Fluke 732A repair, startup, adjustment and characterization. - Page 1
Forum section: electronics / general

author: bdunham7
author_rank: Super Contributor
timestamp: February 08, 2025, 05:44:47 am
content: This 732A has been cold and unused for quite a while, at least 6 months under my bench and who knows how long before that. Its owner has reworked the power supplies and replaced the batteries. We've also made a "forbidden" repair to the ovenized reference circuit, replacing R25 with a 10k resistor to add compensation to U1 and stop it from oscillating. Later, what is hopefully the final repair for now, I fixed an issue with the front panel output board by installing bodge wires bypassing the mechanical connection between the terminal and the rings on the PCB board.https://www.eevblog.com/forum/repair/fluke-732a-ripple-on-output/Since we do not have any calibration or other history on this unit and will have to characterize it from scratch anyway, we decided to adjust it to a nominal 10V and then observe it for some time. There have been other 732A threads, but I want to do one with the intention of following up over time. I'm also looking for some ideas on how to best analyze and characterize its performance with limited equipment--no 3458A or bank of other 732A references here.One of the first things I was curious about was the internal tempco, the amount of tempco that the reference circuit has to actual temperature changes. Obviously the strategy of the 732A is to get that tempco reasonably low and then closely control the temperature, but what are the numbers?I let the unit cool off overnight and then logged its output for a few hours on startup. The unit started off high, went low and then climbed a fairly straight ramp up to the operating point.If you look at the slopes from the point where the thermistor is 7000R to where it reaches its final value at about 4500R, there is about a 1.6ppm rise in voltage. Looking at a generic 10K2 thermistor table, this represents a change from about 33C to 45C, or 12C change. So 1.6/12= 0.133ppm/C. Not bad but not unbelievably fantastic or anything like that. The specified maximum tempco of the unit is +/- 0.05ppm/C so the oven is obviously key. How good is it?The next step was to look at the thermistor. At operating temperature, the thermistor slope is about 100R/C if it is similar to other 10K thermistors--which it appears to be. The manual states that the measured thermistor resistance should be within 1 ohm of whatever is marked on the front during initial calibration. No such marking, but I quickly found out that there were several ohm discrepancies in my readings due to crappy leads not making good contact with the posts. I rechecked and used a 4W measurement and got 4.5169k +/- 0.3R very reliable. Using a maximal error of 1 ohm, meaning ~0.01C variation which means the total variation due to temperature of the ovenized reference under these conditions should be 0.00133ppm/C. These conditions are my bench which is pretty stable, certainly within 23C +/-5C. I think there was 4-5C max variation in ambient temperature during my tests.So, the conclusion is that they've used good but not extraordinary design tempco control with a fairly tight oven control to get what should be nearly nil overall tempco. Next up will be adjustment to nominal which will be done with a Keithley 155 null meter and my VREF-10 from voltagestandard.com.

author: bdunham7
author_rank: Super Contributor
timestamp: February 08, 2025, 06:44:32 am
content: After several days of running, the 732A was measuring about 10.00004V using either my Fluke 8846A and it's known -2.5ppm or so deviation at 10V or by using the null meter and the VREF-10. That may sound scary as neither of those instruments is specified nearly well enough to be doing this, but I'll just call them "characterized" and boldly move ahead. I wanted to know where the 10V adjustment, R59, was in its range. It is a 22-turn potentiomer, a Bourns 3059Y-1-101. It turned 12 turns counterclockwise before stopping and this brought it below 10V. 3 turns to the right and my null meter was fluttering around the zero point. There's about 1-2µV p-p noise visible on the meter, probably the output of the VREF-10 as it is just a P-DIP in a plastic box. The noise didnt' impede me from being pretty satisfied with the result. An overnight run at a reaonably constant temp resulted in no discernable change.I received the USA Cal Club FX reference and my plan was to use my HP 34401A in ratio mode to measure the FX using the 732A as the reference voltage. Keeping the ratio just below 1 gives me 7 digits on the 34401A, on the display and over the serial port. This worked with a result so far of about 0.9999961 after a few hours of warmup for the FX. I've never been able to spot the tempco of the FX before and now I can. Since the FX is supposed to be 9.9999642V +/- any drift from the last calibration, that means that my nulling adjustment has put the 732A about 0.3ppm or 3µV higher than 10V.So far so good, I haven't seen any popcorn noise and the first 36 hours after adjustment still look good. The noise from the HP34401A drowns out any subtle changes but even with that everything is within 3µV of the 0.9999961 average. All there is to do now is to just watch it. IDK how long I should wait before thinking it has been put right. A couple of years?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 08, 2025, 08:37:08 am
content: The ratio mode gives quite some noise. The 34401 and many other modern DMMs do the ratio by alternating conversions of the 2 input signals. In addition the 34401 is quite noisy to start with.One would get a better resolution and less noise by measuring the difference of the 2 external references and use the 100 mV range or maybe the Nullmeter.With the 732 ref. no running for a long time it may take some time to get a stable reading again. It may take a week of so to get somewhat stable humidity levels for the higher operating temperature. I still don't know how much effect the humidity has on the 732. It could be a rather small effect as the critical parts are through hole and largely hermetic.The VREF10 would have noise and also humidity effects as slow drift in a DIP case.

