Influence of switch resistance in Hamon Dividers - Page 1
Forum section: electronics / general

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 08, 2020, 03:26:28 pm
content: Hi,I tried to investigate the importance of the switch resistance for the parallel to series switch (calibration to divide) in a 1:10 Hamon divider.For a first investigation I assumed everything else than the switch is perfect. In a perfect trim (0V bridge voltage) the resistors of the upper and lower leg are exactly the same.I started calculating the resistance of the upper arm in calibration mode (parallel) (see net1.png, net2.png btw. is it possible to embedd pictures without an external source?).I wrote a small function to calculate the resistance as a function of R and the switch resistance Rs. I kept all resistors to have room for playing in the end. (rleg_function.png)The result with 40k output resistance and 10mR switch resistance (as described in the Fluke 752A manual) leads to 40.000009k (rleg_calc.png). That is already a deviation of 0.2ppm and not only 0.025ppm as described in the Fluke manual.To verify my calculation I tried the same circuit in LTSpice (spice1.png and spice2.png). That gave the same results.Therefore, I have to ask: Where is my fallacy?Best regardsPhilipp

author: Gyro
author_rank: Super Contributor
timestamp: February 08, 2020, 07:37:56 pm
content: I did the same (well, back of an envelope) calculation and ended up using solid screw terminal barrier strip for my '2:1 switch' on my primitive 10k divider. I've no idea how Fluke manage it [EDIT: for their stated spec].https://www.eevblog.com/forum/metrology/anyone-else-built-a-hamon-divider/P.S. It's not just the (variable) switch contact resistance, it's the interconnect wiring too.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 08, 2020, 08:18:01 pm
content: I go with the shorting bar. Remember that you can parallel wafer switch wafers, so if you start with a good switch you can probably do better than you'd think.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 08, 2020, 08:39:33 pm
content: Thanks for your feedback!My biggest concern is, that I got 0.2ppm error and not 0.025ppm as described by Fluke. The Fluke 752A is very old and therefore, I think it is verified over many years and my calculation must be wrong anywhere.If the switch resistance is really that critical, I wonder why it isn't verified periodically. Fluke wrote the Fluke 752A is a ratio standard and don't need any traceable calibration. It seems it is possible to Null the 752A and it might be off anyway, if the contact resistance has increased.

author: Vgkid
author_rank: Super Contributor
timestamp: February 08, 2020, 09:26:16 pm
content: A really good quality rotary switch, especially one with paralleled connections will be very repeatable.Some test here:https://www.eevblog.com/forum/metrology/4-wire-switch-selectable-resistance-standard/msg1192644/#msg1192644

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 08, 2020, 09:29:53 pm
content: Quote from: Vgkid on February 08, 2020, 09:26:16 pmThanks, I had my various Pt100 simulators in mind. All of them need a restoration of the switches to get back into specification.

author: Vgkid
author_rank: Super Contributor
timestamp: February 09, 2020, 04:54:54 am
content: Quote from: e61_phil on February 08, 2020, 09:29:53 pmThe good thing is that for a rtd simulator, a super low switch contact resistance is not needed.

author: beanflying
author_rank: Super Contributor
timestamp: February 09, 2020, 07:12:26 am
content: About to get around to making one with a long awaited collection of goodies from Edwin. Given the cost of quality rotary switches I was going to use plated shorting bars and good terminals instead for mine.Much as it will slow down usage I am only building it for my own use so speed isn't really an issue in my case at least.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 09, 2020, 09:31:44 am
content: The difference between the Fluke manual and the simple calculation could be the base value for the PPMs. An error of 0.2 ppm for the 40 K resistors would result in an error of about 0.02 ppm if the input voltage.Companies tend to provide the better looking numbers if they can.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 09, 2020, 04:39:57 pm
content: Quote from: Kleinstein on February 09, 2020, 09:31:44 amThe idea is good, but they wrote 1:10 0.2ppm of ratio. And they also wrote 0.2ppm of output.My concern is still, why the switch alone already causes 0.2ppm. The calculation is no shortcut nor something difficult. I can not imagine that the Fluke 752A is worse than specified and it wasn't noticed until today.In the manual, they have the following formular to calculate the influence of the switches:Vo/Vi = 1 / ( N + (dR/R) )where:N = ratio -> 10 for 10:1R = output resistance (40k)dR = switch contact resistance (10mR)That gives -0.025ppm as described in the manual. But where does the formula come from?

