is who's willing to rewrite the firmware in order to fix it. Second question - how do we adjust these afterwards to pass calibration.

author: Rax
author_rank: Super Contributor
timestamp: April 08, 2023, 05:12:29 am
content: Quote from: bdunham7 on April 08, 2023, 03:06:08 amEarlier today: "A verification short with one of these banana copper cables returned a tiny fluctuation around .0000000V (last digit fluctuating up to maybe 5 on both sides of zero)." Though I'm not quite seeing this right now (seems to fluctuate between hundreds and tens of nV at different times of the day?). I wonder if all this is due to some stability issues (leaking, etc.).

author: dietert1
author_rank: Super Contributor
timestamp: April 08, 2023, 06:02:41 am
content: With other DVMs we have Autorange and Autozero. For calibration one usually turns off Autorange. Calibration needs to be done per range. Sometimes one calibrates one range and keeps previous divider ratios, e.g. in AutoCal.With this meter there is a similar problem with zero calibration and/or Autozero. Calibration needs to be separate for the + or in the - range. To calibrate the +20V range one would for example adjust at +100 mV and at 10 V and the - 20 V range at -100 mV and -10 V. So there should be four calibration constants. Looks like they have one constant (slope) instead of four.Maybe one can implement this calibration on the host. Does the meter tell whether a measurement was taken in + or in - range? Just the sign of the result? Does the meter implement separate Autozero for the + and - range?Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: April 08, 2023, 08:50:21 am
content: The meter seems to no support a classical auto zero. One has to do a manual zero with an external short at some time after warm-up (can take quite long as there is no fan, but not sure how much the zero drifts with warm up). After that the zero point should be pretty stable. The Zero may even be stable over a limited time, like a few days or weeks. For a new bought meter it would be a good idea to do a careful zero for all the ranges.The zero should be separate for both polarities (offset from the ADC and offset from the input amplifier) and AFAIK the meter does both togehter - there would be essentially no good way to tell it which polarity.As the polarity reversal is with a relay one may not need a separate calibration for the positive and negative sign. The point around zero is adjusted with the zero adjust and not just calibration. It could still make sense to have both - more like a test, that usually passes.Getting quite difference readings in the 2 V and 20 V range is a bit surprising. The zero adjustment does not look perfect (difference between the polarities), but the zero error still looks smallQuote from: bdunham7 on April 08, 2023, 03:06:08 amThe error that brandic saw is on the order of some 25 µV extra 'step' in the +-50 mV region. So this would still be just in the +-1 ppm of the range.I would consider this rather high to be a design issue, though having nearly the same type of error with 3 meters is suspecious. There could still be a variation of the parts used (e.g. different batch of OP77).The data from RAX look much worse. I did a quick plot of the low end data for the 20 V range. The attached graph shows about 10 x the step. The parts for the pos and neg sign still look linear and with the same slope. Part of the step could still be from an old zero setting or possibly an issue for the KVD (adding some offset) - though not likely.

author: dietert1
author_rank: Super Contributor
timestamp: April 08, 2023, 11:17:17 am
content: If the + and - range is by a good relay with low thermal EMF deviations, then this explains the good symmetry between the + and - ranges.Some way or the other the zero adjustment gets lost during calibration. Apparently their calibration formulas don't preserve the previous zero adjustment but some other voltage, maybe the previous reference voltage.Does the instrument have a calibration constant for the internal reference voltage? If you protect the internal reference voltage as a calibration point and the reference has drifted due to aging, the slope adjustment will destroy the zero.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: April 08, 2023, 01:47:20 pm
content: The polarity reversal is via a relay used for current polarity switching. So even if there is thermal EMF at the relay it would not matter. The important parameter for the realy is low leakage when off. Here relays are usually quite good. A possible weak point with the polarity switching could be leakage current between the supply for the input amplifier part and the ADC+reference part. This could be the transformer and the charge pump part for the ohm mode. A transformer should be resonable well isolated compared to 50 K (20 V range). For the ohm part one could try removing the photomos realys (K13 and / or K14 or the 4 parts to replace them) from the sockets and have a short for the output side (C32) instead. I think this was discussed before, but not sure if actually tested. I don't expect the photomos part to be causing the problems. A quick check of the photomos switches could be by reading a short in the 20 V range and than applying some heat to the photomos switches. If leakage there is an issue heat would normally make it way worse (e.g. > 10 µV of effect for a 10 K rise).It looks like the zero adjustment is seprate from the gain calibration. The zero adjust point is expected to be done by the normal user from time to time, ideally before each use. I don't think the gain calibration would have much effect of the zero. Nomally the changes in the gain from doing a new gain adjustment are small (e.g. < 100 ppm and often < 10 ppm). The zero point of the ADC itself is shifted to the negative side by some 120 mV. So even of the gain factor is used for this shift it would effect the zero by some 0.1 - 1 µV.It would be only a very minor point if one would need to do a new zeroing after doing a gain adjustment. One would have to do it after some time anyway.The INL problem initially looked like an offset problem, like a mixed up zero constant (for the data from RAX there the missing zero adjustment could be a big part). However the more detailed measurments with more points have a nonlinear curve from about -100 mV to +100 mV. The zero adjust works, but is just in the nonlinear range. So it is more a problem of the ADC when measuring small voltages (or possibly low voltage leakage with the photo-mos switches).I don't see a path that the DMM can directly measure the internal reference. At best there may be a way via ohm current source and shunt, but even that is not for sure. So I don't see a way to have a cal constant for the reference voltage. The cal. constants should be more the ADC scale factor including the reference for the 200 mV / 2 V and 20 V ranges. This may be just 1 number each and no need for a separate constant for positive and negative sign. The zero / offset should be a separate constant per range and polarity. Changes are the 6048 has additional corrections based on the temperature measured on the output side. The extra effort for a 2nd ADC only makes sense if it is used for more than just noting the cal temperature. Chances are the temperature dependent part would be a special factory calibration including tests at different temperatures (e.g. 10 C and 30 C ambient).

author: Rax
author_rank: Super Contributor
timestamp: April 08, 2023, 03:07:07 pm
content: Quote from: Kleinstein on April 08, 2023, 08:50:21 amIt looks like the large difference in my case is due to no attempt to zero this prior to the measurements, is that correct? As there are two issues to consider, the intersection with the zero point (large offset, so I need to carefully zero this), and the slope, which seems to be linear and of correct slope?What would then be a reliable and effective zeroing procedure? Last night I did a quick one with a 3' cable, it was on AUTO and so it went through all VDC ranges. But maybe I need to redo it with the low-emf cable I have connecting the meter to the KVD to collect it readings. I could just short the end of that cable.I don't have one of those shorting blocks (wide trace between HI and LO and thin trace between SOURCE and SENSE), so I wonder what would be a well thought out zeroing. The manual is not particularly clear on this.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 08, 2023, 04:23:05 pm
content: Not having done the zero for a very long time and a possibly bad old value from the prior owner could explain the step in the curve. So the large step may be just the missing zeroing.After a proper zeroing, repeating the low voltage test in the 20 V range would make sense. A few even low voltages (0 , +-10 mV. +-20 mV) could also make sene in the curve.For zero one should have V_hi connected to V_lo with a low EMF connection, e.g. simple copper wire. In additon the gurad should also be linked (but low EMF not critical there).A shorting block as used for 4 wire ohm should be OK too, but is not needed.As one may have to repeat the zero from time to time it makes sense to get some shorting connector that is reasonable easy to handle without introducing much heat.One may have to wait for a stable reading before doing the zeroing. For the 20 V range this should still not be too critical. Low EMF gets more of an issue on the 200 mV range.The short directly at the KVD can also be OK, though not ideal for a more normal use. The test with the KVD assumes that the KVD zero is working - so one could also use that for the test.

author: dietert1
author_rank: Super Contributor
timestamp: April 08, 2023, 04:52:25 pm
content: If you calibrate the 20 V range with a 10 V reference with a zero offset of let's say 20 uV going unnoticed, then the slope/gain factor determined by the calibration will be off. As soon as you zero the meter, the 20 uV (2 ppm) will be missing at the upper end.And if you have a hot relay (an error present for example in the Advantest R6581T) that generates let's say 10 uV thermal EMF, then the + and the - range won't meet ends near zero. As the zero that you create with a short may be in any of the two ranges and you don't know which one, there will be no way to correct that on the host. If the overlap of the + and - range is 120 mV, a zero adjustment using a short won't be reliable.If the instrument had an indicator whether it is using the + or - range, this could help. Maybe one can mod this. Using a spare digital input (like some meters provide), one could transfer the bit to the host. For voltnuttery you can just ignore the minus range. Don't know if one should disconnect the relay in order to always measure in the plus range.Regards, Dieter

author: Rax
author_rank: Super Contributor
timestamp: April 08, 2023, 05:41:19 pm
content: Here are some readings after redoing the zeroing earlier today. This was with the attached low-emf cables and my kludged shorting block, and included shorting the GUARD. After all the measurements, I see a .00000064V drift of the zero (see pics).Integration was maintained at 20s to get all the digits.9.99996194V .............. 9.9999712V (this is the closest I've ever seen a reading of this...).1 ratio on KVD ........... .99997780V (2V range, auto).2 ratio on KVD ........... 1.99996355V (2V range, auto).1 ratio on KVD ........... .9999950V (20V range, manual).2 ratio on KVD ........... 1.9999860V (20V range, manual).3 ratio on KVD ........... 2.9999810V (20V range, auto).4 ratio on KVD ........... 3.9999792V (20V range, auto).5 ratio on KVD ........... 4.9999740V (20V range, auto).6 ratio on KVD ........... 5.9999702V (20V range, auto).7 ratio on KVD ........... 6.9999686V (20V range, auto).8 ratio on KVD ........... 7.9999688V (20V range, auto).9 ratio on KVD ........... 8.9999638V (20V range, auto).01 ratio on KVD ........... .0999683V (.2V range, auto).02 ratio on KVD ........... .19999265V (2V range, auto).03 ratio on KVD ........... .29999765V (2V range, auto).04 ratio on KVD ........... .39999582V (2V range, auto).05 ratio on KVD ........... .49999496V (2V range, auto).06 ratio on KVD ........... .59999384V (2V range, auto).07 ratio on KVD ........... .69999080V (2V range, auto).08 ratio on KVD ........... .79998900V (2V range, auto).09 ratio on KVD ........... .89998784V (2V range, auto)I went ahead and stepped down one more:.001 ratio on KVD ........... .01000050V (.2V range, auto).002 ratio on KVD ........... .01999883V (.2V range, auto).003 ratio on KVD ........... .02999932V (.2V range, auto).004 ratio on KVD ........... .03999877V (.2V range, auto).005 ratio on KVD ........... .04999845V (.2V range, auto).006 ratio on KVD ........... .05999756V (.2V range, auto).007 ratio on KVD ........... .06999700V (.2V range, auto).008 ratio on KVD ........... .07999628V (.2V range, auto).009 ratio on KVD ........... .08999589V (.2V range, auto)

author: Rax
author_rank: Super Contributor
timestamp: April 08, 2023, 05:46:12 pm
content: Quote from: Kleinstein on April 08, 2023, 08:50:21 amHow do you go about plotting this? Is there a convenient way (program, website, etc.) to input the readings and have it spit out the graph? I'm dreaming about something like this and there's got to be something out there. I don't have an even "relatively quick" way of doing it.I really appreciate all the pointers.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 08, 2023, 06:06:42 pm
content: Most of the relays in the 6048 are latching types controlled via a series capacitor. So they would not run hot. Normal realys are used a few in the AC part.Adding a LED to show the polarity (e.g. with low current to the drive side before the capacitor) could help a little.The 120 mV overlap is the theoretical value. The actual used overlap is considerably smaller (set by the SW), probably more like 40 mV, as this is the limit of the offset to compensate in the zeroing function. The old INL curve from brandic also has a jump in about that range.The ADC will not work well for very small values as this involves very short pulses. So one expects a somewhat nonlinear behavior for very low ADC readings. The question is mainly how far the nolinear part extends and if the 120 mV of overlap is enough. The INL test show quite nonlinear behaviour when going slightly negative (up to -40 mV) and thus closer than 120 mV to the limit. The rather linear part only starts at 50-100 mV. So it looks like the overlap is a little to small and should have been more like 200-250 mV. However the SW did not accept the much larger overlap.AFAIK the zeroing would do both polarities. It kind of has to as there is no good way to tell the meter which polarity. At least brandic told that a relay is engaging when doing the zero adjust. So there should be no porblem with having the zero for the wrong polarity. The problem is more a nonlinear response at very low readings (e.g. < 100 mV). So I don't think this is a SW problem but more a limitation for the ADC or less likely leakage from the Ohms part / charge pump.Quote from: Rax on April 08, 2023, 05:46:12 pmI used gnuplot. For the last plot the first part as copiing the data to an ASCII file and add the - for the KVD. The inside Guplot the commoand:fit a*x+b 'C:\Users\fuli\Documents\temp\prema6048 lin.dat' using 1:2 via a,bplot 'C:\Users\fuli\Documents\temp\prema6048 lin.dat' using 1:($2 - a*$1-b)The graph is than coppied to the clipboard and exported as .png. file from a graphics program.So the graph is the data minus a fitted line.Gnuplot may be a bit outdated, but it is free and available for many platforms. (Linux, Windows, VMS)

author: Rax
author_rank: Super Contributor
timestamp: April 08, 2023, 06:19:37 pm
content: Wait. Doesn't Excel have this?... (Graphs based upon tabulated data)

author: Kleinstein
author_rank: Super Contributor
timestamp: April 08, 2023, 06:21:37 pm
content: One can also use excel for plotting, at least when the number of data points is small. It gets ugly with 1000s of points.

author: Rax
author_rank: Super Contributor
timestamp: April 08, 2023, 07:22:21 pm
content: Here's a first attempt at this.If anyone had a chance to digest this newer data and has some input, I'd appreciate it. Did zeroing improve things at all?Scrambling through my day, so no real opportunity for me yet.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 08, 2023, 08:06:22 pm
content: With just a few points higher up in the 20 V range there is not much to see. One would need the other polarity to see if the point line up.The critical range is something like 0 to 200 mV in the 20 V range and maybe 1 or 2 extra points higher up (e.g. 5 V or 10 V) to get the slope.

author: Rax
author_rank: Super Contributor
timestamp: April 09, 2023, 03:57:55 am
content: Quote from: Kleinstein on April 08, 2023, 08:06:22 pmThank you for unpacking that, Kleinstein. It looks like I'll have to perform those more deliberate steps past handing over the FX to the next custodian, but that's alright. I don't think this empirical research relies on having the FX, so I can continue it based upon my own 731B and other references I have here. Ultimately, one thing I seem to be able to rely on with the 6048 is extremely accurate near 10V voltage readings, which essentially makes the FX optional.

author: TUMEMBER
author_rank: Regular Contributor
timestamp: April 09, 2023, 03:21:10 pm
content: Something seems odd about the data you provided. Such a rapid change of the error sign in one measurement step is improbable. Maybe it's a bad linearity of the meter or maybe a data error.

author: Rax
author_rank: Super Contributor
timestamp: April 09, 2023, 03:28:03 pm
content: Quote from: TUMEMBER on April 09, 2023, 03:21:10 pmThat last step is the direct reading of the FX by the 6048 (not going through the KVD). Not sure exactly how that impacts the plot and drawn conclusions off of it.In fact, one thing I'm learning about this instrument is how important it is to let it do enough passes and have plenty of integrating time to be as accurate as it can be.How I know this is that I am extremely close to Illya's FX value by giving it enough time as per above. After multiple passes at 80s integration, it now displays 9.9999678V, which is probably as close as anyone would ever expect this to be to another extremely stable reference that is calibrated to a high standard (the FX, at 9.99996194V). To my math, that's a little over half a ppm of 10V?Not sure how to interpret all this data but one thing I will do next week is to redo all these readings with plenty of integrating time and multiple passes. I feel that's how the most accurate data will come out.To clarify, "Output" is the actual reading on the meter. "Stimulus" is what the output from the KVD should be, with an ideal ratio and treating the FX reference value as absolutely true. The last reading bypasses the KVD altogether, and is the direct of the FX on the meter, with the same integration (20s) and only a pass or two at reading it.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 09, 2023, 04:30:13 pm
content: When using only a 2 wire connection, the loading from the KVD can make a difference. So the 10 V point directly of the reference can be different from 100% setting at the KVD.I would not make too much out of getting the same value at 10 V. Everything better than a few ppm would likely be by chance, depending on the last calibration of the 6048. The accuracy of the last adjustment can vary quite a bit, dependen when and where it was done.

author: dietert1
author_rank: Super Contributor
timestamp: April 09, 2023, 06:33:56 pm
content: If you think you are measuring the FX to some uV: What happens when you reverse polarity, so that the meter shows a negative 10 V? This is what people do in metrology as a consistency check and to reduce some sources of error. Then the average between the two readings gets used as a result.Thinking about the 6048 plus and minus ranges: Would be nice if the meter got some means of manual selecting which of the two ranges, i mean similar to "Autorange off" and then selecting a range manually.Regards, Dieter

author: Rax
author_rank: Super Contributor
timestamp: April 10, 2023, 01:14:13 pm
content: New data from yesterday, below. Tempco of the stimulus has got to be much less stellar, being from my DP8200 this time. Temperature has been 24.4C when I started, not entirely sure at the end (forgot to check...).Two different graphics styles used, hopefully for legibility.Tried to provide good granularity between -100mV and 100mV, then a couple other points on each side of that at -.9V, -.5V, .5V, and .9V.

author: Rax
author_rank: Super Contributor
timestamp: April 10, 2023, 01:38:13 pm
content: Data in .xlsx format is anyone wants a more convenient way to grab it and visualize it better (I am unsure I'm doing that right).

author: Kleinstein
author_rank: Super Contributor
timestamp: April 10, 2023, 01:39:54 pm
content: The graph is not really telling much. The meter is still linear enough to high the INL error in the plots. So see the error one would have to plot something like the reading minus the stimulus or the reading minus a linear fit.Here is the plot for the output-stimulus.Looks pretty much similar to the data from brandic.

author: Rax
author_rank: Super Contributor
timestamp: April 10, 2023, 01:58:50 pm
content: It looks like the non-linearity at the zero point crossing is about 1.5ppm of reading, then? Which is three times the claimed .5ppm.If my math is right - and I'm still digesting this - but I don't think I have a huge problem with that. It may not be a $10k competing meter, but it is what it is, and my personal investment in it misses a lot of digits from that other sticker.It may also be possible it can be modified and improved... We shall see.(enclosed is my version of the more legible graph generated earlier)

author: Kleinstein
author_rank: Super Contributor
timestamp: April 10, 2023, 02:10:35 pm
content: The nonlinearity error is around +-12 µV for the extremes. This is only a little more than 0.5 ppm of the 20 V range.With the somewhat common an rather similar INL error the question is if this a design fault (maybe only after some design change - the photo-mos switches for the ohm source seem to have changed from 2 x 2 channel one to 4 x channel ones) or a kind of common aging or damage / aging.The error seems to be relatively easy to find so I would have expected to find such a problem earlier - ideally aready in the design phase, so that they could have fixed it (e.g. with more overlap)If the error is known and reasonably constant over time and temperature one could apply a numeric correction. No ideal, but maybe some workaround.

author: Rax
author_rank: Super Contributor
timestamp: April 10, 2023, 02:19:25 pm
content: I also think my sample is right on the dot at 10V, so I assume its last cal used that stimulus to software-adjust it. From that perspective, I think I need to be watching my expectations with its accuracy as I depart from that value, and what happens at 0V is essentially a worst case scenario. Also, this is in the 20V range, is it expected that does this applies similarly to all ranges? I guess I can map that out later this week.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 10, 2023, 03:05:49 pm
content: I would expect a similar behaviour in the other ranges too. For the 200 mV range it may be a bit difficult to see as the zero offset can be relevent in this range.A test in the 2 V range could really make sense. If the problem is from the ADC side the error should also shift to smaller stimulus values (so +-5 mV instead of +-50 mV range for most of the nonlinear part). If the error would be from the photomos switches the stimulus range should be about the same. So still +-50 mV in the 2 V range.

author: Rax
author_rank: Super Contributor
timestamp: April 11, 2023, 01:28:39 am
content: I am trying to take a step back and gain a bit of a wider perspective on this issue.I can't really find much on the expected linearity (+,0,-) of the meter. The tightest spec I can find states .5ppm of reading, and/or of maximum reading, (cumulative), which makes the observed magnitude of the error failing at worst just by a hair (does it even, really, fail? I'm not quite sure I can clearly find that in the specs). It also seems like better calibrations (factory, etc.) can make this marginal failure either marginally pass or fail less. In any case, it seems to me all these cases are marginal failures.Am I misunderstanding this?

author: Rax
author_rank: Super Contributor
timestamp: April 11, 2023, 02:02:13 am
content: I've also checked my meter against David Partridge's test, and get:step 4: 10.000069Vstep 6: -10.000050Vstep 8: 0.000092V (!)step 11: 9.999974Vstep 13: 9.999973VSo I don't think my meter is inflicted by that issue, is that correct?