author: iMo
author_rank: Super Contributor
timestamp: February 08, 2025, 11:09:38 am
content: The 34401A does the "ratio" as follows: it is measuring V1 against its internal LM399, and then V2 against its internal LM399, and then it divides the numbers such you get the ratio..So it does not use the V1 (max 12V) as the "reference", a pity..

author: bdunham7
author_rank: Super Contributor
timestamp: February 08, 2025, 03:55:27 pm
content: Quote from: Kleinstein on February 08, 2025, 08:37:08 amActually I don't know of any DMMs that don't do the ratio this way, certainly none that I have. The 34401A is noisy but I used it because it gives me that 7th digit and I can filter the noise out later mathematically. I'll try some of the other methods later but right now I think I've dialed it in as close as I can possibly expect given the references I have. Even the FX is not really certain to 0.1ppm at this point.QuoteThat's what I hope to show in this thread. We've started the 732A up before and let it run hoping its noise (oscillation) and instability (front panel connection) would go away as it baked in, but as shown in the repair thread the problems didn't go away. OTOH, once repaired properly--which I hope it is now--the output pretty much went directly to its stable point in about 90 minutes and has stayed there for a week. I did see what could have been some 0.3ppm popcorn noise early on but nothing since. That also very well could have been a test lead issue because I wasn't taking great care at that point. Also, after adjustment the 732A stabilized in a few seconds although I still gave it 10 minutes to stabilize after every turn of R59.QuoteThe VREF-10 is 16 months old and is obviously spot-on here, there's less than 0.3ppm discrepancy between the 732A nulled by it and the FX. As mentioned, it does have noise that is visible on the null meter and that is probably a limiting factor on using it this way. Still an impressive showing, IMO.

author: guenthert
author_rank: Frequent Contributor
timestamp: February 09, 2025, 11:01:52 am
content: Quote from: bdunham7 on February 08, 2025, 03:55:27 pmI think @MickleT built one himself (a DMM which can optionally feed the ADC from an external reference instead of an internal one -- I believe some Fluke DMMs offer the same). Then there is the (now rare) -hp- 3420B Differential Voltmeter/Ratiometer.

author: jfphp
author_rank: Regular Contributor
timestamp: February 09, 2025, 01:09:32 pm
content: 1. Change all the capacitors in the power supply.2. In all of the cal labs I have seen in France the 732 are sent each year to the french national reference lab and there is NO adjustement : only the difference to the national standard is recorded.By personal experience, playing with the pots induce an instability and you have weeks or months to wait before getting again the "normal" drift, therefore he procedure used.