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 09, 2020, 07:20:45 pm
content: You need to calculate the ratio, that isRatio = R/(10R + Rswitch),with R =40k, and Rswitchis the failure from the 1:1 balancing of the Hammon bridge arrangement.The error of the ratio is then(Ratio - 0.1)/0.1 with 0.1 being the default ratio.This error then calculates into-Rswitch/(10R+Rswitch) which is about Rswitch/10R, which gives 0.025ppm with Rswitch=10mOhm.The only thing I could not find out, how the switch resistance only counts once, not 2 or more times.It could be important to adjust one of the outer chains instead of the middle one.Frank

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 09, 2020, 07:31:30 pm
content: Quote from: Dr. Frank on February 09, 2020, 07:20:45 pmI calculated the ratio and that gives also 0.2ppm with 10mR switches.They wrote one have to add the 10mR in series to the resistance. That is roughly correct (8.8mR is more precise for the two switches). But you have to add that to 40k and not to 10x 40k.They idea of the formula might be for the 1:10gain = Rout / (Rout + 9R + Rswitch)And as long Rout = R it is the same asgain = R / (10R + Rswitch)but that seems to be incorrect.Can anyone show that the formula given by Fluke is correct?PS: I tried the "nulling" with every of the four resistors. All experiments results in the same 0.2ppm

author: Kleinstein
author_rank: Super Contributor
timestamp: February 09, 2020, 08:52:05 pm
content: The Fluke formula (3-1) in the manual seems to apply to a different switch somehow in series with the divider , not the series / parallel switches.In this case the fluke formula makes sense - though I don't see the switch involved.Under 3-30 they handle the series / parallel switches (RS1, RS2) separately - though without many details.So the formula in question is for the wrong problem.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 09, 2020, 10:06:31 pm
content: Quote from: Kleinstein on February 09, 2020, 08:52:05 pmTHANK YOU!!That explains everythingIt seems there is some black magic involved in the 752A to achieve the impressive specifications. They wrote "The design of the instrument is such that most of the effects of this errors are reduced by adjusting the interconnections resistances." (F752a.png)I already saw during playing around that some detunning of the Rout (intentionally setting Rout higher or lower than the parallel resistance of R1, R2 and R3) can compensate for the switch resistance (see dirty.png). Maybe that is what they are doing. The curve will change with the switch resistance of course.PS: The graph is achieved by setting Rout to Rout + detunning and than nulling the bridge by variation of R2.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 09, 2020, 10:40:40 pm
content: A more coloful graphIf the switches really have 10mR for the parallel to series switch and they do it by detunning (or different selection) of Rout, the switch should still stay within ~2mR to keep the 0.042ppm.It would be also possible to increase the switch resistance intentionally and compensate later for that. That might give more stable behaviour over time. It would also be interesting how guildline solved this issue in their automatic divider. With relay contacts the problem might be even bigger.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 10, 2020, 10:24:50 am
content: One could compensate for the known fixed part of the switch resistance (e.g. the wires), but there is still the possibly variable part, e.g. due to oxide and the exact switch position than is not easy to compensate. So it needs a construction the is very repeatable.Before critical measurements checking the switches (for low resistance) is definitely a good idea.A way for compensating for some fixed switch resistance would be some intentional asymmetry in the 120 K resistors. This would lower the resistance of the parallel combination a little.

author: Lesolee
author_rank: Regular Contributor
timestamp: February 10, 2020, 05:21:15 pm
content: Quote from: e61_phil on February 08, 2020, 03:26:28 pmI would say that at least part of the problem is that you are showing a 2-wire connection, when a Hamon build-up resistor requires a 4-wire connection to function correctly.This image is from Hamon's 1954 paper.The idea ofaddingresistors into the potential leads dates back to 1912 (according to Hamon's references).

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 10, 2020, 05:25:44 pm
content: Quote from: Lesolee on February 10, 2020, 05:21:15 pmYou're talking about about a complete different thing.The fallacy was already revealed by Kleinstein.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 10, 2020, 07:47:13 pm
content: The circuit from the old Hamon article could still be used to a certain degree to compensate some of the switch resistance. I don't know if this works all the way, but it could compensate at least some and would be active during the 10:1 divider adjustment step.The main idea is using 4 way junctions and separate switches for voltage and current. The voltage sensing switches would only carry a small current cause by errors in the approximate current sources through extra resistors. It needs about 2-3 times the number of switches and extra, less critical resistors, and could compensate something like 99% (depends on the accuracy of the extra resistors and adjustment range) of the switch resistance.I don't know if the 752 uses this technique, but it is plausible.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 10, 2020, 08:07:22 pm
content: Quote from: Kleinstein on February 10, 2020, 07:47:13 pmHow do you bring the current and voltage path together to form the bridge point?