author: dietert1
author_rank: Super Contributor
timestamp: April 11, 2023, 07:16:57 am
content: Your nonlinearity curve near zero looks like what branadic was showing: The plus and minus range don't meet ends at zero, with an offset of about 24 uV in your case. The shape of the curve indicates that the instrument does not remain in one of the ranges while within a +/- 100 mV or +/- 40 mV margin, but it is using both ranges.How is that possible? To start with, the instrument does use both ranges for zeroing. When you get close to zero the instrument can use the sign of one measurement to determine the + or - range for the next measurement. So it preserves the +/- margin around zero for some AC in the input signal. If one takes averages, very close to zero one will have the observed nonlinearity shape with an edge as sharp as noise allows.The 20 uV to 24 uV offset appears very similar for several instruments - likely a constant (error) built into the firmware. Dirt effects like thermal EMF of the polarity relay might add a uV or two.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: April 11, 2023, 08:24:28 am
content: I don't think the curved from between some -100 mV and +100 mV is due to averaging over using the + and - relay setting. Most of the data shown here are with a rather long integration time of some 20s / 40s and thus not averaging very much. The jumps would also be visible and I would not expect a relatively smooth curve. It is more that the meter stays with the old sign until it reaches some -40 mV and than changes the sign. The switching takes time for settling and thus wants some hysteresis to avoid overly frequent switching.One of the older curves from brandic showed the contineous curve down to -40 mV and only than a jump.When using the KVD one may have forced the switch over already near 0 mV, e.g. from the sequence the points are measured.My best guess on the nonlinearity is from the ADC getting nonlinar when getting close to the low limit of the hardware (some -120 mV theoretical, maybe -100 mV if pulse skipping is excluded). At the very edge the pulses for the reference are short (like < 1.5 µs for 120 mV from the edge). Due to noise and hum some pulses may end up even shorter (the clock is not very high), and depending on the HW possibly are even missing. It is expected to get some nonlinear response very close to the edge - the question is only if the 120 mV distance to the edge is sufficient. I understand that they want to keep it small, as there is some drift with the shift.To me pulses of some 1.5 µs look rather short for an not that fast (typ 600 kHz GBW) OP77 in the integrator.I don't think the nonlinear part is from some numerical correction that is going wrong. The more logical countermeasure would be a slightly larger shift. A numerical correction could be tricky, as I would expect hum to change the INL (make it worse). It would however need quite some hum (e.g. > 20 mV) to make a difference.When assuming an ideal curve through zero, the data would be slightly out of spec (~0.6 ppm off). If one could have some residual offset for a short, one could get parameters to reduce the INL error to maybe 0.3 ppm and thus in spec. It would still be rather unconventient to have an offset and it would also need extra effort to set it that way.

author: Rax
author_rank: Super Contributor
timestamp: April 11, 2023, 02:54:46 pm
content: Quote from: Kleinstein on April 11, 2023, 08:24:28 amI think it should be a pretty easy test to drop an OPA277 in there and see where linearity lands. I assume calibration would be thrown off (but undisturbed when the old op amp is being swapped back - thoughts?), but I think the plot going through zero should be visible.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 11, 2023, 03:04:29 pm
content: Yes, changing the OP-amp would effect the calibration: the offset of the OP-amps is subtraceted from the 7 V reference. Changing back to the old OP-amp would largely bring back the old offset and calibration.One should still first to a comparison to avialable other reference before the test.

author: branadic
author_rank: Super Contributor
timestamp: April 11, 2023, 09:14:02 pm
content: Nice to see, that my observations were confirmed independently, so it's neither a fluke resulting from my measurements nor something specific for the meters on my desk.QuoteI have an OPA202 coming along and can give that a try too. As far as I understand no adjustment is required for the investigation process, it's enough to see if the shape of the INL curve has changed or not. So pretty much straight forward.I was also in contact with someone else having a white 6048, which was part of an INL-testing at Prema as he said. At least in the 2 V range its INL was <0.2 ppm. I'm still waiting for number of the higher ranges though.The so far seen ±0.5 ppm INL in the 20 V range is not too bad, but not exceptionally good either, my Solatron 7081 shows ±0.3 ppm, still way larger than a typical 3458A and thus limiting the instrument for voltage transfers.-branadic-

author: Rax
author_rank: Super Contributor
timestamp: April 14, 2023, 02:43:54 pm
content: Quote from: branadic on April 11, 2023, 09:14:02 pmI'm happy to contribute any useful empirical data that is relevant to the assessment of this unit.One aspect where we may not have the advantage of a completely randomized set of lab data inputs (between my bench and yours) is the fact that, as far as I know, your source (AN3200, correct?) and mine (DP8200) are AFAIK essentially the same unit. I've collected data with the FX while I still had it here, and will seek to generate the INL plot with those too. Also, the fact that I used a KVD (not sure what your methodology has been, branadic) may introduce a bit more randomization of the test conditions, which would contribute positively to the relevance of the results. For instance, I assume both the linearity and the offset at zero of the DP8200 are not a factor.But I'd just make sure the offset on your AN3200 is at a reasonably low value (the SM stipulates under 10uV). Not entirely sure if a factor for this (depending on your test setup) but may play a role.

author: branadic
author_rank: Super Contributor
timestamp: April 14, 2023, 03:14:38 pm
content: As you may haven't noticed, but on the x-axis I have the reading coming from a Solatron 7081, not what you call stimulus. That means, whatever the AN3200 is throwing out in reallity doesn't influence the x-axis.However, the AN3200 was fully maintained and adjusted against a 3458A.-branadic-

author: Rax
author_rank: Super Contributor
timestamp: April 14, 2023, 05:26:33 pm
content: Quote from: branadic on April 14, 2023, 03:14:38 pmI did read your graphs, branadic, though I can't say I have clarity on your test setup and data collection means. I assume there is a stimulus involved (I think you stated that to be an AN3200?), though I also assume you're collecting the x axis data with your Solartron? Such that the x would become sort of source-agnostic? But please explain this in as much detail as you're willing (we can take it offline if you prefer, but I'd appreciate getting a good understanding).From that perspective, I'm not sure I understand how you're making sure the x data is at its turn agnostic of the Solartron's own non-linearity.But my comment was more general, in due diligence relative to our test conditions. I think disclosures are important to make this work useful to others.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 14, 2023, 06:04:49 pm
content: Much of the nonlinear effect seen near zero is in a rather limited range (e.g. +-200 mV). So one does not really need a high grade instrument to compare too. If really needed a HP34401 in the 100 mV or 1 V range would be good enough to see the deviation of some 10 µV.Unless there is a problem with the calibrator, the AN3200 or DP8200 should be good enough for the tests too.It would need a good instrument of other methods / tricks to check the larger range for possible additional more soft INL errors, like a U² or U³ contribution. So for they are not visible. This part would be one where the quality of the source or reference meter can be an issue.

author: Rax
author_rank: Super Contributor
timestamp: April 14, 2023, 07:23:47 pm
content: Quote from: Kleinstein on April 14, 2023, 06:04:49 pmSo, in effect, the core argument there is the enhanced granularity and accuracy of alower rangeof measurement of the instrument used as a reference. I get that. Even in that case, though, the errors would still be cumulative, correct?Quote from: Kleinstein on April 14, 2023, 06:04:49 pmI'm not so sure I follow you here, though. Those calibrators are +-10ppm of rdg + 10uV on the 10V range (the 100mV range need not apply for related and other reasons). Getting the offset to be much less than 10uV is a challenge too (probably explains the formula above). This is why I'm using the KVD which is a .1ppm accuracy instrument. I just don't see how those calibrators would cut it, but maybe I'm not focusing on the right criteria.

author: branadic
author_rank: Super Contributor
timestamp: April 14, 2023, 07:57:44 pm
content: There are two ways of measuring the INL:1. Use a well known, highly linear source2. Use "any" source and compare it to a well known linear meter (such as 3458A or similar)I do the latter, as I don't have such highly linear source. But as you already have proven using methode 1. we both come to the same conclusion, that is, there is a linearity issue with the Prema 6048 at least in the 20 V range.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: April 14, 2023, 11:15:15 pm
content: So OPA202 arrived yesterday. I gave the unit one night after I've replaced OP77 with OPA202, zero'ed it and measured INL again. Attached the file of the measurement as well as a diagramm with the outcome of it.-branadic-

author: Rax
author_rank: Super Contributor
timestamp: April 15, 2023, 12:49:50 am
content: Quote from: branadic on April 14, 2023, 11:15:15 pmI appreciate the work put in this to test it, branadic. Do I see this right - does it seem about maybe a bit over half the spread of what the stock op amp was giving out? So a path worth taking, an improvement?...

author: Rax
author_rank: Super Contributor
timestamp: April 15, 2023, 01:05:14 am
content: I'd probably also like to see an assessment of the impact of this on the state of accuracy of the instrument. In case the INL performance improvement throws the baby with the water...(I may also be able to perform this assessment, having an OPA277P in my supplier's cart... Not sooner than next weekend though.)Of course, one could always re-"software adjust" the instrument given some artifact capabilities, but if that's not available, this is an aspect to consider.

author: branadic
author_rank: Super Contributor
timestamp: April 15, 2023, 06:49:46 am
content: I wouldn't think that the OpAmp made an improvement, it's more like the process of penetrating the unit made the difference. If you followed the thread you have seen that the unit which I call 6048b changed it's INL behavior during the process (paralling resistors) and even changed polarity, although I brought it back to its original settings (removed parallel resistors), only interchanged both OP07 within the unit.1.https://www.eevblog.com/forum/metrology/prema-6048-teardown/msg4657387/#msg46573872.https://www.eevblog.com/forum/metrology/prema-6048-teardown/msg4659679/#msg46596793.https://www.eevblog.com/forum/metrology/prema-6048-teardown/msg4668043/#msg4668043-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: April 15, 2023, 08:59:06 am
content: Quote from: Rax on April 15, 2023, 01:05:14 amThere are a few side effects with the change of the OP-amp: the OP-amp at the integrator effects the calibration (scale factor - offset relative to 7 V and thus some 10 ppm of possible change). Offset drift of the amplifier would also add to the long term drift. The old OP77 in a ceramic case is likely quite good in that respect. A plastic case may add some sensitivity to humidity.A software correction is an option, as the INL part seems to be quite stable. This would likely be correction of the readings, likely in an extra step on the PC side. It looks a bit too complicated for the integrated math functions.The curve with the OPA202 looks a little better, though still not really good. Part could be from the offset adjustment. It would need a closer look at the data closer to zero to really tell. Even if only a partial fix, it supports the amplifier as a possible weak point and it could be worth trying a different amplifier. An AZ OP-amp (e.g. LTC2057 with adapter) could be somewhat tempting, though the ones with a +-12 V supply range tend to be a bit on the fast side. So there would be a chance for instability (oscillation), but also the chance for good performance (e.g. reduced noise and INL and drift) if the integrator stays stable. Already the OPA202 should have slightly lower noise than the OP77 (but likely not much - the very low frequency data are a bit uncertain).

author: branadic
author_rank: Super Contributor
timestamp: April 15, 2023, 10:53:12 am
content: QuoteI'm still not convienced about that. As I stated above, the unit that I call 6048b changed INL behavior and "polarity" during installing and removing parallel resistors and for some odd reason the INL improved (dropped by a factor of 2) along the process, but still having the very same OP77 in ceramic package installed. How do you explain that, magic healing of OP77?Unlikely. I wish we could have a more systematic step-by-step approach to figure out what's going on.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: April 15, 2023, 12:20:16 pm
content: The curve with the 6048b showing the opposite sign is missing the low voltage points. The 0 V reading in the curve looks like it is quite a bit off (-25 µV). So chances are that the opposite sign was just due to a missing or somehow wrong (effected by noise or thermal EMF or EMI) zero adjustment. Though only 1 point in the curve the reading at 0 V suggests a problem with the zero adjustment. The 0 V point is even about similar 10 µV lower than the extra polation from the positive values - so just like the later bahavior and the other meters.So far the curve with the OPA202 only has points at 0 V , +-50 mV and +-100 mV. Here the +-100 mV still look like they fit to the higher votlage part. So it is a bit hard to see if and how much the nonlinear part has moved to lower voltages. For the original HW there is a curve with 10 mV steps. I may make sense to also get similar data, e.g. from some 100 mV down to -40 mV. This would allow for a more direct comparison.The OPA202 is only somewhat faster (1 MHz typ compared to 600 kHz typ). From the limited data so far and the old curve it looks like there is a comparable error for the OPA202 at 0 V and the OP77 at some 50 mV. This would be distance to the theoretical ADC range of 120 mV and 170 mV. So some 30% reduction in the settling time needed. There was some hope for more improvement, but the GBW is no the only factor to determine the settling time. I still consider some 40% reduction in the INL error encouraging.For the HW side some part of the integrator is inside the ADC chip (pins 11 and 13). We don' know what is hidden there - it could be just a resistor, a fast buffer or maybe some amplifer.With no extra access to the summing node it can't be the classical 2 OP-amp integrator or the circuit as the Prema 5000 and similar.So it is unclear how fast the external OP amp for the integrator can be. Similar it is hard to simulate the settling with part the circuit unknown.

author: Rax
author_rank: Super Contributor
timestamp: April 15, 2023, 01:37:54 pm
content: Quote from: branadic on April 15, 2023, 10:53:12 amThat's why I am so hung on methodology and approach (I am a physicist by background, so my mind is hardwired this way). I am very satisfied we've followed two independent methodologies and arrived at the same result. That's ideal.

author: Rax
author_rank: Super Contributor
timestamp: April 15, 2023, 01:43:56 pm
content: Quote from: branadic on January 29, 2023, 02:11:56 pmI missed this message, it looks like the improvement was already present at this point? I compared the OPA202 readings with the earlier, initial ones. Maybe the magic sauce is just conditioning the legs of the ICs and reseating?

author: Rax
author_rank: Super Contributor
timestamp: April 15, 2023, 02:52:38 pm
content: Well, that's interesting. It seems I just lost C61 - I thought there's an odd smell at the bench, but I have no idea when this happened. I've seen dipped tantalum caps fail and it's not pretty. I'm thinking low-esr, 8-10,000hrs service electrolytic - I don't typically replace tants with fresh tants. Any reason to go a different route here?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 15, 2023, 03:24:35 pm
content: C61 is at the supplies, between an 7815 and 7812. A low ESR Al electrolytic should be OK. I see not need to have a tantalum, unless one wants to operate the DMM well below freezing temperature.

author: Rax
author_rank: Super Contributor
timestamp: April 15, 2023, 03:35:07 pm
content: C61 looks pretty toasty too, I typically use a WIMA film for something like this. And on further investigation R1 and R2 seem to need to be replaced. A little PS project on my hand.

author: bdunham7
author_rank: Super Contributor
timestamp: April 15, 2023, 03:56:58 pm
content: Quote from: Rax on April 15, 2023, 02:52:38 pmNo, not in an LM78xx circuit. It was probably a lower voltage version? You could use 35 or 50V tantalums, but Al El-caps will work just fine here and you likely don't need anything too special. $0.14/ea should cover it.https://www.mouser.com/ProductDetail/Wurth-Elektronik/860240572001?qs=sGAEpiMZZMvwFf0viD3Y3aZipiehufnXaZqtKrG5UnKry1vdQEzUvA%3D%3DHas its neighbor C4 already been replaced? I'd look at C62, C4 and C5 as well just to be thorough.

author: Rax
author_rank: Super Contributor
timestamp: April 15, 2023, 04:58:56 pm
content: Quote from: bdunham7 on April 15, 2023, 03:56:58 pmNope, C4 is still there (the only two tantalums in sight - why this choice?...), but I am now thinking this needs a full recap (of the PS). I'd typically bump the values up to twice the value for the larger ones (220 -> 470, etc.), though stay with value for the lower (<=100uF or so). I typically use Nichicon HE, PW, or other rugged, low-esr, long life (>5k hrs, but more like 8-10k) caps in instrumentation. I also like Panasonic (FR, FM) and Elna for some extremely low-esr (as measured @ 150kHz) caps, but typically Nichicon offers longer life at my regular supplier.

author: bdunham7
author_rank: Super Contributor
timestamp: April 15, 2023, 06:41:49 pm
content: Isn't that C4 right next to your pile of ashes? It looks a bit toasted. And isn't the other tantalum going to be C62? I'm just going by the diagrams I have.I wouldn't dream of telling an audio guy what capacitors are best, but my point was that it is probably impossible to make a bad choice here unless you shop on eBay or something. Those caps are between regulators (no ripple) and probably run at 35C if that.

author: Rax
author_rank: Super Contributor
timestamp: April 15, 2023, 07:58:16 pm
content: Quote from: bdunham7 on April 15, 2023, 06:41:49 pmWith instrumentation, my priorities change, and high reliability becomes a top factor. I keep these things running all the time oftentimes. Aside from that, once adjusted/calibrated, I want it to never need repair.Not with the vanilla 78XY/79XY series, but others (LM2940 comes to mind) the ESR of the output cap is critical for stability. I was wondering why in this case, the only two caps that are tantalums are those two. There's other caps in this PS in equivalent positions they chose regular electroytics for.The other thing here is that the U1 heatsink is very close, and there's two other un-heatsinked regulators nearby. I've seen many designs from the '80s and '90s where they ran them far too hot. Many failures due to that, and typically not the regulators themselves, but the electroytics nearby.

author: bdunham7
author_rank: Super Contributor
timestamp: April 15, 2023, 08:29:28 pm
content: Quote from: Rax on April 15, 2023, 07:58:16 pmThe only thing I can think of is that they did a bad job of snubbing the rectifiers and the resultant noise was getting through the first set of regulators. I suppose low-ESR caps might help a bit there, but if there is such noise a better fix would be a well-tuned snubbing.

author: branadic
author_rank: Super Contributor
timestamp: April 16, 2023, 06:37:15 am
content: Attached a repeated measurement overnight with smaller voltage steps around zero. Obviously no improvement at all.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: April 16, 2023, 07:40:36 am
content: I have plotted the data close to zero together with the old data from 1/21 (green).The old data are only going down and thus start with the positive relay setting and likely swtich to the negative setting at -50 mV.The new data have both directions and the range from -40 mV to +40 mV with both relay settings.It looks like there is some drift over time (could be the Prema meter or the Solartron), so the last points (just on top of the green curve) don't fully match up with the early ones.The curves are indeed not very different - more like surprisingly similar. There is some vertical shift, but that could be drift and maybe a zero adjust before fully warmed up.

author: branadic
author_rank: Super Contributor
timestamp: April 16, 2023, 07:55:30 am
content: Could also be a change in ambient temperature, I haven't checked the data for that yet.I was looking through the thread again and found your feedback:https://www.eevblog.com/forum/metrology/prema-6048-teardown/msg4668250/#msg4668250QuoteI then checked images of the boards and found that while the schematic of the ADC board in the manual refers to 6048-I-3, my unit has board revision 6048-I-2 and the other two units here have board revision 6048-I-5.On revision 6048-I-2 there is the option to populate K13 and K14, but they decided to populate 4x optocouplers instead (both footprints are present). On board revision 6048-I-5 this option was removed and we only have footprints for the optocouplers (see images in the very first post).Unfortunately, Prema has no documents of the different revisions anymore, already asked for that years ago.Edit: Since I can hear relays clicking on polarity change, the responsible relays must be sitting elsewhere, most likely on the amplifier board.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: April 16, 2023, 08:51:17 am
content: The relay for polarity switching is K7 on the input amplifier board.The photomos relays K13, K14 or the 4 replacements are part of the Ohms circuit. C31 is a floating capacitor and tranfering the reference voltage from the supply domain of the ADC/refrence to the input amplifier part. This circuit part looks relatively simple and most likely there are mainly 2 separate photo-mos switches replacing the not so common dual versions. The change could be simply due to parts getting hard to get. This may have lead to higher leakage than in the original design. Another point may possible be looking for lower leakage. The HP parts in the photos look not that bad with 20 pA typ leakage.The leakage of semiconductors can be nonlinear in the low voltage range (e.g. 25 mV) - so leakage though K13/K14 could cause nonlinearity.It would be rather surprising to have the same leakage level over multiple meters - so I don't thing the photomos leakage is causing the INL, but it is also not impossible. 20 pA * 50 K would be only 1 µV. So it would need a leakage current of some 100 pA per switch to cause the observed INL error. So about 5 x higher than typical - maybe if running relatively hot.Since it is relatively simple, one could still do the test with the optocouplers removed (and a short over C32 to get a defined value for the ohm source). This could exclude this leakage path as a possible INL source.

author: branadic
author_rank: Super Contributor
timestamp: April 16, 2023, 08:54:29 pm
content: QuoteDid that.-branadic-

author: Rax
author_rank: Super Contributor
timestamp: April 16, 2023, 09:12:46 pm
content: This is transforming a bit on a two parallel tracks - I hope now one's finding that too annoying. Both I think serve in different ways anyone working on one of these units.My PS story reveals further trouble. I wonder if the failing C61 took a heavy toll on R1/R2 (see pic) - or maybe something else has happened? I'd be pretty surprised if it's just that. Such as along the lines of typical of the times under-design and no derating of components on PS duty. Could be a bit of each, but I'd be curious to see if anyone else has been baked R1/2s like I have here.I am thinking my CMF/RN55 should do fine here.

author: Rax
author_rank: Super Contributor
timestamp: April 16, 2023, 09:14:45 pm
content: Quote from: branadic on April 16, 2023, 08:54:29 pmInconclusive? Or just not much improvement, if at all?