author: Rax
author_rank: Super Contributor
timestamp: February 13, 2025, 03:01:37 am
content: Quote from: jfphp on February 09, 2025, 01:09:32 pmI think this is the general wisdom with the adjustment of the unit, but as we don't seem to be seeing this here (at least, not yet - maybe there's need for more observation to notice a trend) maybe it doesn't apply. I believe bdunham7 applied a methodology where looking for an adjustment position was by working initially large, then smaller loops (back and forth) closer to the point sought, to stabilize at the adjustment point. Mechanically, this seems a good approach to clean the contact (wiper and resistive layer) such that there would be less instability over time.__________________________________________________________________________________________________________________________________Bill158 just reminded me offline that (one of) his 732As had a similar issue with the front panel. We should probably cross check our serial numbers to see if this is a consistent issue with a batch of units. I remain of the mind that a certain "crop" of these saw a half-thought/half-baked transition design of those front connectors, as described here and in the old thread. Which didn't age well.

author: bdunham7
author_rank: Super Contributor
timestamp: February 16, 2025, 12:06:21 am
content: OK, so I've improved my test lead situation to be more appropriate for what I'm doing. I tried some custom AB precision cables that are very nice and good for single connections, but they didn't work as well for the types of 3-way connections I'm trying to do. So, I made some Y-cables that use 28AWG twisted pairs. One setup was for ratio measurements where my real issues were the connections to the DMM. The issue is that the low sides of the ratio measurement need to be shorted at the meter but my nickel-plated shorting plug was far from ideal. I used my low-EMF silver connectors and just crossed over the two thin wires near the negative connectors. A solder-blob tetrajunction (SBTJ) if you will. The other, for null measurements has two interwoven pairs so that there is a direct connection from the negative sides of each reference and the two positive connections go the the meter plugs, but without any free single wires--they're all twisted in. All of the reference connectors are bare copper.My procedure here is to compare the FX and the 732A by multiple methods using multiple instruments and then see how consistent--or not--they are. For the ratio measurements I've started with the HP 34401A and for the null I'll use a Keithley 155, a Fluke 8842A and then an 8846A. For the digital meters a major issue is getting stable zero point. The best one of the lot--despite being only 5.5-digit--is actually the 8842A, as it has an extra-low 20mV range and reaches a stable and consistent +/-3 count (300nV) zero after about a 90 minute warmup. However, can't log with it so I'll make do with the 8846A.It's been about week since I did the final adjustments on the 732A. The initial results look very good and I think I'm at the point where the uncertaintly of the FX, as small as it is, dominates the other factors. 24 hours of comparing the two and averaging all the data points gave me a ratio of 0.999996359 and a std dev of .000000235. Assuming the FX is exactly 9.9999642V, that is a result of 10.00000061V +/- 235nV.0.6ppm0.06ppm high. Of course that's reading the ninth digit of a six-digit DMM so some rounding is in order, but hopefully it is still precise enough that I'll be able to spot any drift in the 732A sooner rather than later.As for other methods, using the null method with the Keithley 155 gave what appeared to be a reading of about 37µV, but testing it with a precision source showed that exact same needle position to correspond to more like 35µV and the back output was 0.354V, also indicating less than the needle. The Fluke 8842A gave me a fairly reliable 34.8µV +/- 5 counts. I tried both meters in both polarities by reversing the plugs at the meter. Here the value of the FX, 9.999964 plus 35µV give us 9.999999, or 0.1ppm low. Again, that's the 7th digit of a 5.5-digit meter but it is repeatable and stable so if there is an error IDK where it is.I'm running the last test now, which will be the 8846A measuring the difference between the two references. While the other tests were running I did a 24-hour null test with a 4W short installed, the results are shown here. The average was -179nV +/- 99nV. Any suggestions on how to improve on this with the equipment I currently have would be welcome, although I'm not unhappy with what I see at the moment.

author: bdunham7
author_rank: Super Contributor
timestamp: February 16, 2025, 03:38:52 am
content: One additional test I forgot to mention--after the 24 hour run with the 34401A ratio measurement, I ran it again with the references reversed. The data here gave me a ratio of 1.0000035197 +/- .000000229, that indicates a calculated 732A voltage of 1.0000035197 x 9.9999642 = 9.9999994 or0.6ppm0.06ppm low. So there's a little offset or gain error between the two inputs for the ratio mode. IDK why, but there is is. Graph of test:

author: Kleinstein
author_rank: Super Contributor
timestamp: February 16, 2025, 09:54:41 am
content: The 34401 like man modern DMMs does ratio measurements by measuring both voltages relative to it's internal reference. The ratio is than calculated.The ADC in the 34401 is just not able to use a different reference voltage.Switching the inputs does just swap the inout paths, but no real change in how the ratio is calculated. So no surprise to get essentially the same result. The only small difference would be from the usually small offsets per input (e.g. thermal EMF at the therminals)

author: bdunham7
author_rank: Super Contributor
timestamp: February 16, 2025, 02:42:30 pm
content: Quote from: Kleinstein on February 16, 2025, 09:54:41 amIs this dual measurement comparison method somehow worse than the 'true' method? I would expect reversing the inputs to reveal any error in measuring ratio (if there were any) as well as any offsets or thermal issues. Is there any reason to suspect that these results aren't accurate?I've corrected the error calculation, the two results are +/- ~600nV which is 0.06ppm from nominal, not 0.6ppm.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 16, 2025, 06:11:47 pm
content: Getting the ratio from 2 measurements and math makes it more noisy. It also takes twice or at least 1.5 times (if the same zero can be used) the time than using the external reference to the ADC. However some of the modern ADCs just don't work with a different reference voltage.On the up side the 2 measurement mode is more flexible and allows a larger range. It still suppresses much of the reference noise, though not all. The popcorn noise jumps in the reference would translate to isolated points that are quite a bit off (like up to 0.6 ppm).The accuracy should be good - obviously limited by the linearity of the meter. For the manufacturers it is also a cheap way to implement - hardly any extra hardware needed, just the inputs for 4 wire ohms.The smaller difference makes more sense, as the error should be mainly from relatively small offsets.

author: bdunham7
author_rank: Super Contributor
timestamp: February 16, 2025, 11:58:46 pm
content: Quote from: Kleinstein on February 16, 2025, 06:11:47 pmUsing ratio also suppresses zero/null drift in the input circuits. The short term noise is easy to deal with, random drifting over a few hours is much worse. You can see the invariance of the 34401A ratio log with time and temperature, the null of the 8846A is not as even and shows clear temperature or gradient dependence. The tests I'm running now, which I'll post tonight or tomorrow morning, are not as good and I'm still working through some issues with test lead reversal. I may have to do a reversed-at-reference run using the 34401A ratio method just to reassure myself that I'm on solid ground here.

author: guenthert
author_rank: Frequent Contributor
timestamp: February 17, 2025, 10:41:41 am
content: By all means don't let me stop you, but I'm a wee bit puzzled why you would use ratio measurements for two so similar voltages.When the difference is so small that a lower range can be used then the difference measurements yields a lower error, doesn't it?EDIT: Well, I suppose the short term error in the ratio measurement can be smaller (and be averaged out) then the total of short and long term error of even the lowest range. Don't mind me then.

author: bdunham7
author_rank: Super Contributor
timestamp: February 17, 2025, 09:31:18 pm
content: Quote from: guenthert on February 17, 2025, 10:41:41 amIt's a completely reasonable question and one that I'm trying to answer by doing it multiple ways. The whole subject is probably worthy of another thread and it has likely already been discussed many times, but I beyond the usual advice on how to minimize thermal voltages, avoid AC interference and so forth, I don't have anything definitive that I can use to decide which methods are most effective for what I'm trying to do. My main goal is to have a measurment that is repeatable to 0.1ppm if possible so as to monitor the drift of this newly awoken 732A without having to wait several years. Having that measurement be accurate would be a bonus.The two issues I ran into right away trying to do this by measuring the difference directly are a lack of precision with the K155 at non-zero measurements and a lack of stability in the zero point with multimeters. This is my first time having two references to compare that are stable enough to actually make these relatively tiny errors problematic. The K155 might have an error of 3-4µV on the 100µV scale (I need to look into tweaking it a bit) but that's 10% of that difference or 0.3-0.4ppm of the 10V. The zero/null point of the 8846A is shown a few post back, it is really not bad but clearly a bit tempco dependent. Unfortunately, when I took the extra step of reversing leads in 4 different configurations, I got more variance that expected. I'm not sure why and I'm not sure I can put the effort into tracking it down, but as you'll see in the next post or so the ratio measurement seems to have done the best job.If you look at the noise in the ratio tests, it looks like a flat line with random noise. That can be averaged out and I probably can use much shorter captures and still get good results. That 24-hour plot using the null/difference method seems more difficult to deal with.