author: Lesolee
author_rank: Regular Contributor
timestamp: February 10, 2020, 09:27:25 pm
content: Quote from: e61_phil on February 10, 2020, 05:25:44 pmI agree. I downloaded the 752A manual and the simplified circuit is exactly as you have it.I have not seen “Hamon” mentioned in the manual at all, and the circuit seems entirely unrelated to a Hamon transfer standard like the Guildline 9350 for example.Frustratingly the last few pages showing the actual schematic diagrams are missing from the manual (downloaded from the Fluke site).I am suspicious of their simplified circuit, since it seems likely that you would want to run each equal resistance at the same current in order to minimise self-heating and voltage coefficient errors.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 10, 2020, 10:06:44 pm
content: Bringing the voltage and current paths together can work a little similar to the kelvin bridge. It is a little tricky and limited in how much of the resistance can actually be compensated. At the bridge side the current sharing resistors have to be quite accurate and small (e.g. 1 Ohms range).When used as the 10:1 divider some of the current sharing resistors would be shorted out by an additional switch. This could explain why there is a switch in series with the full bridge at all.attached is a LTspice picture and sim file, that used JFETs for the switches. Due to the high resistance the sharing resistors are larger.

author: Lesolee
author_rank: Regular Contributor
timestamp: February 11, 2020, 09:21:55 am
content: I have written it out fully, just for completeness.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 11, 2020, 10:14:43 am
content: Quote from: Lesolee on February 11, 2020, 09:21:55 amThanks!Just for the clarification: You're describing the series resistance from the switch to the input of the divider.I thought they are describing the switch resistance for the series- to parallel switch. But for this switch (and the simple hamon circuit) my calculations from the first posts are correct. Therefore, if you build a hamon divider with the textbook design and you use 40k for Rout and 10mR switches, you will end up with 0.2ppm (not 0.025ppm) uncertainty just from the switches alone (everything else will add up).The Fluke 752A has a 1:10 uncertainty of 0.2ppm including all effects. Therefore, they must use some black magic like Kleinstein describes (or simillar).

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 11, 2020, 12:24:41 pm
content: Philipp,no, in Lesolees calculation (it's exactly the same I have done) that dR is NOT describing the series switch resistance.This dR comes from the Hamon adjustment .. the series/parallel switch resistance is compensated by the adjusted leg of the three 120kOhm resistor chains.That means, this leg is mathematically reduced by the switch resistances.Either you simply assume that all three 120k (plus 2 * 10mOhm) are in parallel, or you elaboratley calculate the star/ triangle transformation, that might give 3 *10mOhm, maybe, but the Hamon adjustment condition says, that this paralleled circuit has to match exactly the 40k of the lower leg.This Hamon adjustment condition you always miss in your calculations!Therefore there is no black magic behind, but simple math.Btw.: there is also no difference between the described switches in the assembly, because all switches / wipers are sitting on the same switch and are made from the same material.The schematics are also available.Frank

author: Lesolee
author_rank: Regular Contributor
timestamp: February 11, 2020, 12:31:16 pm
content: Quote from: e61_phil on February 11, 2020, 10:14:43 amYes, just as Kleinstein noted earlier. They didn't show the switch(es) in their simplified circuit.

author: Lesolee
author_rank: Regular Contributor
timestamp: February 11, 2020, 12:35:53 pm
content: Although not requested, I saw a note in the manual (3.21) claiming that by adjusting the (ideal) parallel value you make the (ideal) series value exact.I found this a bit dubious so I tried it, and wrote it down in case anyone else is interested.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 11, 2020, 12:56:38 pm
content: Lesolee,if you evaluate your calculation for 2nd order terms, you'll derive that this 0.01% = 10-4mismatch transforms into a quadratic error of the ratio, i.e. to 10-8That's why the SR-1010 ratio standards can give <1ppm ratio for 100:1 transfers, with only 20ppm precise, fixed resistors.Frank

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 11, 2020, 01:05:49 pm
content: Please don't mix up the series to parallel configuarion and the switch resistance.For the ratio the combination of the resistors isn't that important. But the switch resistance has exactly the influence I described in the first post. The is no magic hamon stuff here. It is a very simple linear network.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 11, 2020, 01:08:05 pm
content: Quote from: Dr. Frank on February 11, 2020, 12:24:41 pmI don't miss it. It was shown as an example in the Spice simulation.Frank: If you think there is a wrong assumption or something else, please do a detailed calculation from the first picture of my first post. Without some magic hamon formulars.Or: Do you disagree with the LTSpice networks? What do you think, needs to be changed to be correct?Edit:Just for demonstration I adjusted every of the four possible resistors in the textbook circuit. (R1-R3 series/parallel resistors; Rout lower 40k resistor).I printed all values, so it should be easy for you to verify that.R1 adjusted:R2 adjusted:R3 adjusted:Rout adjusted:

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 11, 2020, 03:57:39 pm
content: Here's my manual calculation, first with the 3 legs directly in parallel, which effectively gives 2 times the switch resistance of 10mOhm, or 0.05ppm error as expected, and 2nd with star / triangle calculation which yields 3 times Rsw, or 30mOhm (not intuitively) or 0.075ppm.All w/o any magic, only the Hamon adjustment criterion used.Sorry for the bad quality, the camera has got mumps, obviously.YourSPICEcalculus yields even 8 times the switch resistance, as the adjusted 120k leg is 80mOhm off.There's something fishy, very obviously, but I can't figure out, where this happens.Maybe the triangle / star ansatz is already wrong, as the 3 times error that I have calculated, is also counter-intuitive.. I would expect some reduction /compensation for this switch error, instead.Frank

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 11, 2020, 04:19:11 pm
content: Hi Frank,in the very first line you add dR, but I cannot see why.Maybe, we should try to solve it with LTSpice only to not run into some calculation error.Therefore, my first question is: How should we trim the bridge? Is one of the resistors in the upper leg ok?Edit:I attached a Spice simulation and I used the value I calculated for a trimming of R2 (see above). As you can see the bridge is perfectly nulled and after switching into the 1:10 mode you will get an error of 0.2ppm.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 11, 2020, 04:48:23 pm
content: That dR are simply the switch resistances of 10mOhm each. The middle leg is trimmed so that that X is the correction value which later turns out to be these - 2 dR.Very obvious.I prefer old school paper math, as just simply putting it into a simulation tool often swallows / covers errors.Your Spice should yield the same result, as my paper stuff, so there's obviously something wrong. 8 times deviation of the middle resistor leg makes logically absolute no sense.. where should that come from?Frank

author: iMo
author_rank: Super Contributor
timestamp: February 11, 2020, 05:56:33 pm
content: Be careful with LTSpice results. With the default settings you will not see expected results when messing with high-res inputs/outputs.See below examples how to get high-res results.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 11, 2020, 06:04:51 pm
content: Quote from: Dr. Frank on February 11, 2020, 04:48:23 pmMy paperwork agreed from the beginning. Spice was in the first post only used to double check the results.I attached the same calculation again. This time everything with paper and calculator (HP48). The result is still exactly the same. I wrote down many steps in between to make it easier to verify.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 11, 2020, 06:35:44 pm
content: Quote from: Dr. Frank on February 11, 2020, 04:48:23 pmOr even worse, introduces new errors specific to the tool...

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 11, 2020, 09:53:25 pm
content: For additional verfication I made a quick and very dirty experiment. I took three 100k resistors (100ppm/K ones only) from the drawer. Soldered them together and meaures every one with a 34401A. I also used two 1.2R resistor (intentionally higher value to make the effect more visible). The "shorting" resistors are applied with cables and clips. I measured the shorting resistors togehter with cables and clips of course.After applying the shorts I measured the resistance of the parallel configuration and the measurement agrees within 20ppm with the calculated values. The Spice simulations again agreed perfectly with the calculation, which makes numerical problems very unlikely.PS: If you calculate the deviation between the simple parallel circuit of R1, R2 and R3 and compare that to the measured one, you end up with ~33ppm. That is roughly 100x more than 0.2ppm which was calculated with 100x smaller shorting resistors.On the other side it is still only 33ppm. Perhaps, I should repeat this with even higher shorting resistors.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 11, 2020, 11:32:14 pm
content: I couldn't resistThis time I used 100R resistors for the shorting. That gave an error of ~2700ppm versus ideal shorts. The measurement and the calculation agreed again within 18ppm. The Spice simulations also agreed again perfectly.And again: 2700ppm / 100R * 10mR ~ 0.2ppm

author: Lesolee
author_rank: Regular Contributor
timestamp: February 12, 2020, 03:47:22 pm
content: Please excuse me for repeating what has already been done. I just wanted to check it independently.I have used SIMetrix 8.20. This is SPICE-like but has its own custom computational engine.Part A:"First test your test-equipment"The upper chain is 0.01 ppm high. The result is 0.005 ppm low at the divided point. This agrees with the Windows 10 scientific calculator (and is obvious anyway).In standard convergence mode the divider read +2mV on the x1,000,000 output. (0.002 ppm high).NOT GOOD ENOUGHfor our use. Therefore it was put intoExtended Precisionmode to get the correct result. Test method validated.Part B:Calibrate the parallel configurationI tuned the top resistor. The Fluke manual doesn't say which exact resistor is to be trimmed. Yes, I did go a bit over-the-top getting it spot on!Part C:Division!I am now very happy that whatever Fluke have actually done is not shown in their description, because the actual error is 5x bigger than they claim.Yes I know that has already been clearly stated (several times), but there were issues about the validation of the result.

author: iMo
author_rank: Super Contributor
timestamp: February 12, 2020, 04:35:14 pm
content: With LTspice you may get the same results..