author: branadic
author_rank: Super Contributor
timestamp: April 17, 2023, 06:08:34 am
content: Leackage is not the problem, the INL curve remains the same.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: April 17, 2023, 06:24:38 am
content: Quote from: Rax on April 16, 2023, 09:12:46 pmR1 and R2 don't need to be stable / accurate types. Ideally this would be fusible resistor types, so types that fail somewhat graceful on overload. It also helps to have them mounted a little higher above the PCB, so that in case of failure the PCB sees less thermal load.

author: Rax
author_rank: Super Contributor
timestamp: April 17, 2023, 01:34:34 pm
content: Quote from: Kleinstein on April 17, 2023, 06:24:38 amGreat idea, thank you Kleinstein. I actually have fusible type CMF55s in exactly 10 ohm value at hand for this type of thing.Given this, I should probably go back to taking a closer look at what's going on with the current mode and see if that issue can be tracked to a PS fault.

author: Rax
author_rank: Super Contributor
timestamp: April 18, 2023, 01:29:06 pm
content: Well, in recapping and fixing the PS, I seem to have lifted a trace - I think it's one of those "guard" traces - and I repaired it by using a piece of component leg (I usually rely on those for this type of purpose). Maybe I'm imagining this, but I do feel I've shifted my 10V reading by a few ppms, darn it (Kleinstein would say - you don't really know that, and he'd be right!). Would anyone care to comment how I can fix this? How sensitive these guarding traces are to fixing, and what to look for. I'm thinking of redoing the bridge with a lighter hand - maybe the current trace has just become too fat and too close to the power supply lines of those two resistors.I could also use some pics of the same area from a different unit. There was a pretty good amount of prior repair on this unit (other traces having been lifted, etc.), so there's that excuse (being self-deprecating here...).Edit: added pic with markup for bridge repair. I did clean everything with IPA etc.

author: Rax
author_rank: Super Contributor
timestamp: April 19, 2023, 01:15:34 am
content: Well, further examination returns expected readings from my DP8200 (relative to the FX, assuming the DP8200 has not drifted at all since I had the ability to compare it with the FX), so as of right now there's no "few ppms drift." Reading too much into the "last ppms" may be what applies here. Things are not rosy, though, as I seem to be getting a new error (or maybe I'm just zeroing far more persistently than I had before) out of this ("Error 4," offset related error when attempting zeroing when too high an offset is present at its input), which I'm not quite sure how to connect with my prior saga.Edit: clarification on zeroing.

author: Rax
author_rank: Super Contributor
timestamp: April 20, 2023, 12:50:46 am
content: I wonder how stable have others found the zeroing on this unit? How often needs to be done? What seems to be making it necessary? (time, cables swap, temperature, something else?)

author: Kleinstein
author_rank: Super Contributor
timestamp: April 20, 2023, 06:30:14 am
content: There are several contributions to the offset drift (20 V range):U9 (OP77) bias dirft (effective with 50 K).Drift in the leakage currents of 2 of the DG308 reference switchs (effective with 50 K).Drift in the charge injection of the switches and drift in the difference of on/off speed.R34,R35,R36 as part of the zero shift of some -120 mV. So 1 ppm drift causing 120 nV.The drift of U11 and U9 is attenuated by a factor of 15 and thus not a big factor.Drift in the input bias current, multiplied with R30+R33. So 1 pA resulting in 100 nV.The input amplifier (ICL7650) - usually pretty small contribution.A few relay contacts (K2,K3,K4 and K5) via thermal EMF - should not be much with latching relays and thus low power.Thermal EMF at the input terminals and maybe R30.If the charge pump for the Ohms part is also active during a voltage measurement, the bias of U13 and charge injection of K13,K14 could contribute.If the charge pump is not active, there could be leakage current of K13,K14 contributing a little (possibly nonlinear, but brandics test did not show a visible effect)Some of the drift parts are at the input and some at the ADC. So they have a different effect on both polarities.For me it would make sense to repeat the zero adjust before critical measurements or with a large temperature change (e.g > 5 K) or maybe once a month. Experiance will show how much the offset actually changes in the units.So conditions a bit comparable to using ACAL with the 3458.

author: dietert1
author_rank: Super Contributor
timestamp: April 20, 2023, 08:41:23 am
content: With branadic's sweep data one can calibrate the + and - range separately. The line fits give0.5 to 10 V gain=1.000000922 offset=7.71 uV-0.5 to -10 V gain=1.000000919 offset=-1.49 uVThe two ranges don't meet ends at zero with a 9.20 uV offset. We can see that in branadics diagram.From the low voltage sweeps we can see that the 6048 switches ranges in the down sweep at the -40 mV to -50 mV step and in the up sweep at the 40 to 50 mV step. For each measurement we know whether it was taken in the + or - range. So let's apply the proper (separate) calibration and plot the corrected measurements. The diagram shows INL values compatible with zero (+/- 1 uV) plus noise, in the ranges 0.05 to 10 V and -0.05 to - 10 V. Within the -50 to 50 mV interval there are errors up to -14 and +18 uV, depending on which range the instrument is actually using. One doesn't know except in a sweep.When using the builtin common calibration and zero adjustment of the + and - ranges, these errors affect all measurements except when measuring near the calibration voltage e.g. 10V.In order to correct the nonlinearities near zero one would probably extend the negative end of the + range somewhat, maybe to -0.5 V or so, while keeping the range switching threshold near 40 to 50 mV. To test this one would fix the instrument in the + range (disconnect relay). Then you can watch the meter during the low voltage sweep and see what the range limit really is.Regards, Dieter

author: Rax
author_rank: Super Contributor
timestamp: April 25, 2023, 02:48:27 pm
content: I need to be switching to more mundane topics - does anyone have some part number of other information on the power switch?Mine seems to have failed after a short period of intermittence. I will slice open the heatshrink sleeve that covers the switch and collect more information (and see if I can repair it), but that needs to wait a day or two.

author: dietert1
author_rank: Super Contributor
timestamp: May 05, 2023, 07:40:35 pm
content: if you search ebay for "Netzschalter main switch für Marantz 2216B-2218-2226-2238 - 2238B- SC 06" you will find a power switch that should fit. I think at the time everybody used these. I think the make was RAFI.The 6048 has a rather elaborate circuit for measuring its internal temperature on the digital board, implementing a LM335 sensor with it's own ADC. In the manual they say the temperature measurement is used for drift corrections.Anybody knows how to read that temperature using GPIB?Concerning the Null adjustment: I think the instrument was meant to be used with a scanner plugin with mechanically latching relais (low thermal EMF). With the scanner one can use one channel to implement autozero.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: May 05, 2023, 09:23:17 pm
content: QuoteUnfortunately not, it's documented nowhere, if at all possible.-branadic-

author: Mickle T.
author_rank: Frequent Contributor
timestamp: May 06, 2023, 09:55:48 am
content: Prema 6048 firmware have a nice NES-like code. I have not yet explored the parser in detail, but it is very likely that it will not be difficult to add technology command handlers to it.

author: dietert1
author_rank: Super Contributor
timestamp: May 06, 2023, 12:36:15 pm
content: A disassembly i looked at showed 563 RTS and 1542 JSR. Already found repeated instances of the same procedure - a bit strange. Many layers are required to make a 6502 into something interesting.After reengineering the firmware their know-how will yet be missing. E.g. how to calibrate the temperature drift correction. As far as i understand we cannot replace a single precision resistor without loosing that part of the calibration.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: May 06, 2023, 02:13:03 pm
content: The temperature drift correction can indeed be tricky. It depends a bit on how much they correct. The obvious parameter to correct with the temperature is the overall gain per range. This would be the reference itself and mainly 1 pair of resistors per range to effect the gain. Other precision resistors, like the ones for the zero shift (R34,R35,R36) should have essentially no effect on the gain. So even a larger change like reducing R36 to half should not change the gain.A 2nd possible point the correct is the offset. Here there is an effect of R31,R34,R35,R36 and a little from the OP-amps. Here it is however not clear if this is actually done or if the offset assumed to be stable enough. The offset part could also be separate for both signs.The firmware will show how many correction parameters are used. It may also give a hint on how to program them.

author: dietert1
author_rank: Super Contributor
timestamp: May 06, 2023, 10:29:26 pm
content: I'd guess reading of internal temperature of the instrument can be invoked by one of the program numbers not discussed in the manual. The disassembly shows lots of functions not referenced from inside the firmware. There is a long jump table at 0x884C.There are several jump tables with lots of indirections meant to make reengineering more difficult (code executed from SRAM). Or maybe they just added wrappers to fit in existing software. In total there are about 520 subroutines in the firmware, but few of them with more than one or two references. There are about 20 functions with more then 10 references.Regards, Dieter

author: Mickle T.
author_rank: Frequent Contributor
timestamp: May 07, 2023, 06:59:26 am
content: There is no need to parse the 0xCA9E gateway system calls and other LUTs. It is enough to look into the procedures of the parser of the GPIB buffer line and the procedure for communicating with the auxiliary ADC of the temperature sensor through the PIA.

author: branadic
author_rank: Super Contributor
timestamp: May 07, 2023, 07:06:31 am
content: Thanks, but are there any undocumented GPIB commands hidden?-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: May 07, 2023, 08:03:23 am
content: The undocumented GPIB command will be a Pxx program call with an undocumented number.I found these: P77, P91, P92, P93, P94 and P97.The jump table mentioned above by Mickle T has 75 dummy pointers (pointers to RTS instruction = empty subroutine). If necessary one might mod one of those dummy pointers to make an existing subroutine accessible by program number. The Eprom has empty space in case a little wrapper is necessary.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: May 07, 2023, 12:02:34 pm
content: QuoteQuickly went through the programs with input shorted, but couldn't observe anything on the display.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: May 07, 2023, 06:54:39 pm
content: Well, i'm glad if nothing bad happened. Maybe the P9x calls were blocked by the calibration switch.It's a bit early. I hope i will have a logic analyzer setup for the P6048 after some days. As Mickle T wrote, one should log CPU activity during certain application level events.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: May 09, 2023, 05:51:16 pm
content: Meanwhile i looked at the relay drivers.The Prema C32 integrated circuit is a 12 bit shift register with latch and totem-pole output buffers. The P6048 has two of them on the amplifier board and one on the optional scanner. They make a 36 bit shift register that works like SPI. So there are three optocoupler isolated input lines. The CPU starts shifting out 12 bit of MUX control, then 12 bit for U2 and finally 12 bit for U1. Clock period is about 636 usec (!!). The whole transaction takes about 23 msec.I wired U2 for the logic analyzer and later included the Din of U1 using an analog channel. One can see the 12 bit delay.I think one can make the scanner using a 4094 shift register. Only the lower 8 of 12 bits are used. Does anybody know how to enable scanner operation? There may be a solder jumper on the CPU board.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: May 12, 2023, 04:14:04 pm
content: Work in progress..The LA record shows the temperature ADC readout procedure. The three signals of the temperature ADC are in LA channels 30, 31 and 32. In the zoom the shift clock (pin 7 of 6821, analyzer channel D30) goes high when writing $F3 to $6000. This is the subroutine at $CDE2. If we could give this task to chatgpt...Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: May 13, 2023, 01:07:32 pm
content: So you are owner of a Prema 6048, too? Interesting, not sure if I should say "congrats" or "Sincere condolences.".Have you tested the INL of the 20 V range on your unit already? Maybe we should also test INL in the 2 V range, I guess this is the range with the Prema 6048 showing its strength.QuoteQuoteHaven't done it myself yet, but maybe worth a try to show the differences between the three units I have, as long as I still have them.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: May 13, 2023, 01:17:15 pm
content: I would not expect the linearity to be much better in the 2 V range. This is still not that bad,as there is at least also no added INL from an extra amplfier stage added as with most other meters.Knowing about the weak spot around 0 V is already a big plus. Outside that small range the linearity looks quite good.

author: Rax
author_rank: Super Contributor
timestamp: May 13, 2023, 02:10:05 pm
content: Quote from: branadic on May 13, 2023, 01:07:32 pmc'mon, branadic. tell us how you really feel.

author: dietert1
author_rank: Super Contributor
timestamp: May 13, 2023, 04:17:54 pm
content: The discussion here convinced me that the Prema 6048 is good - with a minor problem at zero input voltage, but that should be possible to solve. We have a MAX5719 programmable voltage source that can be used with a 100:1 divider to sweep the -100 mV to 100 mV range.I checked that our meter switches 20 V plus and minus ranges at -41.5 mV (to negative range) and at +41.5 mV (to positive range), like observed on other instruments. After nulling the instrument with 30 mV on its input, the display shows 0 at 30 mV in. Yet range switching happens at the same input voltages - now displayed as -71,5 mV and +11,5 mV.In the 2 V range the overlap interval is -4.15 mV to +4.15 mV. Near zero one would use the 200 mV range. In this range stability over some hours was observed to be +/- 30 nV. For comparison: A Keithley 3706 with a 3720 muliplexer yielded an average RMS of 15 nV over many channels wired with low thermal shorts. Of course this uses autozero and recalibration by a spare multiplexer channel.I want to study the mods and tests i proposed above.Anybody here has that Prema 6048 scanner board? The scanner can be used to implement Autozero and Autocal.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: May 13, 2023, 06:24:00 pm
content: QuoteNo, all three units that are currently with me are missing the scanner card.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: May 15, 2023, 10:25:40 pm
content: Meanwhile i could confirm my recent conclusions from branadics low voltage sweeps. Using a MAX5719 based voltage source with a divider i got exactly the same image. 4 complete sweeps are measured in 20 V range and averaged. We can see the same 9 uV difference at 0 V between + and - range and the same maximum nonlinearity errors of about 15 uV. So we can safely assume that this behaviour is typical for a 6048.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: May 16, 2023, 08:20:10 am
content: Here is a similar test of the Prema 6048 2 V range. This time i changed the voltage divider to sweep between -50 and +50 mV and averaged 7 sweeps. A similar analysis separating measurements in + and - range. Range switching occurs at 4 mV when going up and at -4 mV when going down. The difference at 0 V is 2 uV while from the 20 V range result i expected 0.9 uV. This may be thermal EMF of a relay.The test voltage source with its divider has a specified nonlinearity of 0.16 uV.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2023, 09:30:28 am
content: I don't think thermal EMF from a relay should make a difference. The relays for the input before the amplifier would effect the horizontal scale and I very much doubt they would cause an error larger than 1 mV.The relay K7 for the polarity reversal is doing a reversal in the current flow direction and even if there is thermal EMF at this realy, it would not matter, similar to an effset error of U11.The points that are more like to make a difference would be leakage currents between the supplies and maybe the different resistor value (5 K vs 50 K) to effect the actual mechanism behind the nonlinearity (e.g. if this is some pulse reaching back to the input amplifier past the filtering from C90).Another point that can make a difference could be mains hum. Mains hum would modulate the input voltage and this get a mix of voltages more positive and more negative. Added hum would get a bit more nonlinearity from the times when lower in voltage. It would still need quite some hum (2 mV range) to explain the difference.The 2 V range may actually have a slight linearity advantage over the 20 V range. The 5 K resistors R32 would see less self heating than the 50 K resistor R33 for the 20 V range. This would be a small difference in the U³ type contribution to the INL. However this parts seems to be not the main problem due to good quality of these resistors.

author: dietert1
author_rank: Super Contributor
timestamp: May 16, 2023, 11:25:03 am
content: The agreement with branadics data made me wonder whether the nonlinearity might be some problem in the Prema 6048 conversion method. Above i had the trace of the temperature ADC with its 16 bit readout. The main ADC is the same chip, so in order to arrive at high resolution one needs to add lots of conversions.Maybe there is a rounding problem. With low input voltage their modulation scheme makes the integrator cycle at low frequency. One would expect something like 20 V / 65536 = 3 mV or a fraction of that. If one stays away 40 mV the rounding error might be 3/40*300 uV = 22 uV. Something to check by measurement.Regards, DIeter

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2023, 11:53:50 am
content: The problem is at the very end of the range, so more like close to the hardware zero. I don't expect rounding problems. It would very strange to have it just at the lower edge of the hardware.It is more that the PWM ratio may start to reach zero for a few cases with the help of some hum. The ADC uses a quite small (5K) resistor for the reference current. So from the hardware side of the ADC the input voltage could go quite a bit higher than the nominal 2 V or 20 V, more like 7 V or 70 V as theoretical full scale. The shift from R34,35,36 should be some 117 mV. So this would be only rather short PWM puses to happen and a logical canditate for linearity problem is when the pulses get too short. I don't know the exact modulation frequency, but it should not be super low. Asuming some 1 kHz the pwm pulses would be some 1.7 µs for zero input voltage. The clock frequency is also not very high so the 1.7 µs is something like 5-10 clock cycles only.Normally 1.7 µs should be fast enough for the LT1007 reference driver circuit to settle, so I don't think this should be the problem. The reference off phase is allways long, so no problem there.Brandic exchanged U11 (the integrator) to a fater OP-amp with essentially no change - so chance are this OP-amp part is also not the problem.The R29 and C90 RC combination at the integrator input has a time constant in the 6.8 µs range and should thus not be that relevant with the shorter pulses. If at all I would expect an effect with slightly longer pulses / high voltage.From my ADC build I found the clock generator somewhat prone to interference. Though I don't see a link to short pulses the supply for the PLL part would be a possibly path for strange effects.

author: dietert1
author_rank: Super Contributor
timestamp: May 16, 2023, 02:35:31 pm
content: If i do the calculation, i am getting -11,8 mV, not -117 mV.R35/R36 = 10K/40K = 1/4 Measured: 1,76 V/7,07 V = 0,249R31/R34=5K/750K=1/150. Checked R34 Label. In-circuit 190 KOhm.7.07 V / 600 = 11,8 mVWhere is the error?Regards, DieterEdit:Just measured some timings. At 0 V input the 7.07 V reference pulses to the integrator are mainly 17.635 usec or 15.485 usec at a fixed rate of 10 msec (100 Hz). On average this is about 1/600. CD4046 PLL output is measured as 3.684 MHz. The screen dump includes the histogram. The scope counts how often each pulse length happens. It does not look like a gaussian (noise), but there are two peaks.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2023, 05:15:17 pm
content: The difference in the zero shift come from the range use. It is some -11.8 mV in the 2 V range and -118 mV in the 20 V range. (the difference 118 and 117 is rounding).A modulation frequency of only 100 Hz is surprisingly low. I had not expect it to be that low. in this case the OP-amps are likely plenty fast.The histogram part is interesting. Chances are the separation in the 2 groups could be due to main hum (50 Hz). There are than 2 cases for the comparator trigger and feedback to happen, both with a 180 deg phase difference for the 50 Hz mains frequency. This could be mains hum from capacitive coupling at the transformer (e.g. between the 2 secondaries for the ADC and input amplifier), as the ADC input relies on the isolation there. So not surprise to find some 50 Hz hum, even of the input of the amplifier has no hum voltage.A relevant point could be the phase between the 50 Hz signal to the PLL and the PWM signal. With the PWM signal in phase with mains, there could be a point that coupling from the PWM / reference swiching towards the PLL could cause problems, like some FM modulation and this way an INL error.

author: Andreas
author_rank: Super Contributor
timestamp: May 16, 2023, 06:07:21 pm
content: Hmm,I am asking myself.What about the measurement value (noise) distribution when approaching 0V at the input.Does it stay gaussian or does it get unsymmetrically due to clipping at zero?With best regardsAndreas

author: dietert1
author_rank: Super Contributor
timestamp: May 16, 2023, 06:47:40 pm
content: Meanwhile i noticed that the two pulse length peaks at 0 V input vanished and became one with gaussian character when i nulled the instrument. The polarity relay clicked and the scope started counting at 16.5 usec (in the middle). Nulling the instrument again, it returned to the double peaking.The same happened when i watched the histogram while sweeping the input voltage. The minimum pulse length observed before the instruments switches the polarity relay was 10 or 11 usec.In this moment i am not shure in which position of the polarity relay the double peaking happens.As the ADC cycles the integrator at 100 Hz and in each cycle the maximum count is 65535 (16 bit), the maximum total in 1 second is 6 553 500 counts. This would be the full 70.7 V range in the so-called 20 V range. With the measured PLL clock frequency of 3 684 000 Hz the actual range limit of the 20 V range is at 39,7 V.At 20 V input voltage the duty cycle of the reference current is about 50 %.Regards, DieterEdit:Meanwhile the scope shows a trace of measured pulse times. While the negative range looks clean, for the positive range there seems to be 50 Hz hum. Both records are with DVM inputs shorted.Edit 2:At 20 V input the current from the front end is 20 V / 50 KOhm =0.4 mA. The DAC generates 7 V / 5 KOhm = 1.4 mA pulses. If you do the calc, duty cycle of the DAC will be only 30 %.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2023, 07:41:29 pm
content: Seeing the split histogram only with 1 polarity setting could be an artifact from hum coupled in via the scope ground. The polatiy realy changes how the grounds are related and this is expected to change the amount of hum.The splitting to the hum is still rather small and this should not cause clipping. There is still quite some headroom (e.g. some 10 mV in the 2 V range) before this happens. The splitting due to the hum in only in the 1-1.5 mV range. In addition is looks like much of the hum seen is due to the scope and likely not present in normal measurements.The 2 groups would be equally frequent and alternating between them. With intgration of full PLCs there should be always the same groups involved and thus normally no extra noise.The noise may actually change with the input signal, as the reference path is only connected when actually needed. This may lead to more noise when measuring a significant voltage. Still this wold not be with a few 100 mV, more like a thing going half the range and than it is tricky to separat it from reference noise.