author: bdunham7
author_rank: Super Contributor
timestamp: February 17, 2025, 09:46:02 pm
content: Here are some results using a Fluke 8846A to measure the difference between the FX and the 732A. This was done using twisted wire pairs in a Y-configuration (no loose single wires) , connecting the negative posts of the references together and then connecting both positive terminals to the meter. Bare wires at the references, soldered-on silver Z-plugs at the meter. As you can see, there's 2-3µV of noise in there that isn't random or 0.1-10Hz, it's short term drift and tempco, maybe other stuff. The numbers work out to an average of 34.17µV with a std dev of 0.51µV. 9.9999642V + 34.17µV = 9.9999984Vm or 0.16ppm below 10V with a std dev of ~0.05ppm.Now if I add back in the -179nV that I got with the 24-hour zero/null test I could convince myself that this result was absolutely perfect down to a few tens of parts per billion.So that's nice, but is there a systemic error and what will my repeatibility be like if I want to do this in a month? With a measurement like this I think one major worry is that some sort of bias will creep in and shift the results one way or another. FOr example, AC interference can be rectified by the non-linear impedance of the references. There are probably many places a few nanovolts can creep into the equation. I decided to run the test again with the wires all reversed. This means that the two positive terminals of the references are connected and the negative ones go to the meter. The two wires to the meter are also reversed so that the resulting reading is still positive. I just ran this for 3 hours, 24 hour runs are just too long and I see no evidence so far that they are helpful. The results were 36.30µV +/- 0.37µV. 9.9999642V + 36.30µV = 10.0000005V or .05ppm over 10V.Adding that -0.18µV null offset gives us 10.0000023V, or 0.23ppm over 10V.Scratch this, I'm off by x10.OK, so the null on the 8846A isn't so bad. But the result changes noticeably when I reverse the leads.Finally, I did two additional runs with the references connected at the negatives and measuring the difference between the positives with the meter leads reversed (measuring a negative voltage) and then the other way, positives connected and measuring a negative voltage at the meter. I won't post the graphs, but the results were:References to + and +, difference between - and -: -36.32µV +/- 0.39µVReferences to - and -, difference between + and +: -34.17µV +/- 0.35µV.So it is pretty clear that there is an small but observable and repeatable offset based on which way the references are connected. Next up I could try retesting using the ratio method with the references reversed to see if the same effect is there, or I could run null/difference tests with the 34401A or I could do the same with the K155 (after recalibration) using the recorder output on the back. Suggestions welcome.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 17, 2025, 11:14:01 pm
content: There is a possible systematic error from meter ofset and rectified EMI. This applies to the difference measurement, but also to any other measurement. The sensitivity to some AC background is a limitation of the referenes and this would also apply to the ratio measurement. E.g. the input current spikes can be different for the inputs at the 34401 or a 3458 when used for a ratio measurement.The different values one gets for the different polarities / configurations give an idea on the order of magnitude for the extra error. This may may or may not be perfectly reproducible, e.g. from different temperatures and cable positions or external EMI.It could make sense to also try the K155 or 34401 for the difference tests to have more points. The difference is so small that the calibration of the K155 is not that critical and one may get a way with just reading by eye a few values.

author: bdunham7
author_rank: Super Contributor
timestamp: February 18, 2025, 01:52:16 am
content: To plow ahead, I decided to go back and do the 34401A ratio measurements with the references reversed (positives connected). Since the ratio measurement requires the two negative input terminals to be at the same potential, this means that the ratio will be calculated from two negative readings. I expect this to make no difference in the result. Unfortunately I accidentally saved one of the graphs in the smallest format, but it doesn't really matter here. The first one, with the 732A as the reference, gives me a result of 0.9999965677 +/- .000000229, that translates to a result of 9.9999985V +/- 0.23µV. That's 0.15ppm below 10V.Doing the same thing with the ratio/reference connections swapped, still + to + on the references and reading a negative voltage, I get 1.0000032285 +/- .000000241, which is 9.9999965V +/- 0.24µV/ which is a whopping 0.35ppm below 10V. Since I previously observed a 0.12ppm or so difference between the two ratio measurements, that sort of adds up. The first set of results looked better, but that's the sort of irrational exuberance that gets one in trouble.If I have time, I might run all 4 ratio measurements again tomorrow just to see how repeatable the results are. So far all I can say is that I appear to be solidly in the sub-ppm domain. I might be able to have some idea of the initial drift of this unit in just a few shortyearsmonths.