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 12, 2020, 04:40:26 pm
content: Quote from: imo on February 12, 2020, 04:35:14 pmAnd with Python, Excel, HP calculator...

author: Lesolee
author_rank: Regular Contributor
timestamp: February 12, 2020, 04:40:40 pm
content: I had to try it. I adjusted the middle resistor to see if it made much difference. The value was reduced by 26 nano-ohms to get the new balance. The series mode was barely different.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 12, 2020, 04:49:04 pm
content: Quote from: Lesolee on February 12, 2020, 04:40:40 pmIn reply #29 I solved it for every resistor. You can verify the results within your simulation.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 12, 2020, 05:39:30 pm
content: One can calculate the error to the parallel connection approximately to first order relatively simple:The current through the switches is very close to 2/3 the total current. So the voltage drop for every switch is 2/3 * I* R_switch.The upper and lower resistors sees one such voltage drop, while the center resistor sees 2 of the switches. So the average extra voltage is 4/3 * 2/3 * I * R_switch. So the extra resistance for the parallel connection is very close to 8/9 times the switch resistance. No real need to calculate more accurate as the three 120 K resistor may very likely differ more than R_switch from each other. This result is very close to the initial number - withing the rounding used.The Flue formula (3-1) was for a different error. So for the parallel connection Fluke either used better switches or some form of the compensation circuit. The resistors for the compensation only need to be larger than the switch resistors by something like a factor of 10 to get a reasonable good compensation - this may be just some cable / PCB trace parts. They don't give the details, but vague indication:

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 12, 2020, 06:23:37 pm
content: Quote from: Kleinstein on February 12, 2020, 05:39:30 pmVery nice analysis!You couldn't be closer with the added resistance

author: Lesolee
author_rank: Regular Contributor
timestamp: February 12, 2020, 09:33:33 pm
content: The extra resistance needed in the series configuration is just under 8x the switch resistance, so that's 80 milliohms. We then have to suppose that the switch resistance is absolutely stable.I can’t think that adding 10 series contacts in the series path is reasonable. On the other hand adding 10 parallel contacts in the parallel circuit (and one in the series circuit) would make a more robust system.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 13, 2020, 07:48:51 am
content: And now that we've verified that the equations here match, the influence of the leakage resistance.10mR during calibration and the sweep during dividing.

author: Lesolee
author_rank: Regular Contributor
timestamp: February 13, 2020, 08:07:26 am
content: Quote from: e61_phil on February 13, 2020, 07:48:51 amConfirmed at 100G and 1T. Your curves are very useful.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 23, 2020, 05:52:26 pm
content: I finally got time to check my calculation again, and found the error, third line on the 2nd page. It's 63*R*X on the left side, instead od 33*R*X, so that really gives a mis-alignment of -8*Rs, what I already found for the other configuration also, and what e61_phil found by means of his SPICE calculus.Next step was to measure this Rs. On my 15 year old precision grade ELMA switch, all measured contacts (still) showed 5mOhm only, with a slight variation by switching actuation of about +/- 1mOhm.These 5mOhm therefore give about -40mOhm adjustment error or ~ 0.16ppm. (with 100 * 25kOhm resistors)That's still good enough, but obviously can be compensated, by the remark in the Fluke manual "... adjustment of the interconnection resistances.."This switch resistance variation gives +/- 8 mOhm, or an estimate of +/- 0.032ppm, which might be the root cause for these 0.042ppm worst case, Fluke mentioned.Maybe they also include in this parameter a permanent increase/ageing of the switch contacts.All three parameters, the low nominal switch contact, its variation over actuation, and its deterioration, I would expect inside the specification of their custom made switches.Third step was to compare my findings on my box with the 752A schematics, and to identify this compensation / adjustment of the -8*Rs, and of further cable / switch resistances.I have to admit, that I didn't have the schematics available 15 years ago, and I did not check this paragraph 3-30 carefully enough.Long story short, this compensation is done very obviously by a strange long cable, denoted as: '36.75 inches long AWG 22 solid Cu'.That exact specification gives 49.4 mOhm, or 0.12 ppm compensation.This cable-resistor is connected in series with the upper 360 kOhm leg of the divider, but only in the unfolded, divider mode.So that's the way, how to compensate these said effects.The value of this resistance also has the correct order of magnitude, so one can conclude, that the switches of the 752A also have about 5 mOhm contact resistance.Fluke obviously made a precise analysis and summation of all involved cable lengths and number of switch contacts and then determined this specific length of this ominous cable.It should be located between both switches, coiled up, although I think there is another failure in the schematic, as this compensation is not effective in the 100:1 divider.Anyhow, I think that I now have fully understood this aspect. Sort of black magic inside the 752A, which is engineered very well, over all.I also think, that it's possible to replicate its high accuracy using volt-nuts grade facilities.Frank