author: dietert1
author_rank: Super Contributor
timestamp: May 17, 2023, 07:22:32 am
content: I thought a bit before connecting the scope. The Prema 6048 front end has two isolated parts, the amplifier and the ADC. The amplifier output connects to the ADC input via the polarity relay, without any other external connection. Hum may come into play via the common mains transformer. Scope Gnd is connected to the Gnd of the ADC as i was looking at the reference source. Don't see how the scope introduces hum, as its Gnd is the only external connection of the DVM frontend.Anyway, i will connect another DVM to the ADC input in order to check whether the low voltage nonlinearity stems from the ADC.Having a closer look at the HC4046 VCO in the ADC, i found a design problem. They made the VCO range adjustable instead of the offset frequency. This means the VCO range is huge (maybe +/- 25 Hz) in comparison to what is needed (maybe +/- 2 Hz) causing unnecessary phase noise. And the VCO can get its own 5V voltage regulator. I found a LM2950-5.0.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: May 17, 2023, 01:58:52 pm
content: For the hum part in normal use the amplifier ground part including the input terminals acts a bit bit shielding around the ADC part. With external hum much parts are floating and could move in parallel. With the ADC part grounded via the scope hum that couples to the amplifier part can now act on the ADC input, at least for 1 polarity.Because of the relatively low modulation frequency of only 100 Hz the phase noise is not as critical as with other MS ADC with with fast modulation (e.g. HP34401 and 3458 use some 330 kHz). Some of the 100 Hz synchronous hum causing the frequency modulation may however effect the linearity. This would howver not be much for the near zero range, but worse the larger the voltage. This may be the reason for not using a classical LC based VCO, as this could react to an 50 Hz magnetic field or mechanical hum.I am not sure if setting a higher low frequency limit for the VCO in the 4046 would help very much. There are intrinsic limits to the accuracy of the low power relaxation type VCO.Changing the resistor also changes the PLL loop gain. So changing to a smaller R55 and larger R56 could change quite a bit.Having the logic part of the ADC chip and the VCO on the same supply without much filtering could indeed be an issue. A separate regualtor or better filtering of the VCO supply could be a possible point to try. The 4046 VCO has quite some sensitivty to the supply.

author: dietert1
author_rank: Super Contributor
timestamp: May 18, 2023, 07:52:18 am
content: No sorry, the HC4046 VCO gain and offset confusion they made actually caused variable VCO gain - bad for control loop stability. I am correcting that mistake.In my current setup the VCO base frequency adjustment has 3.19K. The other resistor that determines VCO range is 100 KOhm (R1 in HC4046 datasheet). So the VCO range is much smaller than before. The control loop remained stable but can be improved. For example the integrator U6 needs a diode to avoid integrating to the negative side, where the VCO limits. Frequency noise as measured by the scope reduced by 3.5.Regards, DieterEdit:Just found a little trap while nulling the instrument. When you press the null button, you can hear the polarity relay without any delay. Seems they assume you already had the inpot short for some time and there is a valid null measurement for the current polarity. So they switch the relay to acquire the null reading of the other polarity. With long integration times, if you insert the short and press the null button you may get "Error 4" if the last valid reading is outside limits, like open inputs.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 18, 2023, 08:54:04 am
content: You are right, the PLL range adjustment changes the loop gain.The way the PLL adjustment procedure is, they call for a certain VCO set voltage and this way also get an about fixed loop gain. The trimmer would mainly make up for variations in the VCO. Chances are all the trimmers would be set to rather similar values anyway. Changing to the new setup with a fixed relatively large R1 and adjust R2 (from the 4046 datasheet) would be a step to a different (e.g. a factor 30) loop gain that is than fixed. If it works the smaller VCO range can be good and this could be a change that is well worth it.If the frequency noise can be seen with the scope, this is a bad sign or a very good scope. Especially some FM modulation (e.g. from hum effecting the PLL) could be an issue and effect the linearity.In one photo I have seen U6 (the PLL loop filter) changes to an OP07. This could be change worth while testing. One is less LF noise and the other point is having some bias current, so that the phase comparator output is not in the dead zone. We don't know the details of this output of the prema chip, but chance are it is similar to the 2nd phase comperator in the 4046 and thus giving short pulses of either polarity.Are the INL measurements shown with the origial PLL circuit or already with the modified PLL circuit ?

author: dietert1
author_rank: Super Contributor
timestamp: May 18, 2023, 11:16:53 am
content: The frequency/phase comparator output of the BK7 produces positive and negative pulses (2x open collector). During lock there is a small positive pulse of stable length, so that was well done. The Lecroy WR104 i am using was reporting a frequency RMS of 5.3 KHz, now it became 1.5 KHz. This is over short records of about 10 pulses = 5 usec. Another mod to try is moving C56 closer to the HC4046. The VCO mods did not resolve the nonlinearity issue.That issue is in the interface between the amplifier and the ADC. We have seen that in one position of the polarity relay there was a hum issue, which means extra impedance where it isn't expected. My suspicion is with U11 and CR18/CR19 on the amplifier board. I have seen opamps with a dead zone near zero output current. Will do more tests.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: May 18, 2023, 03:21:15 pm
content: I know that OP-amps may have some cross over distortion with near zero output current. However the ICL7650 should have very little of this due to it's high open loop gain, typical of AZ amplifiers.U11 should have even less such effect as it is in a local feedback loop and inside the loop of U10. So I think we can exclude the distortion from U11.An error due to the amplifier should also be about the same for the 2 V and 20 V range. So this can not explain the observed INL near zero that is more similar with respect to the ADC counts.Getting more hum in one polarity setting than with the other is not such a surprise for me. The polartiy relay changes which current path counts to the ADC input an which not. So it can effect the hum, especially if the scope is connected to the ADC.For the VCO I see 3 paths to pick up interference: one is via the VCO input signal, the 2nd is via the supply and a 3rd is the reference switching part effecting the 50 Hz input signal to the PLL. The last would especially be an issue if the 50 Hz input signal to the PLL is switching right at the end of the reference pulse - which would be kind of a design blunder.With the reduced VCO gain the sensitivty to the supply could well be larger than to the VCO input. A modulation in the frequency by 1.5 kHz looks way to high to give a good ADC. If it is white noise this would cause quite some noise. If this is hum, a pure 50 Hz modulation may not be that bad as the effect on both reference pulses per mains period would about compensate. Still the mains waveform is no longer a pure sine and especially 100 Hz hum could be bad and causs an INL error, though not so much the more localized to the extremes of the range type of error seen.I think chances are that without the scope atached the VCO signal would show less frequency variations.It is still a bit odd to me what is so special and bad if the reference pulses are getting shorter than some 15 µs or the input voltage getting below some 5 or 50 mV.Maybe it is the R29*C90 time constant (6.8 µs), as the input amplifier is relatively slow with it's speed set by r30 and C22. So while the input amplifer in principle is a kind of current source, it could not compensate fully for a change in the integrator input voltage. For a test one could try something like doubling C90 and see if the problematic range gets larger

author: dietert1
author_rank: Super Contributor
timestamp: May 18, 2023, 07:59:43 pm
content: Here i have some ADC signals of the Prema 6048 at 0 V. Channel 1 is the integrator input, Channel 2 is the switched reference source and F1 is a 100 trace average over the input signal of R29. That's the voltage over C90, which is meant to cover the missing bandwidth gap. It receives error voltage of about 7 mV from the ADC operation. 7 mV times 10 usec makes 7 uV in 10 msec, so that's about the right magnitude.I increased R29 to 1 KOhm. This is possible, as the amplifier is an active current source. It will compensate for the extra voltage drop of up to 0.4 V. At the same time i added a 680 pF capacitor over the integrator input. In the HP 3456A ADC this capacitor is 470 pF. The "modded" scope dump shows the result. The error voltage on C90 and the high frequency spikes on the integrator input disappeared. Instead we can see a damped oscillation of the Integrator, caused by the 680 pF capacitive load. Just running another linearity test.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: May 19, 2023, 06:38:47 am
content: The signal seen at C90 looks a little that it could cause the INL problem. The amplifier is relatively slow and can not fully compensate for the changing voltage at C90. In addition there is a slight chance it may show some overshoot / ringing too. Per se ringing would not be that bad as it may not add area under the curve to add up to an error.A RC combination in addition or instead of just a capacitors (e.g. the 470 pF) at the integrator input would likely dampen the ringing. The original R29+C90 may have done that job in addition to keeping the peak away from the input amplifier.There is also the chance that the recovery / ringing at the integrator input could change with the load current to the OP77. With very low input it could be in a kind of dead zone and there react a little slower.

author: dietert1
author_rank: Super Contributor
timestamp: May 19, 2023, 08:53:48 am
content: The last mods did not resolve the nonlinearity issue. The nonlinearity is small. At zero input voltage in the 20 V range the ADC is measuring essentially the 118 mV offset. An error of about 20 uV is a 1.7E-4 effect, difficult to see on the scope. I think soon i will connect another DVM.For the time being i looked at the switched reference. I added a p-channel JFET J175 and a 4.7 KOhm resistor to have a near constant current of 1.4 mA out of the reference buffer. The JFET gate is connected to the same signal as the four DG308 control inputs. It turns off when the DG308 switches turn on. I'm injecting the dummy current near the non-inverting input of the integrator.One may also try another switch scheme with 2:1 muxes, replacing the two 1 MOhm resistors R53 R54 by switches. They were meant to reduce leakage. It works, but only after about 20 or 30 usec. The spare third MUX can replace the JFET.Regards, DIeter

author: dietert1
author_rank: Super Contributor
timestamp: May 21, 2023, 08:21:29 am
content: Meanwhile i used the MAX5719 voltage source with two resistors of 208K to feed test current directly into the integrator current node. Thus i confirmed that the nonlinearity issue is in the integrator board and not in the amplifier/front end.Next i will try to measure the current the time division DAC feeds into the integrator. I want to find out whether the nonlinearity stems from the DAC or the integrator (BK7 microcircuit).There is an improved version DG308B of the DG308A with charge injection reduced from 10 pC to 1 pC. Also i ordered some DG309B to replace the two 1 MOhm resistors by switches. R63 in the DAC sense path causes a 30 Ohm / 1 MOhm = 30 ppm offset.In another test i found that the scanner appears in the front panel display after closing the solder bridge labeled "SC" on the CPU board. Already ordered parts to make the scanner.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: May 22, 2023, 02:49:18 pm
content: When making the scanner i tried to follow the original schematic as shown in the manual. If found that a CD4094 serves to replace the Prema C32 microcircuit. Extra features of the C32 are 4 more bits and relay driver outputs, both unused in the scanner.In contrast to what they describe in the text of the 6047/6048 manual they did not use mechanically latching relays for the scanner. In the other boards relays are labeled S4L-6V and S2L-6V while in the scanner schematic they are labeled S4-12V. The drive circuit looks different, too. A CD4049 won't drive a latching relay due to its insufficient pull-high strength. Needs more work..Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: May 22, 2023, 08:46:00 pm
content: If the original circuit uses non latching relays the control would likely need also extra effort. This would likely mean using a µC to do the actual control. The SPI interface is kind of similar to the shift registers, but some may have trouble with 12 bit data.

author: dietert1
author_rank: Super Contributor
timestamp: May 22, 2023, 10:21:09 pm
content: The schematic of the scanner board is the last page of the Prema 6047/6048 user manual.It shows a resistor and a diode in addition to the capacitor as relay coil interface. This works because the pull-up of the CD4049 output is weak.Meanwhile i think they used non-latching relays, as with latching relays there is a risk of creating shorts between different inputs of the scanner. During power-up their favorite simple capacitor method cannot switch off all relays to begin with. A scanner with latching relays driven by the capacitor method needs a power supervisor to disconnect the active channel during power-down. Then on the next power-on a short won't happen. Probably this is the way to go.Regards, Dieter

author: Rax
author_rank: Super Contributor
timestamp: May 29, 2023, 04:31:14 am
content: Quote from: Kleinstein on April 06, 2023, 06:16:53 amI'm only getting occasional opportunities to look at this. And I've never established if my current mode is working, so I'm trying to take advantage of this extended weekend in the US (national holiday) to register some progress.The manual is a bit confusing (to me). It seems the blue function levels (where mAdc and mAac functions are) are activated by pressing the "2nd" key on the right keyboard (pressing the "TEMP/2nd" key on the left seems to always enable the temperature measurement function?...). Therefore, I assume if I see both the mAdc/Vdc and the "RATIO/2nd" LEDs on I am in the mA DC mode of operation, correct?This is confusing because the "RATIO/2nd" seems to be the only way to enable the blue/top modes on both the left and the right keyboard clusters and I am not entirely sure I'm still in "MEAS" as opposed to "COMP" mode when the LED for "RATIO/2nd" is on. Does it matter the order in which I am pressing the "RATIO/2nd" to register an effect on a blue/top key on the right vs. the left keyboard clusters?If I'm switching everything correctly, my current measurement function is behaving erratically. I'll have to figure the current signal path in the preamp, which exact relays are involved and what is the switching mode (open/close) for the current function.

author: dietert1
author_rank: Super Contributor
timestamp: May 29, 2023, 06:23:52 am
content: Yes, i also found it confusing. Until i noticed both groups have their own 2nd key. When a 2nd function is active, the corresponding panels 2nd key has its indicator led on. In order to measure mADC the VDC and the left 2nd indicator should be on. There is only one range = 2000 mA. Input on right connectors.The ratio function should really be in the left group. You terminate the ratio function by selecting another function, e.g. VDC. Similarly the instrument leaves the compute mode when pressing the VDC key on the left. When you are in AC mode and press null, it switches to VDC to null the instrument and then goes back to VAC.What i am missing is a way to get the instrument into continuous measurement mode after going local. Could be some reset button. It's a pitty that product was abandonded.With my research on the nonlinearity near zero issue our 6048 proved to be rather inert to mods, which is something good. Their conversion method is rocksolid, except i have my doubts about integrating digital control of the integrator (counters, pll control) together with analog control (amplifier and comparator). Also i have some doubts about running the ADC without negative supply. The only negative supply comes from U9 to pin 11. The comparator won't fire on the positive glitch of the integrator when starting run-down. There seems to be a minimum run-down duration of about 7 usec, in case the comparator triggers early. In that occasion summing/averaging errors happen. I found a capacitor missing in the schematic and another one present in the temperature ADC but optional and not present in the main ADC.The nonlinearity effect amounts to about 90 to 200 pA error current at the integrator summing point. With 10 pC charge injection and 10 msec integrator cycle time, charge injection offset current can be 1 nA. Cutting into that at short pulse lengths can produce that nonlinearity. I am still waiting for the DG308B switches with 1 pC charge injection spec.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: May 29, 2023, 08:08:16 am
content: The charge injection part should still be largely constant. The pulses are not that short, usually some15 µs and up. It would be rather unusual for the charge injection to change with pulse length on the that time scale. Checking a different switch chip is of cause an easy option (a good thing of having the chips in sockets).A possibly nasty point could be the quite strong current pulse that the LT1007 sees to charge the switches. Having about half the capacitance with the DG308B can help too.In the current circuit the LT1007 has to make a small step up in the voltage when the switch turns on. This is to copensate the drop on the switch resistance or some 140 mV. one could consider adding some large resistance from the inverting input to a positive voltage (e.g. some 33 M to the 12 V) to already have that extra voltage also in the off state.I don't see a problem of not having a negative supply at the ADC chip. The interator is working with a positive output voltage only and the custom chip is mainly the comparator, logic and part of the PLL. What is a bit strange is that the integrator seems to be a simple 1 OP-amp type with the relatively slow OP77. I don't see a way to have much added gain in the ADC chip, as there is no input for the integrator input node and thus no way to react faster.What I don't like very much is having the PLL together with the ADC part - if the timing is at a certain point the ADC may effect the PLL and errors in the clock can cause INL errors. Not seeing an effect from the change in the PLL indicates that the PLL may not be the trouble. How is the timing of the ADC reference pulses relative to the 50 Hz signal to the PLL ?

author: dietert1
author_rank: Super Contributor
timestamp: May 29, 2023, 09:03:55 am
content: I was not reporting all the mods i tried to let this continue as a Prema 6048 thread. One mod was a switched current source to keep the LT1007 at constant output current (see above). Also i found a simple method with two diodes and a 10K resistor to eliminate R63 and R64 using the PWM signal to provide switched pull-downs.The OP77 U9 drives an amplifier of high voltage gain inside the BK7. U9 output is at -0.7 V and swings no more than 50 mVpp. A simple model would be a npn transistor inside the BK7 with base on Gnd, emitter (maybe with a small resistor) on output of OP77 and collector on pin 13 which is also the comparator input. I guess the other comparator input is pin 14. Pull-up cabability of the BK7 output pin 13 is somewhat limited, just enough to handle the -0.4 mA max input current from the frontend. The pull-down current to handle the 1.4 mA reference pulse comes from U9.At zero input the integrator output remains close to Gnd. So that output transistor works near saturation. The comparator may be compatible with zero input levels, but again a negative supply could help. One could try to lift pin 14 to +1 V or so to get some margin on the output side of the integrator.Regards, Dieter

author: Rax
author_rank: Super Contributor
timestamp: May 29, 2023, 02:42:24 pm
content: Quote from: dietert1 on May 29, 2023, 06:23:52 amThank you, Dieter! This is great. I fiddled with the controls and it seems when the LEDs for both Vdc/mAdc and the left side "TEMP/2nd" are lit is when the unit is in current measurement mode. I need to fiddle with it more to get it. I am not yet sure exactly if the "RATIO/2nd" need to be actuated in any way for this to kick in,* but it's not lit when the current is being measured.The other thing I didn't realize is that there's just one range, so I was further confusing the unit (and myself) with trying to get it into AUTO.I can report success, though, and my unit is able to measure current! Yeah! Nothing to see there.________________________________________________________________________________________________________________Later: * It seems to me the "left cluster 2nd" won't engage unless the "right cluster 2nd" is also actuated prior. I think I read something to this effect in the manual - though I can't seem to be able to find it now - but if so, this is really odd and unintuitive. Maybe my unit acts up in some way, though I lean to believe it's just weird controls logic.

author: Rax
author_rank: Super Contributor
timestamp: May 29, 2023, 06:53:38 pm
content: Quote from: dietert1 on May 05, 2023, 07:40:35 pmI think I'll take a slightly different route with the switch. I enclose pics of mine, and I'm pretty sure this is one of those "ME5A" types used in some Commodore, Atari etc. monitors. I'll try one and post here if successful.

author: dietert1
author_rank: Super Contributor
timestamp: May 30, 2023, 08:48:10 pm
content: The first scope dump shows the timing relationship between the BK7 PLL input pin 8 and the integrator run-down. The timing reference has a small asymmetry between positive and negative edges. The positive half is a little more than 10 msec. So i would say the PLL lock onto the negative edge and run-down starts at the same time. After i removed the input short, run-down was slowly getting longer. At full range it reaches about 3 msec. With open inputs hum is so large that it is visible at this time scale: every second run-down is longer.The second scope dump shows the LT1007 output during a short reference pulse. The reference voltage is 7,075 V. During the reference pulse the LT1007 output rises to 7.170 V under control of the sense connection (DG308 pin 11 to pin 6). After DG308 turn off, we can see the charge injection into a high impedance, some parasitic capacitance and the 1M resistor R32. The first rising pulse has an area of 10,1 V usec. Divided by 1 MOhm this makes a charge of 10,1 pC. The slower negative response with a duration of about 35 usec has a similar area and gives -12,5 pC. Something similar can be expected on the integrator summing point, where we have R34 750K as DC resistance. The net charge will be small and constant (e.g. -2.4 pC) if one waits long enough.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: May 30, 2023, 09:28:09 pm
content: The PLL timing could be a slight issue. I see 2 possible weak points:1) the reference switching may interfere with the PLL and possibly cause some frequency modulation.2) the 50 Hz input signal could effect the comparator or intergrator. This would be the slope not used for sync.However there is very little one can do about the timing. This should be more or less fixed in the chip. Having U6 populated with an OP07( as in some photo) instead of the LF411 in the plan could make a small difference as this could shift the timing a little from the bias current that would require a small phase shift at the PLL. Still I would not expect a significant shift from this.The settling of the LT1007 looks quite slow for the off part. However as this is the off phase there should be plenty (e.g. > 5 ms) of time for settling. The peak is quite large, but it should stay the same, no matter how long the reference pulse was. The on phase settling looks fast.

author: dietert1
author_rank: Super Contributor
timestamp: May 31, 2023, 06:40:12 am
content: Changing the timing relationship becomes an all digital mod when using the HC4046 built-in phase comparators with the BK7 PWM output for comparison. Just a 9*8192 counter to divide 3.6864 MHz down to 50 Hz. A small CPLD can do that.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: June 01, 2023, 09:12:29 am
content: Meanwhile i found how to make a nonlinear integrator. As a low pass filter between PWM and integrator i added two parts: C31A and R31B in the schematic. This time the linearity test showed the opposite nonlinearity effect.If this is the effect of a low pass filter, the P6048 nonlinearity is the effect of a high pass filter (charge injection). Using a DG308B did not help. In fact the parasitic capacitance of R31 gives more charge. Its metal can measures 5 pF to the resistor, so charge injection will be some fraction of 7 V * 5 pF = 35 pC. I will try and ground the can.For the nonlinearity to disappear it needs to become a capacitive divider. Later one can add R31A to restore calibration.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: June 01, 2023, 11:29:57 am
content: Some charge caught at the metal can of R31 could indeed be an issue. So grounding the can is a really good idea to test.I don't think adding R31A would make much sense - a new calibration is likely simpler.