author: guenthert
author_rank: Frequent Contributor
timestamp: February 18, 2025, 08:46:29 am
content: I'm not quite sure about the rectification of AC at the reference. There's a lot of circuity between the Zener and the reference's output. I've seen this mentioned many times, including some publication from NIST in the eighties, but I always wondered whether that's not a mis-attribution of the error caused by the pump-out current of the DMM. Some, including some national labs, bridge the input of the DMM in such measurements with a small capacitance (which should help with either).Other than thermal EMF, chemical effects on not-perfectly-clean connectors could cause small errors which show up when reversing leads.Going for sub-ppm precision in a home lab is no small feat.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2025, 09:26:33 am
content: The pump out current from Az switching in many DMMs are a special form of AC current. How bad they are depends on the DMM. Testing of the pump out current is sometimes done at zero voltage, but the current pulses tend to get worse with non zero voltage.The interference should not reach the zener reference, but could still effect the amplifier.Chances are both the 34401 and Fluke 8846 are not that bad with the charge pulse as they have quite some resistance in the protection that acts as filter.Adding a capacitor to the input can help, but not all ref. sources like capacitive load.Another part of AC current is common mode current from the DMM power supply that would normally flow mainly though the common/low side.A good reference should handle to mains frequency part OK. AFAIK the fluke 8846 still uses an old style transformer.

author: bdunham7
author_rank: Super Contributor
timestamp: February 18, 2025, 09:02:21 pm
content: Quote from: guenthert on February 18, 2025, 08:46:29 amI've actually seen this previously with a 731B and a very large line-conducted EMI. IDK, but I'm imagining there could be some similar but much smaller effect here as I do have two SMPS units running on the bench--the computer and a UPS. Perhaps I'll get the K155 tweaked and try an all-battery all-isolated test.Yes, sub-ppm is hit and miss and now that I'm at previously unexplored levels of precision here, I'm finding out what works and what doesn't. The otherwise excellent Fluke 8846A seems to have reached its limits here as the ratio tests aren't as good. It only gives me an extra digit if the ratio is just below 1, above 1 it reverts to 1ppm resolution. Averaging can only reaslistically add one digit unless you use some precision triangle dithering of some sort. There's also a signficant difference between the ratio and reference measuring inputs as I get more than a 1ppm delta when I switch them.Fluke 8846A ratios measuring 732A vs FX with negatives connected, then positives:-/- 732 as ref: 0.99999591 => 10.0000051V-/- FX as ref: 1.00000279 => 9.9999921V (▲1.3ppm)+/+ 732 as ref: 0.99999750 => 9.9999892V+/+ FX as ref: 1.00000423 => 10.0000065V (▲1.7ppm)Averaging out all 4 gives 9.9999982V or -0.18ppm, but that seems a nice but questionalbe result. There are clearly two different sources of offset here. Swapping the inputs does not cause any significant disturbance of the layout of the wires or anything else, so that would be an imbalance between the two inputs of the meter. Reversing the reference connections seems to always shift the measured voltage down about 15µV if you take polarity into account--when doing the +/+ ratio the meter is measuring negative voltage.