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 23, 2020, 06:05:45 pm
content: Quote from: Dr. Frank on February 23, 2020, 05:52:26 pmNot only spice. Spice was only to verify the calculations done in python.Did you measure the 5mR including the cables?What do you expect for the leakage resistance? With 1e12 Ohms you would end up with roughly 0.35ppm in my calculation with your configuration (75k instead of 120k). At 1e13 and higher the error is dominated by the 5mR contact resistance.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 23, 2020, 06:10:41 pm
content: I only made a brief resistance measurement for the PTFE cables I've used, that may give some additional mOhm in total.Leakage / resistance might be available in the ELMA spec, but that will be analysed and measured, later. For this aspect, as it was described as being the dominant error, I've chosen 25kOhm as the basic value, instead of 40k.Frank

author: Kleinstein
author_rank: Super Contributor
timestamp: February 23, 2020, 09:47:59 pm
content: So Fluke "only" used a simple switchable series resistors, not sets of additional current compensation resistors like in a Kelvin bridge. Using such extra resistors can get quite messy and possibly causes more trouble than good due to additional errors from leakage. It may still be an option for lower resistance (e.g. for use as a 10 to 1 V step and not as a 100:10 V).A lower base value would also give more self heating - one of the other limiting effects.For the 1:100 divider mode the switch resistance is less critical, so they seem to get away without extra compensation.

author: Lesolee
author_rank: Regular Contributor
timestamp: February 23, 2020, 09:53:44 pm
content: Quote from: e61_phil on February 08, 2020, 08:39:33 pmSo basically the truth is finally out. The switch resistance is critical (and corrected for), but is not measured as part of their calibration procedure. If an ant crawls into the contacts and dies, taking the resistance up by a factor of 10, the absoluteness of the divider ratio is no longer correct, and you have no way to know, other than to have another better standard. One can say they have designed and manufactured the switches so that even after years of life they are still excellent on contact resistance. But if you live near the sea, and the corrosive salty air takes its toll, your reference standard is in anunknownstate of mis-calibration.I can just imagine the engineers saying "that needs to be in the manual", and some manager saying "the switches are good, and not suitable for the customer to measure, so just shut up about it."

author: notfaded1
author_rank: Frequent Contributor
timestamp: February 24, 2020, 01:08:43 am
content: Quote from: Lesolee on February 23, 2020, 09:53:44 pmThat's funny... we've seen some residue that looks worse than an ant dying before. Sometimes looks like someone lived by the ocean and kept it in their shed outside or a soggy basement for 15 years.

author: notfaded1
author_rank: Frequent Contributor
timestamp: February 24, 2020, 04:15:19 am
content: Quote from: Dr. Frank on February 23, 2020, 05:52:26 pmI've been following this closely... can we take this to mean we may be able to replicate this black magic soon Dr. Frank? Nice highlight on the 36.75 inches long AWG 22 solid Cu btw. Also I like the replicated calculations and seems everyone is in agreement now.B

author: Kleinstein
author_rank: Super Contributor
timestamp: February 24, 2020, 07:21:50 am
content: I check of the contact resistance would be a really good idea. However the requirements are not that dramatic - more like make sure they are not off by a lot (e.g. more than 100%).Chances are the users at the primary labs will know that and do such a check. They will also not store there precious instrument in a damp basement or near the shore (which is quite a bit away from the NIST in Bolder).

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 24, 2020, 07:48:21 am
content: Very nice analysis, Frank!For this kind of compensation it is obviously necessary to compensate one of the upper resistors.I attached a graph which shows the error due to a changing switch resistance with the 49.4mR compensation.@Kleinstein: I'm very sure, that there a primary labs, which don't do such checks.@notfaded1: Noone, will stop you from replicating

author: Kleinstein
author_rank: Super Contributor
timestamp: February 24, 2020, 04:38:06 pm
content: The tricky part to check is leakage: here it is nearly impossible to check at exactly the same positions. In addition contamination from deposits (e.g. nicotine, grease) and worst case a fungi growing or a small spider inside is a way to produce leakage. The best one could do is probably to keep the case well closed, clean and dry and do a leakage test for some dummy parts inside.For a high precision divider there is a little the question if the Hamon divider is still the best choice: The alternative is a chain of some 10 equal resistors that are checked / measured before use. With modern instruments that allow for a reasonable fast and accurate measurement this may be a real alternative. Equal resistors may have some advantage as self heating could also compensate. Also checking for leakage is easier (i.e. no longer essentially impossible) with a simple string.A good resistor measurement bridge with the associated switches is still not easy. Chances are it would be more switches, but less critical.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 25, 2020, 02:13:20 pm
content: The resistance is specified as minimum 1012Ohm.Indeed, I measured about 60pA @ 100V between adjacent pins, that gives 1.7 * 1012Ohm.So good enough for my original design goal to get about 1ppm 10:1 and 100:1 transfer accuracy.Frank

author: Lesolee
author_rank: Regular Contributor
timestamp: February 26, 2020, 01:40:39 pm
content: Quote from: Kleinstein on February 24, 2020, 04:38:06 pmInterestingly, they do say that the switches are "sealed units" (section 4-15), and they stress the guard circuits in 3-23, 3-24, 3-25, and figure 3-9. But as you say, difficult toverifytheir claim of 0.057 ppm error due to leakage on the 10:1 range.