author: dietert1
author_rank: Super Contributor
timestamp: June 07, 2023, 09:07:42 am
content: Meanwhile i solved the riddle of the nonlinearity and to make it short, the main problem was U9 OP77 that isn't fast enough to handle the PWM reference properly. 75 % of the nonlinearity vanished when i replaced it by an OPA140 (about 60x faster). I'm still using the filter with a 10 nF cap. As the time constant is about 1usec, it won't affect linearity in the range the instrument actually uses.The other 25% of nonlinearity vanished after replacing U10 OP07 by another OPA140. This time the problem was in the input protection diodes of the OP07. When the four switches of the DG308 close at the beginning of the PWM pulse, it happens in arbitrary order and the OP07 can't follow anyway. A strong current pulse flows from the inverting input through the protection diodes to the non-inverting input of U10, hitting the reference output and causing U20 pin 7 to jump multiple times like a rubber ball on the ground (20 mV during 10 usec). The schematic shows a buffer cap C49 100nf not present in our P6048. For good reason as it causes instability of U20. I inserted a 10 uF cap in series with a 56R resistor. Also i added 100nF across diode CR14.And another 100nF across R35.The last sweep appears clean. INL axis is +/- 1 ppm of 2 V range.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: June 07, 2023, 09:28:38 am
content: Was U10 really an OP07 ? In the plan there is a much faster LT1007 for U10.The OPA140 is a great JFET OP-amp, but in the LF range may still be a bit more noisy than the OP77 / LT1007. It would not show up much with a shorted input, but could be a slight issue when measuring a significant voltage.

author: branadic
author_rank: Super Contributor
timestamp: June 07, 2023, 10:40:27 am
content: That would be some serious improvement. What about the 20V range?-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: June 07, 2023, 10:48:57 am
content: With U10 the problem wasn't speed but the input protection diodes. LT1007 and OP07 both have those input protection diodes, so they are no good for U10.As far as i remember OPA140 noise specs are all better than OP07. Anyway, since the SO8 OPA140 only sells in China right now, i will also try to use OPA189 chopper stabilized amplifiers (where we have some). In the OPA189 datasheet they discuss using the part with analog switches.Yes, i will look at the 20 V range, too. That's why i did not want to compensate the nonlinearity but look for the origin.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: June 07, 2023, 11:07:11 am
content: The very low frequency noise data are not very relaiable, but from the curves shown in the datasheets the OPA140 is a bit higher in the 1/f noise than the OP07 / OP77 and definitely higher ( ~ 5 x) in the voltage noise than the LT1007. The OPA140 may still be good enough.If the OPA140 is hard to get, one could in most cases use an OPA141. The main difference should be more stringent testing and a usually better offset (may also be just testing but could also a more accurate trim).

author: branadic
author_rank: Super Contributor
timestamp: June 08, 2023, 01:23:20 pm
content: Could you add all of your changes into the schematic? It seems I couldn't follow all of them completely. I would then try to recreate them and provide confirmation.-branadic-

author: Rax
author_rank: Super Contributor
timestamp: June 08, 2023, 01:56:01 pm
content: Quote from: branadic on June 08, 2023, 01:23:20 pmdietert1: +1.

author: dietert1
author_rank: Super Contributor
timestamp: June 08, 2023, 04:24:17 pm
content: The nonlinearity test with -160 mV to 160 mV sweeps in the 20 V range also looks clean. Vertical scale of the diagram is +/- 1 ppm of 20 V range. The MAX5719 test DAC has an INL specification of 3 ppm, so that would be 0.48 uV in 160 mV.Yes i will prepare a step by step document of the two mods that made a difference.I also have an image of a DIY scanner (work in progress). A friendly person supplied SDS latching relays - similar to the other relays in the P6048. Instead of the Prema C32 microcircuit there is a CD4094 shift register with latch and the CD4049 buffers were replaced by complementary emitter followers. There is a supply supervisor made from discrete parts and a CD4011 to produce the inhibit signals for the two decoders.Regards, Dieter

author: Rax
author_rank: Super Contributor
timestamp: June 08, 2023, 04:32:57 pm
content: Quote from: dietert1 on June 08, 2023, 04:24:17 pmDieter, this looks very promising, and I'm quite a bit blown away by your results. Fantastic work!I'll seek to run these trials with my unit so we'd confirm the results in at least three different samples.

author: branadic
author_rank: Super Contributor
timestamp: June 08, 2023, 05:34:39 pm
content: What is now left open is a full sweep to see overall INL. However, the result looks very promissing.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: June 08, 2023, 06:58:52 pm
content: I know you can do that with your calibrator and your other meters.It's team work anyway.Let's mention perdrix (David Partridge) who discovered the nonlinearity in the first place.And Kleinsteins comments have been important. Concerning OPA140 noise, probably the zener noise dominates in that part. I just used what i had.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: June 08, 2023, 07:24:40 pm
content: I had a few thought on the noise. The op-amp U10 is only relevant for the time the reference is connected. U9 (the intgrator) is also mainly important when the reference is active. Both OP-amps work togehter with the reference signal that does to the input of U10. So one kind of get a combination of the noise of the 2 OP-amps and the LTZ1000 reference. In this set the reference is usually the larger noise source by quite a bit.Another point is that the time the 3 are connected depends on the input signal / voltage. With the variable time this gives a noise that gets higher the higher the voltage, not just from the principle low frequency reference noise, but also from higher frequencies ( > 99 Hz). The crude picture gives an added noise part proportional to the square root of the voltage.With the relatively slow 20 s integration the extpected contribution is small - more like hard to see. If really an issue the point would be more adding a little filtering to the reference. The expected noise constribution is still small and though simple it is likely not really worth it.The 2 main expected noise sources are jitter from the PLL - so the modification with the VCO range may help somewhat, though probably not much as there are also internal limitations.The other point could be quantization, depending on what filtering is used in the custom prema chip. For the 2 V range the input amplifier can also contribute quite a bit, though it is still not that bad.I have not seen detailed noise data on the 6048, but from the few curves shown at the start of the thread the noise is not that bad and with a 10 V test signal not that much higher than the 2 references involved.

author: Rax
author_rank: Super Contributor
timestamp: June 09, 2023, 02:56:09 am
content: Quote from: dietert1 on June 08, 2023, 06:58:52 pmDieter,Please use the "Quote" feature of the forum. I personally don't know who you mean to respond to there.It is teamwork, and if it all pans out, this may improve on the OEM performance of an 8.5 digit meter (read that a couple of times, and let it sink in). I don't want to jump the gun - and let's definitely do the work - but if this works, it'd be pretty groundbreaking. I'll congratulate everyone contributing when we get the peer review completed.

author: dietert1
author_rank: Super Contributor
timestamp: June 09, 2023, 09:44:24 am
content: Here i made a quick writeup, let's see whether it serves.Regards, DieterEdit: One image slipped away..

author: dietert1
author_rank: Super Contributor
timestamp: June 09, 2023, 02:17:41 pm
content: Here is a schematic including mods of 50 Hz => 3.6864 MHz PLL.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: June 09, 2023, 04:45:40 pm
content: Thanks. In the end only the opamp replacement made the difference, is that correct?-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: June 09, 2023, 06:02:09 pm
content: You can try that. If it isn't sufficient, next i would try 4 to 6 . Then 12 and 13, then 9 and 10. I documented the other mods as they are visible in the images and they are meaningful. Probably i won't go back to start to find the minimal solution.For example two independent sources (Lee Essen and an expert at Systems Engineering in Stolberg) talked about voltage regulator failures. The instrument works without fan! Also remember Kleinsteins report of thermal oscillations. What i did certainly looks unusual. An alternative solution would be a common heatsink mounted to the frame that serves to shield the mains transformer at the same time. But mounting all seven regulators on a common heatsink will introduce capacitive coupling and isolation voltage limits between the different power supply sections.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: June 10, 2023, 09:08:53 am
content: Here i did a noise test with the modded instrument versus our 14.1 V 5x2 LM399 array. Vertical scale is 14 uV = 1 ppm of reading. The 20 seconds reading noise is 0.2 uV (= 0.01 ppm of 20 V scale) yet there is some additional process at the time scale of 3 or 4 minutes that makes the total standard deviation 0.87 uV. Probably i will try protective covers for the LTZ1000 reference inside the Prema.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: June 10, 2023, 10:31:03 am
content: The noise does not looks like normal white of random walk type noise. It is more like quite some popcorn noise included. Some of the jumps look a bt large to come from 5 x LM399 in parallel, as this should give jumps of some 0.8 - 1 µV.A cover over the LTZ1000 would definitely make sense, though I doubt it would solve the issue with the jumps.

author: dietert1
author_rank: Super Contributor
timestamp: June 10, 2023, 10:54:24 am
content: One can see slopes with 15 or more readings in a row, so that would be a process of 5 minutes.Maybe the variations we see are caused by 50 Hz mains frequency steering. One could try to run the instrument with the PLL off. Or log mains frequency for comparison. Some meters include monitoring mains frequency.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: June 10, 2023, 12:26:56 pm
content: I've replaced U9 and U10 with OPA189 in the white Prema 6048 (no other modification), as this is what I had at hand. While I'm enjoying the sun, INL measurement is running, so we will know more tonight. By the way, U10 was an LT1007 (plastic DIP package) in that unit, the OP77 was a nice CERDIP.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: June 10, 2023, 01:40:13 pm
content: I don't see a mechanism of the mains frequency or a drift rate in the frequency to have a direct effect on the reading. An unstable mains may add some extra noise, but I don't see a reason for steps.My main suspect for the steps are the references. Mybe there is something to couple the LM399 references to each other and this way get steps of 2 x the expected size.

author: branadic
author_rank: Super Contributor
timestamp: June 10, 2023, 06:09:54 pm
content: INL measurement finished, we can clearly see the massive improvement. The remaining INL error visible might be due to the INL of the S7081 itself, which Frank measured to be ±0.3 ppm in its 10 V range or the effect of temprature changes during the measurement.-branadic-

author: Rax
author_rank: Super Contributor
timestamp: June 11, 2023, 05:22:50 am
content: Quote from: branadic on June 10, 2023, 06:09:54 pmGood stuff, branadic. I think your only mods are U9 and U10 to obtain these improvements, correct? I'm currently looking into what suitable op amps I can source and fit with a DIP adapter to replace those two ICs, but it's going to be a few weeks given I'll be mostly away from the bench due to conferences and work-related duties across the US.Question for both branadic and dietert1: have you attempted to assess how these mods influence the state of calibration of the unit? Does it stay within the same margins as before the mod, or, should these changes be implemented, the unit would definitely need to be sent in for calibration/adjustment? I wonder if in the pursuit of linearity at zero-crossing, one would effectively discard the original/ulterior calibration constants' usability.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 11, 2023, 05:38:50 am
content: A change of the OP amps will effect the calibration. The offset of the OP-amps add to the effective 7 V reference voltage. This is not only the offset of the new OP-amps, but the difference to the old ones.This would be one factor effecting all ranges (including resistance). So there would be a chance to check the scale factor before (e.g. 10 V referene voltage , 10 K resistor) and than at least get an idea on how much change happend.

author: dietert1
author_rank: Super Contributor
timestamp: June 11, 2023, 05:56:02 am
content: My unit definitely lost calibration when i installed the PWM filter (mod 9) effectively incrementing the 5 KOhm precision resistor by 120 ohm. It's a good idea to transfer the P6048 calibration to other standards before applying mods.For me the next step is working on the LTZ1000 reference. The Prema 6048 runs it's reference with a 12.4/1 KOhm divider for the oven setting. I will sweep its set temperature and adjust for zero TC. The test DAC and the software application i made for the nonlinearity sweeps can be used with little change. I will just pull R80 402 KOhm and use it to sweep the set point. TC compensation will also affect voltage calibration.And it will also affect the total TC of the instrument. The real thing will be reengineering the firmware to understand how temperature dependent calibration works.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: June 11, 2023, 06:03:30 am
content: Unless the TC of the reference is really high, I would not change anything with the reference circuit unless there is a real problem. A 12.4 K and 1 K divider gives a relatively low set temperature and this can cause a relatively low maximum room temperature. If it works and is well settled a low set temperature also has advantages. The old age of the reference is a good thing here.Quote from: dietert1 on June 11, 2023, 05:56:02 amThis is another reason of no changing the reference TC - the current firmware should compensate for much of it.

author: branadic
author_rank: Super Contributor
timestamp: June 11, 2023, 06:12:25 am
content: I already replaced the cheaper oven resistor divider by a metal can resistor network in the past, together with a metal can resistor for the 120 Ω, but haven't aimed for zero t.c. back then.QuoteThat would indeed be the real deal to finally fix temperature effects. I guess they used some common temperature compensation constants on all their units, rather than individual temperature compensation, which is a bit odd.I will replace the opamps on my black unit by OPA189 today and perform the INL sweep, but I'm sure we will see the very same improvement. Once done I guess it is worth repeating the INL measurement for both units against our 3485A under controlled and stable lab temps or maybe even in a climate chamber.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: June 11, 2023, 03:26:24 pm
content: The noise test log i had above continued for another 24 hours. Looking at a temperature sensor inside the test DAC, i found a temperature variation of -1 °C during the night caused about 1.5 uV of drift of the readings. So the difference between the TCs of the Prema 6048 and the LM399 array is roughly -0.1 ppm/K. To separate the TCs, one of the devices should move into the temperature chamber.Here i have a first temperature sweep result of the LTZ1000 reference in our instrument - as is. Resistor R80 402 KOhm was not present. For the sweep i used a resistor 403K from the test DAC (-4.5 to 4.5 V). This gives a sweep range of about +/- 5 K. The default voltage of 0.5 V at the base of the temperature sense transistor causes a slight asymmetry of the sweep.The plot looks as expected with a negative TC of the voltage readings of the LM399 array, which means a positive TC of the LTZ1000 reference of about 31 ppm/K. The curvature of the line fit residuals is also as expected. Adjustment for zero TC will require a resistor of about 13 Ohm in series with the zener.Regards, Dieter

author: Rax
author_rank: Super Contributor
timestamp: June 12, 2023, 04:16:05 am
content: Quote from: branadic on April 14, 2023, 11:15:15 pmIn trying to close this loop, I think we're replacing the same U9/integrator as before - just a different part this time around than the one tried before - is this correct? Also, U10, though I'm not sure it's been determined which has the instrumental contribution.OPA140/189 vs. the OPA202 we've looked at before. For all those trying to do this, though, fair warning is calibration will be disturbed.

author: Rax
author_rank: Super Contributor
timestamp: June 12, 2023, 04:29:16 am
content: Quote from: Kleinstein on June 11, 2023, 05:38:50 amI think we've already established calibration is thrown off, I was just wondering by how much, I guess. I think you're kind of going that way too.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 12, 2023, 05:05:51 am
content: Quote from: Rax on June 12, 2023, 04:16:05 amSo yes it is about the same part (U9) as suggested before. U10 is a separate step for a different reason (diodes between the inputs case a spike back to the reference).The OPA202 is only a little (1 MHz vs 600 kHz) faster than the OP77. This was to avoid possible oscillation with gain in the Prema chip. I consider it bad luck that the resulting INL was still about the same.With Dieter's discovery of the extra 10 nF capacitor at the OP-amp this is no longer that critical: the OP-amp does the fast part of the integrator and the extra gain is mainly for the slow part. So a much faster OP-amp is possible and should even allow for reducing the 10 nF capacitor. Both the OPA189 and OPA140 seem to work and chances are neither would contribute significant to the noise.

author: dietert1
author_rank: Super Contributor
timestamp: June 12, 2023, 06:18:05 am
content: When i found that extra capacitor i thought it was a bit large and removed it. Then the integrator was unstable at 1 MHz. so i put a 680pF there. This was still between negative input of the opamp U9 and its output. When i put in the OPA140, i thought a better choice was placing the 680 pF across the amplifier inside the BK7. Now the extra capacitive feedback needed for stability is via the integration cap, but avoiding double integration. To illustrate how the integrator was injecting charge, i have two scope dumps.First one shows the setup with the op77 and the 120R/10nF filter. Top track is the PWM signal as trigger. The two yellow tracks show the two ends of the 120R PWM filter resistor. One can see the 1.2 usec time constant and one can see the error voltage on the integrator input. The difference between the two would be the current from the PWM into the integrator. The nonlinearity is caused by a difference of the error pulses when turning the reference on and off, i mean they don't cancel each other.Second dump shows the same with the OPA140. The error pulses are hardly visible. One could say the OPA140 handles the bandwidth limited PWM pulse correctly. I'd guess without bandwidth limit, there could be nonlinearity again causing "rectification". This time the blue track shows the output of U9.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: June 12, 2023, 08:48:04 am
content: The extra 120 ohm 10 nF filter looks a bit strange and may effect the linearity. For good linearity the filter should be fully settled for the reference pulse ends. So this may be a little too much filtering, though I like the general idea of some filtering there.With the 6048 ADC the reference resistor is removed from the circuit and and in the off state there es essentially no DC path at the integrator. So the 2 pulses for turn on / off at the intergrator input behave quite different. There is no compensation between the 2 pulses like it is with many other MS-ADCs that have a constant input impedance at the integrator. The question is if the 1st pulse is not fully settled before the reference turns off and if the pulse changes in area when the input voltage changes. A constant pulse would only add to the offset.With a BJT based OP-amp some 25 mV at the input is about the amplitude where a long tailed pair input becomes nonlinear and one is leaving the small signal range. A FET based input is often linear over a larger range.There are quite some interferences visible in the scope traces, also away from the obvious switching times. Is this more an EMI effect from outside/scope, or more a thing comming from the DMM itself ?

author: dietert1
author_rank: Super Contributor
timestamp: June 12, 2023, 10:04:45 am
content: These scope dumps are with input short. The pulse length is 15 usec = 12x time constant, so it settles to 10 ** -5. Then the turn on-delay should be compensated by the turn-off residual charge. Each mod was accompanied by a new linearity sweep (with the scope disconnected!) and the visible effect of the 120R 47nF combination vanished when going to 120R 10nF.Without the filter, if one applies the full 7 V pulse, the error voltages are much worse.Yes, the EMI looks a bit disturbing. It was a single shot capture in order to avoid averaging the hum effect on the PWM pulse length. Probably the digital scope with its switched mode power supply needs to move away a little.Regards, DieterEdit: Meanwhile i fixed one of the scope probes and i think that track with more EMI was taken with a "tired" ground lead. Origin of the 40 KHz EMI is the isolating DC-DC converter in the test DAC i am using for the sweeps.

author: branadic
author_rank: Super Contributor
timestamp: June 19, 2023, 08:41:13 pm
content: Unfortunately, my 6048 stopped working. I had intermittent resets in the past, which turned out to be a problem of the bridge rectifier, with resoldering the pins helped in first place. This time I will replace the bridge rectifier, (B40C3700/2200), just in case. I don't like the very small rest-ring at the vias, it's a potential source of failure.Also the flex-connector from integrator board and micro-processor board looks suspicious (small burn marks). Hence why I've ordered a few of those:https://www.tme.eu/de/details/fsn24a-14/ffc-fpc-bander/te-connectivity/3-1474644-8/?f45b=f45band will replace them all, better save than sorry.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: June 20, 2023, 05:52:32 am
content: Will you solder the connections? The original connectors are interesting with their copper contacts - at least the one i unsoldered from the amplifier board to prepare the scanner connection. Don't know where low thermal EMF may be important. One problem with the original flat cables is their position in the socket isn't well defined, so one needs to insert them carefully matching the ribbon traces with the contacts in the socket.I have been looking into replacing the BK7 ADC microcircuit with standard parts and wrote a verilog description of its digital workings with an all synchronous implementation for low jitter. Seems like it fits into a XILINX XC9572XL CPLD. We have old stock of those and it has 5V tolerant IO. Usage of PTerms arrived at 94%. Maximum clock frequency with the 10 nsec part would be 76 MHz.Unfortunately the BK7 uses several different standards for digital signals. 5V for the interrupt and SPI lines and 12V for the PWM and phase comparator output. The 3.6864 MHz clock input uses L=0.2 H=0.5 V levels - maybe emitter-coupled. Instead of implementing those i might make another Prema 6048-alike first, i mean a 8.5 voltmeter core. One interesting part i found is the 74HC7046 that includes a lock detector.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: June 20, 2023, 07:13:05 pm
content: QuoteYes, at least the one between processor and integrator board.That problem seems to exist only in the black units, the white unit I have on the desk uses connectors with clamping mechanism, so the position is well defined.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: June 21, 2023, 03:39:43 pm
content: The Prema 6048 is nice to service. If one opens both top and bottom covers almost everything is accessible without unscrewing the boards.Thinking about the noise record i showed above: I determined 20 second reading noise in 20 V range to be 0.2 uVrms and overall standard deviation to be 0.89 uVrms. From the very start the noise did not look like white noise.What to expect? The ADC runs with a 3.6864 MHz clock and at the 20 V range limit each rundown is about 3,3 msec of the 10 msec integrator cycle, so it counts roughly 1 200 000 clocks in one second and 24 million clocks in 20 seconds. This makes a digitization step of about 5E-8. How can the reading noise be 1E-8? The instrument firmware supports a digital filter but it wasn't active in my log. Are they cheating?Regards, Dieter

author: MegaVolt
author_rank: Frequent Contributor
timestamp: June 21, 2023, 04:05:38 pm
content: Quote from: dietert1 on June 21, 2023, 03:39:43 pmMathematics. If signal samples = 0 0 0 5E-8 0 0 0 -5E-8 then we will get approximately 1E-8 to calculate RMS.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 21, 2023, 04:10:16 pm
content: With a sigma delta ADC one is not limited to using only the first / last timing to get the result. One can get a little extra information from other transitions of the comparator.I can't really follow the calculation of the resolution, but my estimate comes to about the same result: About 70 million clock cycles in 20 seconds. The reference is stron enough for a theoretical 70 V range so a quatization of some 1 µV for 20 seconds and just using the 1st and last comparator time.The RMS quatization noise is step size divided by root of 12. And with a start and stop part one would still have the root of 6 as a factor and thus some 0.4 µV of quatization noise.It would be interesting to know how much averaging they use, but it would not need that much to gain another factor of 2 or 3 from a kind of oversampling. This is kind of digital filtering, but on a much shorter time scale, maybe blurring the start and stop of the integration by some 200 ms or so to get 20 comparator readings each and theoretical a factor of about 4 lower quantization noise.