author: bdunham7
author_rank: Super Contributor
timestamp: February 18, 2025, 09:07:01 pm
content: Quote from: Kleinstein on February 18, 2025, 09:26:33 amThe 8846A has a small periodic current surge that is observable on the 100pA range of my picoammeter, so in the tens of pA in addition to the bias current. However, when the analog filter is engaged this gets smoothed out and I just get about 22pA of constant bias. But now that I think of it, I believe that enaging the ratio measurement disables the analog filter and sets the aperture to NPLC10.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 19, 2025, 10:04:10 am
content: The input current spikes of meters like the Fluke8846 or HP34401 are start as short (some 1 µs range ?) pulses from AZ switching (e.g. 3 Hz repeat for 10 PLC). There is than some internal filtering from the protection: some 700 pF or 220 pF at the 34401. The 2 inputs in ratio mode are slightly different (less protection for ohms sense). I don't know the details of the F8846, but there should be similar capacitance and resistance.The pA meter will likely not resolve that on the time scale an apply extra filtering.It makes sense to turn off the filter for the ratio mode. As the meter may have the switch between the inputs. At least the ref input (usually ohm sense H) would not see the filter.

author: CalibrationGuy
author_rank: Unknown
timestamp: February 24, 2025, 02:53:01 am
content: When I saw this thread, I looked at our vref bank data from 6 months ago and from yesterday. Drift measured was under 1.4uV, in 6 MONTHS! So, I just want to underscore that even with multiple 3458a, 34420a, and 732 units, it's extremely difficult to accurately characterize absolute voltage and drift of an aged voltage reference unit like the 732a/b/c units. As long as you control temperature, humidity, and emi/rf influences, and follow a set measurement procedure, you should be able to get useful data over time. If you need help, let me know. Good luck.TomG.

author: bdunham7
author_rank: Super Contributor
timestamp: February 24, 2025, 06:36:25 pm
content: Quote from: CalibrationGuy on February 24, 2025, 02:53:01 amThanks! Yes--it may be difficult to finally characterize the unit once it is working properly and has re-aged for a while, but the actual goal is for that to become difficult because the 732A should eventually be more stable and less noisy than anything else I can get my hands on. So for now I'm looking at the basic specs of 0.5ppm/month or 6.0ppm/year and making sure it is inside those, then as time goes on I hope to be basically measuring the drift and noise of everything else while the errors of the 732A become invisible to me--and then it goes back to its owner. So far it has been 3 weeks and I haven't seen any movement, so the only question is how little I'd be able to reliably detect. Probably not 1.4µV!

author: 1audio
author_rank: Frequent Contributor
timestamp: March 01, 2025, 12:46:10 am
content: My experiece comparing the FX to my 732A was that there was no identifiable drift between them over 3 weeks of measurements. The noise is dominate at the 1 uV level. With some software processing it may be possible to improve the resolution. I checked using the QA350 (which is well suited to this) and 3 different Fluke 8506A's and while the absolute voltages are different they tracked really closely even over significant temperature swings. This was all differential measurement.Here is a plot of the difference over time and the ambient temp (Temp is green and voltage is orange in uV)(I have not plotted in Excel in a long time and it seems they just made the process harder. . .)

author: calenguk
author_rank: Newbie
timestamp: March 03, 2025, 09:44:13 pm
content: Your test leads are not good enough. You need good quality test leads so invest in some. Your connections are giving you errors.

author: bdunham7
author_rank: Super Contributor
timestamp: March 09, 2025, 01:17:45 am
content: Quote from: calenguk on March 03, 2025, 09:44:13 pmI've tried a set of very good test leads that weren't specifically designed for null tests and I didn't get an improvement at all, quite the opposite. Can you tell me where and what type of errors you see in my data that you think are related to test leads? Thermal voltages/noise/?I've had some suggestions on how to make shielded leads, are you suggesting I can buy them? Which ones?

author: bdunham7
author_rank: Super Contributor
timestamp: March 09, 2025, 01:25:22 am
content: Has anyone logged the thermistor values on their 732A? I set up a 4W measurement and I'm pretty sure I'm seeing either a cyclical or random variation of a bit more than 1 ohm (p-p), not quite 2. This happens over a daily period, so quite slow. Ambient temperature changes 3-4C in that time.

author: CalibrationGuy
author_rank: Unknown
timestamp: March 10, 2025, 06:22:05 am
content: Quote from: bdunham7 on March 09, 2025, 01:25:22 amWe monitor the voltage of all the 732 standards in the array. The only time I would look at the thermistor, is if there is some kind of voltage variation that is out of the norm for that particular standard.TomG.