author: quarks
author_rank: Frequent Contributor
timestamp: April 25, 2020, 08:16:44 am
content: bookmark, because I missed it until now

author: Lesolee
author_rank: Regular Contributor
timestamp: April 28, 2020, 10:14:20 pm
content: magic did some analysis of the series parallel stuffhttps://www.eevblog.com/forum/metrology/anyone-else-built-a-hamon-divider/msg2612673/#msg2612673I found the maths a bit intimidating in his analysis, and I did a simpler version with just the three resistors.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 31, 2022, 07:26:21 pm
content: Consistency Check with HP3458AHighVoltage kindly lent me his 752A to check it and then to calibrate the HV ranges of my 5442A.The 752A is from about 1993, 29 years old.That vintage bears the risk that the resistance of the switch contacts increased considerably, busting its specification of 0.2ppm uncertainty @ 10:1 ratio.The intrinsic problem with such Primary Ratio Standards is that they can not be calibrated by “something better” instrument. It’s written in the manuals, that they are working correctly “by design”, elaborate error analysis, and by auto-adjustment.Therefore, I tried to check the 10:1 ratio by use of the 3458A. Its transfer uncertainty on its 10V DC range is specified to 0.55ppm, but its 10:1 transfer capability by linearity (INL + DNL) might be even accurate to about 0.3ppm of output, following hpj 4/1989, p.23.The 752A was auto-adjusted following the setup from the manual addendum and using an 845AR w/o problems.I tested the 10:1 transfer by measuring the 10V input directly at its INPUT jacks, as well as the 1V output, both with reversed polarity, which is critical concerning thermoelectric voltages. I repeated this procedure several times, but always found errors far beyond 0.2 or 0.3ppm ratio uncertainty.So my conclusion was that the switches should have excessively higher resistance, after all these years.HighVoltage allowed me to open the instrument with greatest care and white gloves, of course, to measure the resistances of the switch contacts. To my big surprise the switches were all ok, as described in the next chapter.So I investigated further and finally discovered that the bias current in the 10V range of my 3458A caused these errors. It’s specified to have max. 20pA.The bias current was determined between -10 .. +10V of input voltage, by measuring the voltage drop over an external 1MΩ resistor.These bias current measurements changed over several sessions, probably over temperature, time, humidity and so forth, up to 20pA max. bias current.My 845AR has an input impedance of 1MΩ. By closing and opening the input short, the difference in µV readings gave about 100.. 200fA of bias current. Reading carefully the manual, chapter 2-31 “Increasing Input Resistance”, I deduced that the bias current w/o that 1MΩ resistor would typically be 3µV / 1000MΩ ~ 3fA.The 752A has quite a high impedance of 400 kΩ, so I calculated the ratio error of the divider loaded by a bias current, see xls file below.The error formula is: ε = -R1* IB/ UinA 20pA bias current at 360kΩ and 10V input creates -0.72ppm of ratio error, so this obviously is the root cause.The 3458A can be used, anyhow, for measuring 100V and 1kV at 10V output, as the ratio error is 10 times smaller in this case. Used as a Nullmeter, it still has e.g. +8pA bias, creating errors when using the 752A in its classical bridge configuration for calibrating 1kV, 100V, 1V and 100mV versus an external 10V standard.Therefore, any Nullmeter to be used must have guaranteed bias currents well below 1pA.Here’s another table for my Keithley 182A nV DVM. This shows, that even this class of nV-meters might not be the right choice.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 31, 2022, 07:27:17 pm
content: Switch Resistance Check and circuit analysisI disassembled top and front of the instrument.There were indications, that this instrument had been serviced, replacing several broken jacks, see careless solder joints and flux residues.Probably the divider resistors were re-adjusted by changing bridges on the adjustment board.In the third picture, the black compensation wire, folded inside the cable tree, can be seen.Then I measured most of the switch resistances and several crucial cable connections, using 4W Ohm method.To my surprise, the switches all had very low resistance values between 0.6 mΩ and 3.8mΩ, far away from the 10mΩ as anticipated in the manual. Their initial values might have been 0.5mΩ only. After 29 years now, the resistances of the Hamon relevant switches were between 0.8 .. 