author: Rax
author_rank: Super Contributor
timestamp: June 21, 2023, 04:27:57 pm
content: Quote from: dietert1 on June 21, 2023, 03:39:43 pmI agree. One thing to have in mind, though - for anyone becoming overenthusiastic about this - is that the boards are pretty brittle. Typical of boards of that time.So care needs to be paid not to exfoliate the pads or traces or otherwise damage the boards as one reworks it.

author: dietert1
author_rank: Super Contributor
timestamp: June 21, 2023, 10:48:26 pm
content: In a numerical experiment with 500 random numbers of 0 or 1 both average and standard deviation was 0.5. Don't understand how to arrive at sqrt(12). Also there should be more noise from other (analog) sources of noise.Anyway one could try to check the question experimentally, creating a small modulation of the input voltage synchronous to readout. Lets say one 10 uV different input for every tenth 20 second integration interval. Then logging modulation and measurement results one should see how the readings are averaged, at least after some repetitions of the test.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: June 22, 2023, 06:48:35 am
content: The sqrt(12) factor is a classic result: it comes from assuming a variabel true value that leads to quantization errors uniformly distributed between 0 and 1/2 LSB. This may not apply to the test with a fixed zero input if there is little extra noise. Even with this extra factor just the simple use of the first and last comparator time is not enough. So they must use more information to reduce the quantization noise.Besides averaging over multiple zero crossings another possibilty would be a finer time resolution inside the chip (e.g. using a 2nd interal PLL or multi phase clock) to get a higher resolution to measure the time of zero crosssing.For testing the ADC function it may be easier to do with a shorter integration, like 1 second. This may other noise sources should get a little less critical and clock steps more visible.One may want a relatively large step, to actually see the apperture function.

author: branadic
author_rank: Super Contributor
timestamp: June 23, 2023, 04:45:02 pm
content: QuoteBridge rectifier and flex connector arrived and have been replaced, the black unit is now working again, at least for the moment. I will observe it and once fine I will perform the linearity measurement, that I've planned for quite a while.-branadic-

author: Rax
author_rank: Super Contributor
timestamp: June 25, 2023, 05:23:35 am
content: Again, not sure this belongs in the "Prema 6048 teardown" thread, but I think this needs to be documented somewhere. Maybe this thread is becoming a compendium on this meter.I finally got around to replacing the power switch on this 6048. It seems these "COMMODORE 1084s PHILIPS CM8833 Atari SC1435 Monitor PREH ME5A" switches are exact replacements. The only surgical part of the procedure was transplanting the bracket that attaches it to the front panel assembly. It comes with a plastic bracket with correct spacing between the two mounting screws, but too narrow to fit the Prema original screws. So I decided to transplant the original metal bracket.Other than that - and the rather finicky access (as opposed to the access to the boards, which is excellent as mentioned before) - it's been pretty smooth sailing. My 6048 is again easy to switch on and off (not that it's going to be off for any significant amount of time, but at least I have the choice).

author: dietert1
author_rank: Super Contributor
timestamp: July 01, 2023, 03:12:04 pm
content: Meanwhile i made a test setup with a prototype of a Prema BK7 substitute, see schematic. It performs the ADC integrator cycle, but needs more work, for example debouncer for the comparator. The PLL part needs testing, too.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: July 02, 2023, 06:08:13 am
content: Have you made progress on the software side? I'm asking because that temperature compensation is working and within the specifications on my black unit, but still rather large for my feeling. GPIB access to the temperature sensor data would be a good start.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: July 02, 2023, 07:59:12 am
content: Currently i am studying the BK7 substitute. Meanwhile i got the comparator noise sorted out. I needed to review AGnd to get clean switching. The BK7 has two Gnd pins (4 and 14) for a good reason.Scope dump shows comparator out in yellow and rundown in blue, converting full range input current. Debouncing now works well and there is pretty clean switching with little more than one clock period = 300 nsec uncertainty. After turning off the rundown current source, the integrator error voltage causes an extra pulse of the comparator. It's a TL071..Regars, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: July 02, 2023, 09:23:26 am
content: If the comparator noise is an issue, one may consider the slope amplifier idea found in many ADCs. A similar circuit is also used in precision timing to get low jitter zero crossing signal. The initial gain stage is not only adding gain to get more / faster overdrive at the comparator, but it also is a stage to limit the bandwidth seen by the comparator. The actual BW needed in this case is not that high - it is set by the time from the last reference switching. With normally more than some 5 µs this would mean a suitable BW of some 200-1000 kHz. So no need for a super fast amplifier. Only a fast comparator looks tempting to get a fast transition, but this also means more bandwidth and thus more noise.A little more than 300 ns noise from the comparator does not look that good, even with the not so large slope ( ~ 1.4 mA / 0.5 µF = 2.8 mV/µs).

author: dietert1
author_rank: Super Contributor
timestamp: July 02, 2023, 12:05:53 pm
content: One clock period is the expected uncertainty when sampling an asynchronous event, with a 3.3 MHz clock that gives 300 nsec. It can't be less. Inside the original meter the circuit will perform at least as well as in the ad-hoc test setup.Next i have to make the VCO and the PLL loop filter in order to test the circuit at 3.6864 MHz instead of 3.3 MHz. "Learning by doing."Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: July 03, 2023, 04:45:36 am
content: Here is the confirmation of the INL improvement after the OpAmp upgrade with OPA189 on a second unit. Unfortunately, the black unit suffers from larger t.c., still within spec though and slightly higher overall INL.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: July 07, 2023, 10:11:40 am
content: Meanwhile i looked at the Prema 5017 schematic that is very interesting. It has the dedicated voltage regulator LM2936 for the HC4046 VCO. It also uses the BK7 microcircuit, but with an additional modulator as a refinement. They call it noise shaping and as far as i understand it is about comparator noise.I was thinking about another method. There are fast high resolution ADCs like the LTC2386 (18 bit, 10 MHz) that can be used with a FPGA to determine the zero crossing of the integrator run-down by linear regression, hopefully with less noise and sub cycle resolution. Is this a known method?Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: July 07, 2023, 10:35:52 am
content: The use of a fast ADC and linear regression is use by NI instruments as flex ADC. It is described in patent US 6,243,034.I don't think one would need a super high resolution ADC. Something like 10 or 12 bit may be well good enough, as this would be only for the small range close to zero crossing.Additional noise shaping (a higher order intergrator / additional low pass filter before the comparator) makes absolute sense. The single chips SD ADC chips are AFAIK all at least 2nd order. So the 1st order SD in the 6048 is more like an oddity.

author: dietert1
author_rank: Super Contributor
timestamp: July 07, 2023, 11:37:35 am
content: Thanks, that patent of 2001 used the term "best-fit line" instead of "linear regression" but it shows the idea. Interesting enough they considered the method a good substitute to the multislope method like used in the Advantest R6581 with several different current sources, meant to solve the same problem at end of run-down.Of course, if one can do it with a MCU and its built-in ADC, that's interesting. I thought a MCU built-in ADC might be to noisy and to produce more jitter. Maybe it just needs a slope amplfier (same as comparator). And it depends on how ADC sampling/trigger reacts to the 3.6864 MHz PWM time base. The LTC part is near ideal in that respect (psec aperture jitter).Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: July 07, 2023, 07:57:59 pm
content: Best fit line and linear regression is the same, just different words.The idea with an ADC to measure the charge is also used in the HP multislope 3 and 4, just with a much faster modulation. In case of the MS3 the modulation is with rather large blocks and thus a large range covered by the auxiliary ADC. AFAIK the MS4 ADC uses a more PWM like feedback and thus a much smaller residual charge. The MS3 and very likely (because of the patent) also the MS4 uses a single ADC reading and than a slow measured up front. The linear regression is kind of a way to average over more readings and repeat the the scale factor calibration for every zero crossing.The ADC should still be relatively fast, so that one can get a decent number of readings in the time. However I don't see a real need for very high resolution. The point is getting a little more resolution, like maybe some extra 4 or 6 bits. So enough to get an effective time resolution in the 5-10 ns range. Within limits one can trade resolution for speed. A µC internal ADC may be a bit on the slow side. Some 10 bits at 5 MSPS would ideally resolve the 200 ns in 500-1000 parts. To get a bit more averaging and less noise bandwidth one would however loose some of the resolution. It may still be good enough.The main point of the prema ADC are using reference switching with drive and sense switches. This can be precise with no effect of the switch resistance, but it still adds a bit switching error and needs relatively long settling time. So this requires the relatively slow modulation and large integrator capacitor.Another point with the 6048 is the ADC for only 1 polarity with the reference path switching on / off and not both polarities. This leads to a noise that depends on the input voltage. So with 0 input the ADC can be quite low noise, but with high voltage there can be quite some extra noise. A bit like adding much of the ADC noise to the reference side and not to the input.The simple direct feedback control with the comparator works OK with the rather strong reference (1.4 mA for the 6048), which is not ideal noise wise. My understanding / guess that the MS4 ADC uses the 2nd auxiliary ADC for the Feedback control to get away with a weaker reference. The solartron DMMs use the extra forcing signal to support the feedback. Otherwise they are quite similar.I am not so sure how much it would help, but a possible way to improve on the noise would be some filtering the reference. The LTZ1000 is low noise but still relatively high noise compared to OP-amps. The reference is only used for relatively short (less than 30% duty cycle) pulses and this increses the BW for the reference side. So ref. noise in the 100 Hz + harmonics bands can have an effect. Filtering could reduce this higher frequency noise contribution.

author: Mickle T.
author_rank: Frequent Contributor
timestamp: July 08, 2023, 06:16:38 am
content: Quote from: Kleinstein on July 07, 2023, 07:57:59 pmPrema is known to have several proprietary types of pulse feedback ADCs, including those with a convergence acceleration signal (US5066955).Thaler uses the same topology as in the early Prema patents (1973, 1982).

author: dietert1
author_rank: Super Contributor
timestamp: July 08, 2023, 07:37:50 am
content: As far as i understand one can improve timing resolution at the end of run-down by using a TDC-like circuit to measure the timing of the comparator relative to the integrator timebase. The TDC involves another integrator for a reference current during the delay so one can later digitize the delay with good resolution, to get a fractional, sub cycle time measurement.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: July 08, 2023, 09:23:24 am
content: With a different control hardware one would not be limited to the rather low 3.6x MHz clock for the timing. Just a higher clock for the PLL may be enough for a little more timing resolution. There is anyway the noise limit for the comparator. Due to the relatively large capacitor and thus moderately slow slope the noise could well be limiting:In the 6048 this is 1.4 mA/0.5 µF = 2.8 mV/ µs , which is more comparable to the slow slopes in a normal MS ADC. The noise voltage is given by the integrator ( ~ 10 nV/sqrt(Hz)) and if added slope amplifier with about a comparable noise. The BW is set by the comparator or ideally the slope amplifier. To get reasonble fast response for short pulses one would likely need 100 kHz or a bit more. So one can expect a noise voltage of some 4.4 µV or a timing noise of some 1.6 ns (more with a high BW).With the long integration time one would likely not need that much resolution anyway.

author: Rax
author_rank: Super Contributor
timestamp: October 08, 2023, 07:02:55 pm
content: Quote from: dietert1 on May 18, 2023, 07:52:18 amI accidentally just stumbled into this nugget, dieter1. Very insightful - I've ran into this many times and had no idea what may cause it. I always thought my meter has some sort of low level issue that's rearing its head once in a while (like caused by some minor leakage, or some data being a bit off somewhere, etc.). I always just sort of shook it into nulling by changing the integration times, range, etc. until it'd be willing to null.

author: Rax
author_rank: Super Contributor
timestamp: October 10, 2023, 12:49:28 pm
content: I am registering some success connecting to my 34401A via RS232. I have not yet been able to make GPIB work yet under any of my meters and any of the programs I tried. I mention this because these are my baby steps on this pursuit. I am cursorily controlling my Audio Precision System One via APIB/USB, but that's a whole different story (that's an actively maintained solution).I think some of you have been able to connect to the Prema via GPIB. Can you offer some guidance? If worth a different thread, I'd be happy to start one.I am currently doing some pretty extensive monitoring of a bunch of references - a Fluke 931B, 5440A, a VREF10-001 r9, etc. - the first two displaying incredible accuracy and stability (I've been watching last night, overnight, and this morning the 931B being within something like .02ppm of Prema's 9.9999990V... This is in an unconditioned garage, though the weather here is aways very mild. And the 5440A is in the same ballpark, both comfortably within less than about .5ppm of 10.0000000V for some weeks of observations now, under good discipline of performing INT CALs on the 5440A and "zeroings" on the Prema; the 931B needs no such fluff!).It'd be great if I could log these by monitoring them automatically.

author: dietert1
author_rank: Super Contributor
timestamp: October 10, 2023, 04:16:52 pm
content: Yes, i have a similar setup logging a Prema 6048 and other meters over GPIB. The GPIB interface is DIY based on a STM32 with a USB.I wrote a very small Win32 app that uses a serial comm device to talk to the USB-GPIB adapter and to cycle the meter(s). Results get written to text-with-tabs dayfiles that are easy to use e.g. with Excel/OpenOffice. Or with some averaging app to reduce a large amount of raw data before.Initialization (once)--------------Configue GPIBManual: Connect GPIB cables and USB cable of interfaceGPIB interface setup (open and configure the driver)Bus initialization with IFC bus signalBus and device configuration concerning generation and handling of interfacesignal EOI and/or line terminatorsConfigure P5017The 5017 defines the pace of data acquisitionn, others follow using trigger commands.Manual: Bus address=8, 30 V DC, 4 secondssprintf_s(cBuffer,sizeof(cBuffer),"GD %i",iGPIBAddr_P5017);iStatus=GPIB_Write(hSerialGPIB,cBuffer);iStatus=GPIB_Send(hSerialGPIB,"S0"); // continuous readingsiStatus=GPIB_Send(hSerialGPIB,"CN1");Configure P6048Manual: Bus address=7, 20 V DC, 4 secondssprintf_s(cBuffer,sizeof(cBuffer),"GD %i",iGPIBAddr_P6048);iStatus=GPIB_Write(hSerialGPIB,cBuffer);iStatus=GPIB_Send(hSerialGPIB,"S1"); // single reading, trigger and query resultData taking loop (reading meters, sending next trigger)----------------..GD 8 OK after 0msecGQRD? +10.000072E+0MRVDP01A0R3F0T7D0S0Q0M01B00 (44ms)GD 7 OK after 0msecGR? +01.0158152E+1MRVDP00A0R3F0T7D0S1Q0MOFB00 (3ms)S1 OK after 0msec..This is the bus interface dump:GD n selects a bus deviceGQRD? sends the RD? command to query the device and receives its response.There is a 44 msec delay until the P5017 terminates its measurement and responds.GR? reads a response from the selected deviceThe P6048 already finished its mesurement and responds quicklyS1 is the P6048 command to trigger and query the next measurementThis is the most basic approach and very reliable. Something similar can also be done using the AR488 interface or commercial GPIB adapters.Regards, Dieter

author: Rax
author_rank: Super Contributor
timestamp: November 07, 2023, 01:15:47 am
content: Quote from: dietert1 on October 10, 2023, 04:16:52 pmThank you very much again, Dieter. Can you elaborate on the GPIB-USB interface? Or, maybe better - if you know - would an NI adapter work? I may have access to one on loan. If this works at all, it seems like it won't be a bank breaker. Unless all those affordable ones are bad clones (I'll take a good clone, though...).

author: dietert1
author_rank: Super Contributor
timestamp: November 07, 2023, 07:14:23 am
content: I am using DIY hardware and software for that. For debugging GPIB we have an old HP 59401A.A beginner should rather look at industrial products. Installing GPIB automation isn't that difficult, once you have the hardware (controller and cables). If you borrow it, you could ask the owner how they used it.After connecting the hardware (interface and cables) you will download a driver package and install it on a PC. Then you can use an interactive application included with the drivers (e.g. NI "Measurement and automation explorer") and type in commands and talk to your instrument and listen for the response. To automate the process people use data collection applications, see WinGPIB thread. There you can find examples for logging a DVM and use that as a starting point to setup your own process.GPIB isn't from the AI age. Each instrument has it's own command set. You need to manually assign a different address to each bus device. You need to setup timeouts and EOT convention. GPIB is slow as many of those old instruments have 8 bit MCUs, so each transaction will take some 10 msec or more.I think i needed about 3 or 4 hours to get the Prema 6048 going - without implementing SRQ though. One nasty "feature" of the Prema is that it forgets the assigned GPIB address on power-down. Maybe that can be solved using the calibration switch.Regards, Dieter

author: Rax
author_rank: Super Contributor
timestamp: November 07, 2023, 03:34:19 pm
content: Has anyone reading this successfully implemented xyphro's USBGPIB?https://github.com/xyphro/UsbGpib

author: ch_scr
author_rank: Frequent Contributor
timestamp: November 07, 2023, 03:42:06 pm
content: Quote from: Rax on November 07, 2023, 03:34:19 pmI built a few. They seem to work fine, but no extensive test done.

author: Rax
author_rank: Super Contributor
timestamp: November 07, 2023, 06:48:15 pm
content: Quote from: ch_scr on November 07, 2023, 03:42:06 pmCan you describe your major steps in putting one together? I don't mean hardware. From the point it's all buttoned up and ready to be connected to the computer via USB and the instrument via GPIB.For instance, there's apparently need to "flash the microcontroller," but very cursory directions are provided on Github. I got this: "USB AVR Programmer w/ 6-Pin 10-Pin IDC ISP Connector for USBASP," as I think it allows me to perform this flashing, but I don't know the first thing about if I need it and how to use it.I tried to simply connect one of those built kits to the computer via USB but it's not seeing it.If all these things I'm asking have been clearly mapped somewhere, I'll give that another shot, but the Github guide is completely cryptic to me.

author: Rax
author_rank: Super Contributor
timestamp: November 07, 2023, 06:54:41 pm
content: Obviously, this topic doesn't really belong in this thread, so if you don't mind, we can move it to the one II started specifically for this:https://www.eevblog.com/forum/metrology/prema-6048-gpib-comms-or-maybe-general-thread-on-gpib-readings-harvesting/.

author: Rax
author_rank: Super Contributor
timestamp: November 14, 2023, 01:55:28 pm
content: Quote from: dietert1 on June 11, 2023, 05:56:02 amWith this, do you mean saving the constants out of the unit (maybe via GPIB), or "transfer-adjust" other references to save the various reference points of the meter (such as, for instance, adjust a 731B for the "P6048 10.0000000V" etc?)

author: dietert1
author_rank: Super Contributor
timestamp: November 14, 2023, 03:59:30 pm
content: Please don't adjust any of your precision devices, but do everything on paper. Without notes it's easy to get lost. Some people use a lab notebook to keep important information like that in a safe place.You could just measure the Fluke reference as good as possible and note down the result, maybe together with ambient temperature. After the P6048 mod you could repeat the exact same procedure and determine the new value. This way you can always calculate "unmodded", calibrated results by subtracting the shift caused by the mod. It's easier to revert the mod as long as you don't adjust the Prema calibration.Regards, Dieter

author: Rax
author_rank: Super Contributor
timestamp: November 15, 2023, 01:16:28 am
content: Quote from: dietert1 on November 14, 2023, 03:59:30 pmDieter - thank you for your points, and no! I wasn't planning to touch the Prema calibration in any way. Not even close!The only one I was considering adjusting is the 731B (to match the Prema at 10V), which, as much as it has unusually good tempco, it's a transfer standard, so literally designed for this purpose.That said, your recommendation makes sense.

author: Rax
author_rank: Super Contributor
timestamp: December 31, 2023, 07:30:44 am
content: I finally got tonight to replacing U9 and U10 with OPA 189. I'm getting "Error 1," which doesn't let me go any further (other than, when trying to null the instrument, "Error 4").Any thoughts?

author: branadic
author_rank: Super Contributor
timestamp: December 31, 2023, 09:05:09 am
content: Is this with something connected at the input? If not, that message is not surprising:Quotepage 2/7Quotepage 5/12-branadic-

author: Rax
author_rank: Super Contributor
timestamp: December 31, 2023, 04:32:10 pm
content: Quote from: branadic on December 31, 2023, 09:05:09 amIt comes up regardless. I start the meter with nothing at the input, so it'd run its startup checks undisturbed, and the error comes up right after the checks. The unit settles into the largest range on VDC. If I connect a short or a known voltage at the input, it stays on "Error 1," and if I try to null it with the short on the input it gives "Error 4." So no useful measurement would be rendered with both U9 and U10 replaced.I may try to only replace one at a time and see where that takes me.

author: Rax
author_rank: Super Contributor
timestamp: December 31, 2023, 05:25:22 pm
content: With just U10 replaced with an OPA189, for instance, the instrument responds correctly to commands and inputs. I'll let it warm up, null it, and see if I get more consistent behavior from it.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2023, 05:51:52 pm
content: There is a slight chance that the integrator gets unstable with a different, much faster OP-amp. Here it is not just the GBW, but also the phase reseve under the given conditions that can make a difference. So the OPA189 may not be suitable. With some of the rail to rail output stages it can help to have an additional RC load (e.g. 50 ohm and 100 pF in series) at the output of the OP-amp as this can slightly reduce the gain of the amplifier and improve stability. Even if not actually oscillating a lot of ringing may be worse than the bit slow reaction of the OP77.Candidate OPs to test could be ADA4077 (only 3.9 MHz but still much faster than the OP77), OPA205 (a similar part from Ti), OPA182 (5 MHz brother to the OPA189) and the OPA145 (5.5 MHz) and OPA140/141/OPA1641 (some 11 MHz).

author: Rax
author_rank: Super Contributor
timestamp: December 31, 2023, 06:10:23 pm
content: Quote from: Kleinstein on December 31, 2023, 05:51:52 pmI also have OPA140s at hand - given Dieter1's success with those - so I could try to use that for U9.

author: Rax
author_rank: Super Contributor
timestamp: December 31, 2023, 07:22:04 pm
content: Before I try the OPA140 for U9, I thought I'd run more tests with the OPA189 for U10 and the original OP77 for U9.I'm doing successive nulling operations and then measuring the FX to see if I observe any noticeable jumps in value from one nulling to another. After 5 of these or so, it seems the measurements are pretty consistent. I probably still need to run this more to fully stabilize temperature-wise, but the last few readings have been within less than .02-.03ppm from each other (averaging slight variations of the last count from one integration to another). In other words, it seems that from one nulling to another the readings seem to be very consistent (essentially identical).Does this sound like a relevant test? Aside from a more invested INL plot which would be more than what I can really do today.

author: Rax
author_rank: Super Contributor
timestamp: December 31, 2023, 07:56:36 pm
content: Next step accomplished - it turns out my sample unit agrees with using the OPA140 for U9... Yeah!To summarize, my unit's now using:U9: OPA140U10: OPA189And remains stable and responsive.I'll run some nulling cycles and check for signs of similarly consistent readings as I've seen with the OP77/OPA189 combo.

author: Rax
author_rank: Super Contributor
timestamp: December 31, 2023, 10:01:51 pm
content: Repeated my experiment as above, and I'm getting pretty good (sub-ppm) consistency on the FX measurements from one nulling cycle to another.

author: Rax
author_rank: Super Contributor
timestamp: January 07, 2024, 12:57:17 am
content: Quote from: dietert1 on November 14, 2023, 03:59:30 pmThank you for this, Dieter. I'd really like to save the current constants from the meter, and I may do that before doing anything else.But after having a calibrated 3458A at hand, it seems the Prema - which was about 4ppm off in its original hardware configuration, at least per the 3458A - is now at what seems to be 6.64ppm high at 10V. So, essentially, the difference between the different op amps is these two ppms. I can double check this with my 5440A - which I deem verified by the 3458A - but I'm also considering having the Prema cal/adjusted to the 5440A. I'd bring it to sub-ppm levels on 10V from the over 6ppms it's at now, while I'm very happy for figuring a working combo of op amps curing the INL issue (which I should double check is cured, surely...).Just something I'm considering. Of course, I can always put back the original op amps and restore calibration to factory constants.

author: dietert1
author_rank: Super Contributor
timestamp: January 07, 2024, 02:10:49 am
content: I think a deviation of 4 ppm is completely normal for equipment that hasn't been calibrated for years. The extra 2 or 3 ppm shift from upgrading the opamps are very little, too.If you are able to read the actual calibration info of your Prema 6048 and save it digitally, would you provide some hints how to do it? I tried to find something in the user manual but in vain. Is it by pulling the calibration memory IC?Regards, Dieter

author: Rax
author_rank: Super Contributor
timestamp: January 07, 2024, 06:17:02 am
content: Quote from: dietert1 on January 07, 2024, 02:10:49 amJust about compliant with the 1yr spec, if I read it correctly (5.5ppm of 10V?).Quote from: dietert1 on January 07, 2024, 02:10:49 amWhat I'm thinking is pulling U12, ripping out the data on it and saving it with my xgecu reader so I'd be able to rewrite it on some other IC (such as FM16W08s which I got for this purpose). I think this should transplant the factory cal constants into the replacement contemporary IC. Non-volatile memory not relying on a battery.

author: perdrix
author_rank: Frequent Contributor
timestamp: January 07, 2024, 10:54:52 am
content: You may find the attached useful (it includes calibration instructions) as well as schematics.

author: Rax
author_rank: Super Contributor
timestamp: January 07, 2024, 04:57:06 pm
content: Quote from: perdrix on January 07, 2024, 10:54:52 amThank you, David. I think at least the three of us in this thread doing work on their 6048s have the manual at hand (if you haven't already, please do check the op amps mod!), but I took a quick, cursory look back in the thread, and it looks like the manual PDF hasn't been attached anywhere. Thanks for filling that gap!

author: Rax
author_rank: Super Contributor
timestamp: January 13, 2024, 10:55:27 pm
content: Quote from: dietert1 on January 07, 2024, 02:10:49 amDieter, I think the "floating" calibration info is saved on U12, a 48Z02 which in my unit is a Dallas DS1220Y. I saved the memory out of it with my xGecu T48 to a BIN file (which I can provide should you want it, though I think it only carries my calibrations constants).I'm currently evaluating how I can transplant it on an FM18W08,which seems to be a pin-compatible replacementwith non-volatile memory but without a battery. My plan is to then have a full hardware backup of U12 including the current cal of the instrument. I'd then be able to recalibrate it with the capacity to go back.Now, I hit a bit of a roadblock - I'm not 100% sure how to proceed. I programmed the FM16W08 with the DS1220Y BIN, but there seems to be more to it than I initially thought:https://xdevs.com/fix/hp3458a_u4/#fram. Rewiring and placing of the data on the lowest bank?If anyone reading this has completed or is familiar with the procedure, I'd appreciate some input.

author: Rax
author_rank: Super Contributor
timestamp: January 13, 2024, 11:29:25 pm
content: Some pics of the work today. I had to remove the side panel to be able to pry U12 out of its socket. These f-ers stick like glue after a decade or three. I am using a Brown Dog (Cimarron) adapter to make the FM16W08 (SOIC) into DIP.

author: Rax
author_rank: Super Contributor
timestamp: January 14, 2024, 02:42:12 am
content: Well, for one, the FM16W08 is a 28 pin IC, the DS1220 is 24. Shame on me thinking this is a "drop in."

author: branadic
author_rank: Super Contributor
timestamp: January 14, 2024, 07:44:38 am
content: Attached images of a proper adapter.-branadic-

author: Rax
author_rank: Super Contributor
timestamp: January 14, 2024, 11:27:56 pm
content: Last night, I got as far as finding some adapters (such as those you point out, branadic) available both for direct purchase and gerber files (https://github.com/tebo1000/DS1220-replacement-PCB), which is when it downed on me I blew up my "extended weekend" project. I could probably source some "80s date codes" DS1220s in my neck of the woods during the weekend, but that's about all I can do.I'm planning to simply program the BIN onto the FM16W08 and test in the meter. If I get some Error 8 or whatnot, reverse it (put back the DS1220, though I'll still try building a few with different calibration constants).In terms of programming the FM16W08s with the DS1220 BINs, are there some known specific precautions to take? Given the former are four times the size of the latter. The other thing I'm not quite sure of is if the programmer needs to be set to "see" a DS1220 once the FRAM is on the adapter, but that's what makes sense to me, so that's what I'll do.

author: Rax
author_rank: Super Contributor
timestamp: January 15, 2024, 05:18:44 pm
content: Quote from: Rax on January 14, 2024, 11:27:56 pmI wonder though if anyone can confirm these adapters (those above) are working as intended. I ordered a set from OSH Park, but it'd be great to have them vetted before I go too far with them.

author: Rax
author_rank: Super Contributor
timestamp: January 16, 2024, 11:29:17 pm
content: Quote from: dietert1 on January 07, 2024, 02:10:49 amTo answer this question specifically, Dieter (and as discussed offline), that is correct. I'm not trying to approach this through GPIB. It would be nice to be able to pull this data that way, but I don't think I need that given my specific needs.I am able to see the data off of the DS1220Y (though the programmer interface) but it's not directly legible. Just enough to allow me to assume there's something on there, and make some basic comparisons (for instance, after transplanting this code into the FM16W08, which seems to check out).I'm not sure if there's a way to interpret it, but I assume it would need to rely on the Prema software in some manner, which may preclude that path.

author: Rax
author_rank: Super Contributor
timestamp: January 22, 2024, 06:49:14 am
content: I've adjusted the meter tonight per my 5440A (10V range only). I trust it to about .5ppm/10V. I'm about 1C below everything the 5440A has seen in December when it was running alongside the calibrated 3458A, so I exercise a bit of judgement in the software calibration adjustment.At this point, I have a DS1220Y with these calibration constants, one with the last calibration calibration constants, and will next program one with the factory constants.I'm not sure how easy it's been for others to extract U12 out of its socket, but I'm having a heck of a time doing it. There's an "cardboard HV shield" that's in the way. The FM16W08 on adapter will be so much easier to plug in.My end goal is to have three FRAMs with these three sets of calibration constants. Such that I'd have these three sets in an easy to swap in manner and "battery-agnostic," so they'd last 150 years with no stress.

author: dietert1
author_rank: Super Contributor
timestamp: January 22, 2024, 12:49:43 pm
content: It may be easier to take out the whole processor board. There are the flat cables, two nuts that hold down the board near the front and two GPIB screws in the back. As always, avoid ESD damage while handling the board. When inserting the board back again, assure the calibration switch remains off.Regards, Dieter

author: Rax
author_rank: Super Contributor
timestamp: January 23, 2024, 03:42:32 am
content: Continuing to examine the shifts I get after switching those op amps:2V - a 1V excitation from the 5440A returns .99999595V vs. .99999535V (the latter being the reading on the 3458A from a couple of weeks ago).2V - a 100mV excitation returns 99.99431mV vs. 99.99616mV (which certainly gets into the noise)The above is with 80s integration.I think this is much lower than I expected - or is there maybe a scaling aspect to the shift? In the sense of being proportionally lower at lower readings. I didn't think it would. If I'm reading the specs correctly, this is actually compliant with spec accuracy on the 2V scale but not on the .2V (off by 1.25uV, which is well down in the weeds...).

author: Kleinstein
author_rank: Super Contributor
timestamp: January 23, 2024, 09:28:56 am
content: For replacing U9 , I would expect an effect on the readings near zero, effecting the INL problem there as an extra offset for the positive / neg side. The error near zero also slightly effects the calibration of the scale factor, but this may be in the uncertainty of the usual calibrations.For replacing U10 one should have a small effect on the scale factor: the offset of U10 adds to the 7 V reference voltage.In addition there can be a change in the zero point, but this is usually corrected with the zero function.The changes should scale with the ranges and this less visible in the 2 V range and even less for the 0.2 V range. The gain between the ranges should not be effected by a change in the OP-amps.

author: Rax
author_rank: Super Contributor
timestamp: January 23, 2024, 01:56:00 pm
content: Quote from: Kleinstein on January 23, 2024, 09:28:56 amI think that is what I'm seeing. The shift at a 10V excitation/measurement (20V range) was not small - all said and done about 8-9ppm [including a likely 4ppm drift by the meter due to aging]. In the 2V range at 1V, if my math is correct, just about 1ppm of 1V [much less, actually, judging by values measured after multiple 80s integrations (all the way to 24hrs later). But then, who has time for an integration so long it can be considered "aging?..."]. In the .2V at 100mV excitation I see more shift, or lack of accuracy, but I blame the low level of the excitation which gets things close to noise levels.[...] = ulterior notes

author: Rax
author_rank: Super Contributor
timestamp: January 25, 2024, 02:28:36 am
content: Finally got "Bonetti"-based adapters from OSH Park and soldered some FM16W08s, but I'm getting errors when trying to write the BIN onto it as a "pseudo-DS1220." Both the "program" and the "verify" seem to return errors.When trying to write the 6048 BIN onto the FM16W08 installed on a straight-out DIP adapter instead, the programmer didn't complain. Both the "program" and the "verify" procedure went through without a hitch.Any thoughts as yo why I'm unable to write this successfully? The only thing I can think of is I didn't install the SMD cap the "Bonetti" board has provisions for. But I can't imagine this to be the issue here.Pics enclosed.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 25, 2024, 10:12:24 am
content: The Decoupling capacitor could make a difference, as decoupling should be close to the chip and the adapter / ZIF socket add quite some path.An obvious point is that with the adatper the programmer should be set to a DS1220 or similar chip, no longer the FM16W08.

author: Rax
author_rank: Super Contributor
timestamp: January 26, 2024, 12:51:47 pm
content: Maybe I'm impacted by this:https://www.eevblog.com/forum/manufacture/fyi-xgecu-tl866ii-and-t48-cant-program-fm16w08-fram-chips/.

author: Rax
author_rank: Super Contributor
timestamp: January 26, 2024, 12:53:31 pm
content: Quote from: Kleinstein on January 25, 2024, 10:12:24 amI didn't have an 0805 at hand, so I used a very small leaded MLCC to no avail.Quote from: Kleinstein on January 25, 2024, 10:12:24 amYes, indeed, and that's how I did it.

author: branadic
author_rank: Super Contributor
timestamp: January 26, 2024, 08:25:31 pm
content: Why do you use "pseudo-DS1220" instead of choosing the FW16W08?-branadic-

author: Rax
author_rank: Super Contributor
timestamp: January 26, 2024, 08:39:35 pm
content: Quote from: branadic on January 26, 2024, 08:25:31 pmBecause, once the FM16W08 is installed on that adapter, it effectively is a "non-battery" DS1220 for all intents and purposes. The programmer should see a DS1220 to be able to program it (pins assignments, etc.). And hopefully the Prema also "sees" it as DS1220 once installed in its U12 socket.Am I missing something?...

author: Rax
author_rank: Super Contributor
timestamp: January 26, 2024, 08:42:14 pm
content: Quote from: branadic on January 26, 2024, 08:25:31 pmOn a side note, as I was able to program the BIN on an FM16W08 placed on a BrownDog 28 SOIC to DIP adapter, and verify it successfully, I guess I could transplant that already programmed IC onto the "Bonetti" adapter. But it feels I shouldn't need to do that. I also am not sure the programming would become a successful emulation of a DS1220 so the Prema would like it.

author: Rax
author_rank: Super Contributor
timestamp: February 17, 2024, 12:20:33 am
content: After performing the DCV cal on the Prema on the .2V, 2V and 20V ranges - using my 5440A, which seems to be vetted by a KS-calibrated 3458A - I am seeing what to me like pretty reassuring agreement. For instance, between the 2V "non-DIV" input and the 2V "DIV" input, there's sub-unitary (ppm) divergence between the two hardware paths (preliminary conclusion). Of course, not very easy to say if what I'm seeing confirms the Prema, the Fluke, of both of them in "joint venture." I'd need to do more work to clear my data and experimental conditions.But to me this provides some level of comfort with the linearity of the Prema ADC away from zero crossing, as it compares with the pretty good linearity the 5440A in known to have.

author: Rax
author_rank: Super Contributor
timestamp: February 17, 2024, 12:53:49 am
content: The other provisional conclusion I feel I can draw is that I'm seeing much more consistency from the Prema than before the upgrade.I feel that the zeroing is now pretty reliable and and the drifts I used to see (and not know whether they are caused by the Prema or the 5440A) are no longer occurring. If anything, the zeroing of the Prema seems just about as reliable and resulting in a level of relative accuracy (between the two) as the AUTOCAL the 3458A seemed to provide (and direly need all the time). In fact, for my particular samples, the Prema needs zeroing less than the 3458A needed the AUTOCAL.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 17, 2024, 10:40:39 am
content: The upgrade via better OP-amps can be really woth it. The change OP77 to OPA140 seems to largely fix the weak point in the INL. The OPA140 and LT1007 for the reference are major contributors to the zero point drift. Here there can still be variations in the actual parts used. Even in the original configuration the stability of the zero can vary between units. If the zero is stable there is no need to change things. If it is drifting it can make sense to try an alternative chip (could be just another LT1007 or similar or an AZ OP).How often the zero for the 6048 or ACAL for the 3458 is needed can also vary. Some units can be more stable than others.For the prema meter the gain of the ADC and input is set by 1 resistor ratio. The 2 V range may be slightly more stable in this respect, using 2 x 5 K and thus equal value resistors. The other ranges still also still have only 1 pair of resistors, just exchanging one of the resistors to a different value. So there is no clear prime range with like with the 3458. The 6048 does not look that it could easily measure it's own reference directly. So a kind of ACAL would not work so easy for the Prema meter. The test in 2 ranges is more like a self test, but no way to correct drift.The 3458 has additional resistors in the ADC gain and amplifier gain. So less stable to start with, but able to correct. On the other side it has a stable zero.

author: Rax
author_rank: Super Contributor
timestamp: August 13, 2024, 02:55:27 am
content: A bit of a follow up post (medium term observations, at least in my book) to the issue of the op amps upgrades and zeroing. It partly reconnects with some offline conversations with dietert1.I'm very happy for how it behaves from that perspective. I had it off for a few days and just turned it on. From a cold turn on it started around maybe .00000250V with a short on the input, and after a good one hour warmup it's at about .00000035-45. I don't think that's a bad zero by saved values from the prior run by any stretch of the imagination.I think this is further proof the op amps upgrades are conducive to bettering the performance of this great meter.

author: branadic
author_rank: Super Contributor
timestamp: August 18, 2024, 09:02:07 am
content: We learned something new at MM2024 about Prema 6048. Some participant, let's call him Manfred L., gave us some interesting info. In case you were wondering about the bodge wires in your unit (soldering side), in particular at the 5 V logic rail, this is not someones repair attempt of a broken trace but a factory bodge made by Wilfried Stricker himself to decrease trace resistance. Don't attemp to remove them, but leave them as they are and where they are!Hopefully we can soon report what removing them will lead to-branadic-

author: Rax
author_rank: Super Contributor
timestamp: August 19, 2024, 12:23:29 am
content: Quote from: branadic on August 18, 2024, 09:02:07 amOK. Bring on more novel finds on the 6048.

author: meandeev
author_rank: Regular Contributor
timestamp: August 19, 2024, 05:51:56 pm
content: Quote from: branadic on August 18, 2024, 09:02:07 amHi Branadic,got the broad hint ;-)Background for everyone else: I bougth a prema 6048 3 years ago with a strange error, that wasn´t solved by us until now.In the picture that I made when the unit arrived one can see the bodge wires at the digital board.I removed them, because I couldn´t imagine that they are an "official repair". I didn´t made any measurements before that, but later the meter shows a strange behavior, shown in the second picture:strange jumps of the measured values. The red line are the values from the Prema 6048, blue are the values from a HP34401 that measures PIN6 of U10 (~ the voltage of the Prema LTZ1000). Please ignore the jump at 1:05:00, at that time I switched off/on the prema for a second.If I have some spare time I will start to investigate that again, also with installed bodge wires.

author: dietert1
author_rank: Super Contributor
timestamp: August 19, 2024, 08:08:24 pm
content: When i started working on that board - after receiving our Prema 6048 from former contributor essele - i found those bodge wires, too. As far as i remember it is an improvement of the +5 V supply network.I also remember adding several buffer caps on that board. The original design is poor in comparison to a typical HP device of that time. Those have almost one buffer cap per logic chip.I also bought a new 2 MHz crystal, as the oscillator wasn't in good shape. Don't remember whether that solved the issue or other mods were necessary, like grounding the crystal housing.Regards, DieterSome more information: The processor boarddoeshave one buffer cap per IC, my memory error. Appended two new images of our Prema 6048 processor board:Top:- 2 MHz crystal replaced, its housing connected to Gnd- Three 22 uF elyt buffer caps renewed- New 5V power supply cap is 10 000uF/16V/85°C- Rectifier bridge got 5x5 cm copper sheet for cooling. Still gets pretty hot.- FRAM was installed by previous ownerBottom:- 2x 470 nF across rectifier- Supplementary wiring fixed with glue

author: meandeev
author_rank: Regular Contributor
timestamp: September 02, 2024, 07:20:25 pm
content: I found some spare time and so I investigated the error of my Prema a little bit deeper (with removed bodge wires): with a logic analyzer and decoded the SPI value from the BK7 chip. At 10V one can see nearly only 2 values alternating. I don´t have a clue, what the SPI values are or if I decoded it right.A new timeseries with a stable 10V shows spikes again. Because the lid of the prema was open, the measured values are not that stable. But you can can see clearly the spike of ~120µV. The x-axis is the sample number of my logic analyzer, that I tried to synchronize (around sample 205 000 000)I recorded the SPI-values in parallel, you can see the values start to oscillate at around 205 000 000:The 5V-rail at capacitor C2 was also measured with a 8-bit AD converter. It oscillates all the time, but normally the amplitude is stable:But the amplitude start to oscillate at the same time, when the SPI-values start to oscillate:And also stop, when the SPI values stop to oscillate:So the next step will be: install the bodge wires again and the enhancements that dietert1 described

author: dietert1
author_rank: Super Contributor
timestamp: September 02, 2024, 09:30:37 pm
content: The LM309 datasheet claims that no output capacitor is needed for stability. Then C2 isn't that important.Your "scope" traces suggest that C1 requires replacement. Probably its capacitance is less than 20 % of nominal value.

author: meandeev
author_rank: Regular Contributor
timestamp: September 08, 2024, 06:29:05 pm
content: I reinstalled the bodge wires as well as a new C1 with 10000µF and a new CR1. For CR1 I used a B500C7000, because the datasheet of the former installed B40C3700 doesn´t allow such a big C1. Also the 2MHz quarz was replaced.Unfortunately this was not the solution for my problem.The 5V rail looks clean(er) (maybe also because of better ground) and the amplitude doesn´t oscillate anymore, when the SPI values start to oscillate.So now I´m clueless again where to search for the error :'(

author: branadic
author_rank: Super Contributor
timestamp: September 08, 2024, 06:58:42 pm
content: That's really unfortuneAre the ground wires thick enough, similar to the original ones? Is there anything else you have "cleaned up"?This weekend I've tested another Prema 6048 white, that was dropped here after MM2024, for INL with the original OpAmp arrangement. Similar results to what we've already seen. New OpAmps are on their way and should arrive in the next days (OPA189 but also OPA140), will give them both a try to see, if I can spot a difference.-branadic-

author: meandeev
author_rank: Regular Contributor
timestamp: September 08, 2024, 07:28:26 pm
content: The wires are 1.5mm^2 copper. On the digital side nothing else was changed.Because you have a 6048 on the bench: please can you measure the PLL voltage PIN9 from U7?And make an oscilloscope picture from PIN9 of U8 (the Prema IC)?Thanks!

author: branadic
author_rank: Super Contributor
timestamp: September 13, 2024, 08:54:15 pm
content: Pin9 of U7 measures 0.678 V on that unit. Attached are images of U8 pin9 with display set to infinity over a course of 20 s, as well as U10 pin 6 @ 10 V.-branadic-

author: meandeev
author_rank: Regular Contributor
timestamp: September 15, 2024, 10:12:25 am
content: Thanks for the measurements!I´m more and more convinced, that I have a problem with the PLL locking…It seems that the output pin9 from the Prema-IC (U8) has an extreme high output resistance. It makes a big difference if I use a 10Meg 1:10 probe or a 1:1 1Meg. As you told me, you used a 10:1 probe.For the records here my measurements from my Prema 6048:With 10:1 10Meg the upper voltage of the peak is ~11.4V, the lower „floor“ ~5.92V, the peak width is ~220µs, and the Prema shows 10.243489V for a 10V input, Pin 9 of U7 is 2.13VWith 1:1 1Meg the upper voltage of the peak is ~11.3V, the lower „floor“ drops to ~5.5V, the peak width is now 1.85ms !!!, and the Prema shows 10.261289V for a 10V input, Pin 9 of U7 stays at 2.13VWithout any connection the Prema shows 10.242167V for a 10V input.A rough calculation from the "floor" drop gives 90k - can this be?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 15, 2024, 03:16:53 pm
content: The voltage levels for the PLL signal look about right. The following OP-amp has some 6 V at the non inverting input, as a reference level.For a PLL phase-frequency dedector (e.g. as in the 4046) it is normal to have a signal with 3 states: positive , negative and high impedance, usually with much of the time at high impedance. The prema meter has an integrator in the loop filter and should thus have only short pulses. However the scope probe already disturbes the DC balance, even with 10 M and more with 1 M. The 5.9 or 5.5 V case are likely 6 V from the OP-amp side and the the 100 K and probe causing the voltage drop. The 1.8 ms of pulse length sound about right so that 18.2 ms with 1.1 M to GND is compensated by 1.8 ms via 100 K.From the observed waveform it more looks like the PLL is working. The better point to probe may be the output of U6 or the PLL test point. This should be a relatively stable voltage with little to no detectable ripple. The exact voltage will depend on the current mains frequency and R56 setting.Normally probing the PLL should not effect the reading. Some effect is possible via additional capacitive coupling to the PLL and the larger ripple at the VCO tuning signal. With normal operation coupling to the PLL and the residual ripple can cause part of the INL error.

author: branadic
author_rank: Super Contributor
timestamp: September 22, 2024, 08:59:20 am
content: Precision pinheaders arrived, so I was able to populate U9 and U10 with OPA140. Results attached. The improvement is crystal clear. Next is OPA189 for comparison.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: September 22, 2024, 06:13:27 pm
content: For some odd reason OPA189 performs worse than OPA140 on this unit.So OPA140 is the way to go.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: December 15, 2024, 08:09:43 pm
content: Good evening,this weekend I had a visitor in the lab with a Prema 6047 and Prema 6048. As usual INL in the 20 V range was measured against 3458A and something interesting was found.The Prema 6048, populated with U9 = OP77 and U10 = LT1007, showed the typical INL issue (Prema6048-1.png) before the OpAmps were replaced with OPA140 (Prema6048-2.png).The Prema 6047, also populated with U9 = OP77 and U10 = LT1007 didn't, but showed a slightly different behavior (Prema6047-1.png and Prema6047-2.png).While both meters are absolutely identical except for the fact that Prema 6047 is missing the BK7 for temperature measurement on the digital board and uses LM399 instead of the LTZ1000 reference circuit, I wonder if the INL issue is really due to the OpAmps used for U9 and U10. Instead I think the real issue is something like capacitive loading of the LTZ1000 reference circuit, with U10 being part of the loading.The Prema 6048 I had (see the first posts in this thread) was one of the very first ones, using a Prema 6047 ADC board and a seperate LTZ reference board connected to the ADC board. I remember me trying to measure the t.c. of this reference board and having huge trouble with oscillation, without any difference between using LT1078 and LT1007.We now see a totally different behavior of the ADC with an LM399 connected to it. We also know that LM399 has pretty low dynamic impedance — 0.5Ω.Unfortunately, we didn't have enough time to measure Prema 6047 with OPA140 populated, but this would be something interesting to test. I also think equipping the Prema 6047 with ADR1399 could be an interesting investigation.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: December 15, 2024, 09:14:10 pm
content: As far as i remember the 6048 has a buffer meant to correct for multiplexer resistance in its switched reference current source. That buffer opamp had input protection diodes which would cause transient load/current spikes from the reference source (LTZ1000 circuit), causing the typical nonlinearity near zero input voltage. No surprise that the magnitude of the effect depends on details of the reference circuit.I also remember that the ADC board i was working on at the time had a place for an optional buffer cap for the LTZ000 reference. It was missing in our instrument, probably because capacitive load can cause instability. I remember adding an RC combination similar to the snubber recommended for the ADR1399.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: December 15, 2024, 10:53:17 pm
content: Loading the reference via switching spikes and the diodes inside the LT1007 (U10) is possible. However this would not be limited to the range just around zero, but a possible issue with any ref. switching. AFAIR the point for the INL was that the OP77 (U9) is a bit slow, so that the integrator input voltage is not fully settled for very short reference pulses that happen near zero. So an INL cause be a low speed of U9, not from U10. The way the reference is switched leaves the ref. side of R31 floating when off. This makes the integrator sensitive to incomplete settling with short pulses.Loading the reference with a capacitor (C49 = 100 nF) is a possible issue. The LM399 ref has a better chance to tolerate the capacitor. For the LTZ version one would ideally not have C49 or a resistor of some 100 ohm in series to C49.Using a ADR1399 could be a sensible upgrade for the 6047. The effort is moderate and so far they seem to behave well: not just half the noise, but also low drift seen so far (still a bit limited statistics).

author: branadic
author_rank: Super Contributor
timestamp: December 16, 2024, 09:33:18 am
content: @ dieterAs far as I can see, there is no difference between 6047 and 6048 integrator board schematic except the reference (see attachement).@ KleinsteinHow do you explain that 6047 doesn't show the INL issue, if the schematics are otherwise identical? All that we can see for the 6047 is a t.c. sensitivity, visible around zero.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: December 16, 2024, 10:25:58 am
content: Kleinsteins little theories sound interesting but don't lead anywhere. Discussing at that level of half knowledge doesn't resolve anything. You have to spend some serious time to find out what is going on. I did that work for the 6048 and the facts can be found above in this thread.If the 6047 doesn't have that linearity problem, i don't understand the point of this discussion.Regards, DieterEdit: Yesterday i happened to watch this repair video:A good example of what it may take to fix the problems of a legacy device. Probably the 6047 has enough other problems to work on..

author: branadic
author_rank: Super Contributor
timestamp: December 16, 2024, 10:39:10 am
content: QuoteThe answers to that is:1. Both meters have identical schematics, except for the reference.2. One of them shows the INL issue in the 20 V range (6048), the other doesn't (6047).While the OpAmp (OPA140) replacement you suggested solved the issue for the 6048, I somewhat doubt the root cause (OpAmp speed, ...) and then the question is, what is the difference between both meters? That is what I'm interested in to learn something from that excercise for the future. For me this points towards the reference and its output impedance. But I might be wrong.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: December 16, 2024, 12:31:45 pm
content: Chances are the INL problem in the 6048/6047 may not effect all units but could be a borderline case. If as large as measured in this thread, they should have caught it at Prema in the design / initial test phase. So it could depend on individual parts (e.g. the OP77 or LT1007 or prema chip) within the normal variations.Another point could be a slight difference between the schematics and actual circuit. Dieterts writeup of the mods show an extra capacitor at the integrator (around U9), that could make a difference, though hard to tell without knowing the internals of U8.The impedance of the reference is a clear difference between LM399 and LTZ1000 based. The LT1007 may cause a current spike on switching the feedback / load though not for sure. This would be a part one could check.

author: branadic
author_rank: Super Contributor
timestamp: December 16, 2024, 01:10:03 pm
content: QuoteI wouldn't call 6 out of 6 units (4 meters I had on the table, 1 unit by dieter, 1 unit by Rax) a borderline case.On the other hand, I had a call with the cal lab guys at systems-engineering.eu, who calibrated meters for Prema and confirmed the observation of the INL issue.I hope, that one of my contacts finds the paper article by Prema, which also confirms the INL issue for Prema 6048 and shows the INL error around zero that we observed.-branadic-

author: Andreas
author_rank: Super Contributor
timestamp: December 16, 2024, 06:50:38 pm
content: Hmm,did you check for oscillations on U20 pin 7?A capacitive loading on the output together with a asymmetrical impedance (diode) should lead to oscillations.That is why I introduced C8, R19, R17 to avoid oscillations with capacitive loading in my LTZ1000 designs.with best regardsAndreasNote: for R19 I had to interrupt one pcb trace on my very first PCB.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 16, 2024, 08:16:14 pm
content: The plan has the capacitor C49 loading the reference, but for the 6048 this one seems to be not populated - it is at least not in the pictures and Dietert mentions this. If at all one should have C49 with a series resistor.

author: dietert1
author_rank: Super Contributor
timestamp: December 17, 2024, 07:28:52 pm
content: Quote from: branadic on December 16, 2024, 10:39:10 amYou got confused by Kleinsteins comments. As i repeated some days ago one of the design faults i found in the 6048 were the input protection diodes of the buffer amplifier of the switched reference current source. In my comment i also wrote that a certain depence of this on the details of the voltage reference implementation is no surprise.As far as i understand, you propose to replace the LTZ1000 reference in the 6048 by a LM399 or ADR1399 in order to avoid changing the opamps. Appears a bit strange to me.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: December 17, 2024, 07:43:01 pm
content: QuoteNo you got me wrong, what I propose is to simply add a 100R resistor between LTZ reference and opamp for 6048 to solve the INL issue.I do propose to try ADR1399 for 6047 though as I expect way lower noise.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: December 17, 2024, 08:04:14 pm
content: A resistor at the U10 (LT1007) input makes absolute sense. 100 Ohm could still be a bit on the low side. The DG308 switches already have some 60 ohm and 2 switches may apply.It would be more like 1 K to make a real difference and still not significant noise.The alternative would be replacing the LT1007 with something like OPA140 or OPA189.

author: Rax
author_rank: Super Contributor
timestamp: December 18, 2024, 01:05:46 am
content: Quote from: branadic on December 17, 2024, 07:43:01 pmBranadic,I think your other finding is using the OPA140. My sweetspot seems to be:U9: OPA140U10: OPA189Are you using OPA140 for both? Or just one of those positions?  Perhttps://www.eevblog.com/forum/metrology/prema-6048-teardown/msg5650669/#msg5650669,yes.I'm also not sure I understand if you're saying the 100ohm resistor would completely sort out the INL issues? Without the opamp upgrade?

author: bdunham7
author_rank: Super Contributor
timestamp: May 05, 2025, 07:16:45 pm
content: To continue the saga of Rax's 6048, it has now been updated with the correct new firmware and reverted to factory calibration constants. Here's a chart and graph of the raw data--I haven't reduced it to an INL plot. At +1.000000V the INL appears to be 1.6µV or 0.08ppm/range. I checked the null at several points and it changed by a total of 2.4µV over the course of the tests. That's a whole lot less than the mess I was seeing before the firmware update but I still want to have a closer look at how it behaves. This data was obtained using 80 seconds integration time and letting the meter settle at least 3 periods and longer if more than a few counts change was noted. At lower voltages this meter take a very, very long time to fully settle.Comments and suggestions welcome. Obviously this data includes any INL, offset or other error of the 5440B used as a source. The nulls were done by selecting 0.00000000V on the 5440B, not by shorting the inputs of the Prema. The readings were taken in order from the top to the bottom of the columns although there were some addtional tests done (not listed) between the -10V and the last null.

author: dietert1
author_rank: Super Contributor
timestamp: May 05, 2025, 10:20:17 pm
content: Is that the firmware provided in message 22?Is the factory calibration device specific? Does it cover the temperature corrections?

author: Rax
author_rank: Super Contributor
timestamp: May 05, 2025, 10:40:27 pm
content: Quote from: dietert1 on May 05, 2025, 10:20:17 pmThe firmware update is the 2018 file provided by Prema at the time to David P, which was intended to cure this "moving zero" defect most of us observed. I don't think it was intended to address the INL issue.The factory calibration has to be device specific, and I believe saves the cal constants as performed on day one at the factory as the unit is about to be commissioned. Refer to the procedure in the manual for restoring the unit to that factory cal state (restart with cal switch in CAL position). That's the procedure we followed.Certainly, we're aware restoring this way provides no guarantees the unit goes into a state of unmitigated accuracy a few decades later. But it was worth performing (we have all other other calibration datasets in a set of respective SRAMs).

author: bdunham7
author_rank: Super Contributor
timestamp: May 05, 2025, 11:05:47 pm
content: Quote from: dietert1 on May 05, 2025, 10:20:17 pmI got the firmware from reply #13 of this thread:https://www.eevblog.com/forum/metrology/possible-problem-with-prema-6047-or-6048-meters/The factory calibration was extracted from the EPROM in the device, so I'd assume it is unit specific. IDK about the temp corrections, not sure how those are implemented or where I'd look.

author: dietert1
author_rank: Super Contributor
timestamp: May 06, 2025, 05:51:37 am
content: Thanks! That zip archive contains a remark on how and where to insert the factory calibration of an instrument into the EPROM. The calibration of the undocumented temperature correction should be contained there.As far as i understand, that improved software can not correct the nonlinearity, but it does solve the null ambiguity by a refined calibration model. So the null would no longer depend on whether the instrument is in + or - range. Instead the error voltage of the nonlinearity appears further away from zero where the relative error is less.Regards, DieterPS: By the way, currently a working P6048 is on offer at kleinanzeigen.de for € 1200.https://www.kleinanzeigen.de/s-anzeige/prema-6048-intgrating-multimeter-81-2-stellig-/3067427068-168-1160

author: Rax
author_rank: Super Contributor
timestamp: May 06, 2025, 12:09:51 pm
content: Quote from: dietert1 on May 06, 2025, 05:51:37 amDieter - yes, we replaced the generic "FF" records between registries "4000" to "47FF" with the data from my old EPROM. I assume when you refer to the "undocumented temperature correction" data, it is then included in those calibration values/constants, and we've therefore transplanted it into the firmware-upgraded unit.This far we seem to see far more consistency around zero. Some tests on Sunday seem to show fractions of a uV difference, if even that, but we're still collecting more data and observations. But I think we can preliminarily conclude this firmware update provides a very distinct improvement (and possibly a good fix) in this regard.

author: Rax
author_rank: Super Contributor
timestamp: May 08, 2025, 01:49:22 am
content: Have all the elaborate contributors - branadic, dietert1, etc. - performed the firmware upgrade? I don' think I have that data handily available.

author: bdunham7
author_rank: Super Contributor
timestamp: May 08, 2025, 05:03:46 am
content: Quote from: Rax on May 08, 2025, 01:49:22 amYes, a long time ago--look at reply#61. We're very late to the party. It looks likebranadicconcluded that the software was not really a fix, something I'm starting to see myself.Have you read this thread?https://www.eevblog.com/forum/metrology/prema-bk7-derived-meter/msg4964713/#msg4964713

author: dietert1
author_rank: Super Contributor
timestamp: May 08, 2025, 07:24:25 am
content: Don't know when i can continue that "Prema remake". Closing our business due to age was and still is a tedious operation. Everything is slowing down, yet i am still using our lab as long as we have it.Regards, Dieter

author: Rax
author_rank: Super Contributor
timestamp: May 08, 2025, 12:29:17 pm
content: Quote from: bdunham7 on May 08, 2025, 05:03:46 amYes, though it's been a while, and I also have not personally been at the Prema game in a while. Don't have a fresh memory of all the intricacies of efforts and their specific records.

author: bdunham7
author_rank: Super Contributor
timestamp: August 02, 2025, 05:22:01 pm
content: I think this 6048 might be fixed. Here are the pictures!

author: bdunham7
author_rank: Super Contributor
timestamp: August 02, 2025, 08:45:10 pm
content: Here are the details of how I took these measurements. I'm not sure I have a full explanation for the better overall results (less noise) but the INL problem appears to have been resolved by paying very, very close attention to the null setting.First, I hadZondar'sdying 3458A here and it was well enough long enough to use the ratio functions to help do a full external calibration of a Fluke 5440A. The pictures say 5440B but that's an error, it was the 5440A. In any case, using other available stuff I was able to at least verify that the 22V, 11V and the lower divider ranges were all as accurate as they could be. Unfortunately the 3458A has now drifted off to oblivion, but that is its own saga.Next I very carefully calibrated the DCV ranges on the 6048. I used the 40s integration time for all measurements as 20s was a tiny bit less stable but 80s takes too long and part of the idea was do the procedure as quickly as possible to minimize any tempco or drift. The 5440A and 6048 had all been powered on for several days and I started in the morning with an ambient temp of 22.3 and ended with 22.7. I started by installing a 4W thermal shunt and then letting it sit for 10 minutes. I then nulled each range separately (not using AUTO) and kept doing each one until I had an exact zero count null. Then I connected the 5440A and calibrated each range using 40s integration time. Each time I waited and verified that the 0V output (output turned on and set to 0.0000000) of the 5440A was within 1 count of zero before entering the calibration voltage. Afterwards I verified the 10V and 100V ranges directly with the 732A. This whole process took 20 minutes working with all deliberate speed, no breaks. When I was done I rechecked the nulls on each range with the thermal short and they were not zero, but they were single digits IIRC.The next day I did this linearity test. I did an AUTO null on the 6048 with the thermal shunt and then I connected the 5440A and 6048 as before. I took the measurements as shown in the chart in that order without touching the leads and entering exactly what is shown on the front panel of the 5440A. I only went up to 2V because that is the highest I can go on the 5440A without moving the leads. Higher voltages would not add to this discussion because the INL problems are all near zero anyway. I took multiple zero readings throughout so that you can see how and when the null shifted by any small amount. Also, the little kerfluffle that shows up at about +/- 0.2V appears to be a (not problematic) issue with the 5440A as I was able to see it afterwards on a different meter. Note that I have mis-entered the data an line 41, there is one too many zeros. This makes no difference to the end picture.So there it is. I find it interesting that the two nulls in the center are exactly zero whereas all the others exhibit a slight shift that sort of inversely corresponds with the previously applied voltage. Then again, those single-digit measurements are surely down in the noise. I did notice an anomaly later where my 10V reference was reading about 50 counts (0.5ppm) high. I checked the null and it was about -50 counts--the opposite of what you might expect. I nulled it and rechecked the 10V expecting it to now be off by 100 counts, but it wasn't--it was almost exactly spot on. So I think there is still an issue with how the meter handles nulls, the solution and the way to get results like what I've posted is to make sure not to have any null offset whatsoever. The meter doesn't have autozero (perhaps you'd call it demented zero) and it is a pain to manually zero it before each reading--but zero it you must. You have to swap the leads out, wait for it to settle for the zero and then again when you've reconnected the test leads. You have to make sure the temperature is very stable during this time, etc etc. There is some tempco both for gain and null and it is a signficant part of the performance. I haven't characterized it yet, but I suspect that it will dominate the 5ppm spec.