1.2mΩ only. So they play a minor role only.As shown in the schematic in green, only switches S2D/2 and S2C/8 contribute about 2mΩ, plus the connection to R35, those 4 brown wires, 10.5mΩ, that gives 12.5mΩ in total. This should be compensated by the red items, cable from Input High to and including S1A/5, this mysterious black 49mΩ cable, plus S2D/1, all giving 63mΩ in total. As the interconnection between R21-24 and R31-34 are linked directly at S2D/2, these cables are included in the divider resistors and contribute zero error.Therefore, mostly this 10.5mΩ of ‘4 brown wires’ is compensated, and to a minor degree the switches, including their expected lifetime increase of resistance. Calculating the divider error of the initial state (all contacts having 0.5mOhm) gives about -0.046ppm.If both switches increase to 5mΩ each, i.e. 10mΩ in total (what’s indicated in the manual), that would give an error of +0.049ppm. To check my explanations, please see also the calculation chart by Dr. Philipp.A compensation resistor like the black 49mOhm one is evidently missing for the calibration and use of the 100:1 divider ratio. A 630mΩ resistor would be required, instead of only 28mΩ, as measured between: IN HI – S1A/6 – GREEN Cable – S2C/1.This way, the 100:1 ratio seems to have an additional, undisclosed +0.11ppm error, included in the 0.5ppm specification, if I did not overlook something.I also briefly checked the leakage current, which was ok.As a conclusion, this 752A seems to operate in near mint condition, at least I expect that it fully meets its specifications.Edit:The switches obviously are manufactured by MultiTech, 820 / 2000 family, which VgKid had found:https://www.eevblog.com/forum/metrology/4-wire-switch-selectable-resistance-standard/msg1193999/#msg1193999https://www.multi-tech-industries.com/test-switches.htmlThe wafers look identical, and the initial contact resistance is below 1mΩ.FLUKE probably ordered a special version. 4 decks, 6 positions, 2 poles / deck, 5 stops.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 31, 2022, 07:28:25 pm
content: For calibration and use of the 752A, I use an 845AR as a Null-Meter, which is line-powered.It’s very important to use the grounding scheme of the addendum of the 752A manual, which I append, see fig. 1 and fig. 3.There’s a difference between battery and line powered instruments. Only if these instructions are followed, the null readings will be comfortably stable and usable.I also tried my 3458A as a Null Meter. I did not calculate the error for the calibration of the 752A, caused by its bias current.So I speculate, that only the higher resistance of the 100:1 range might induce noteworthy errors.But the 3458A is not as convenient and stable to use as an 845A, because the readings are very noisy, and sort of ambiguous – you have to test & feel this on your own. This is possibly caused by the 10V common mode voltage relative to earth, and also to the lacking proper grounding scheme as described above.The errors by bias currents for the calibration of the DUT I have already described in the first part.The 5442A has six ranges, i.e. 0.2V, 2V, 10V, 20V, 250V, 1kV, which were calibrated by a differential comparison against a known 10V reference, either by dividing the reference by 100 or 10, comparison on 1V, 100mV level, or by dividing the output of the DUT by 10 and 100, and comparison on 10V level. Due to the bias induced errors of the 3458A, I followed the classical procedure in the manual, i.e. using the differential mode only.The Fluke 7000 first had been trimmed to exactly 10.00000V, derived from the average of my reference group.It turned out, that the 4 higher ranges (i.e. their ratio constants) had stayed very stable over at least 13 years (date of last full external calibration is unknown):The 10V range, i.e. its internal 13V reference, SZA263 based, drifted about +1ppm.The 20V range ratio drifted ~ -0.1ppm, 250V ratio -0.6ppm, 1000V ratio -1.6ppm.The auto-calibration of the 5442A is able to measure and correct small drifts of the ratios at any given time, but a 10:1 cross-transfer between ranges is not possible, as in the 3458A, or the later 57xxA calibrators.That means, that the calibration drift is mostly caused by its voltage reference drift (affecting all ranges in parallel), or by the linearity drift of its DAC, which might basically be on the order of its INL, as the ratio drifts are virtually zero by internal calibration.Anyhow, Dr. Philipp might use this 752A to check the HV calibration of his 5730A.

author: TiN
author_rank: Super Contributor
timestamp: June 01, 2022, 03:11:49 am
content: Dr. FrankIf you (or anyone else in community) have particular things would like to check with 752A for more data, please do let know.We plan to have a CalFest 2022 in bit over a month from now with ppm members attending and will have more than four 752A's to test against. Idea was to perform self-calibration on multiple dividers and compare them between each other from fresh volt.

author: alm
author_rank: Super Contributor
timestamp: June 01, 2022, 08:41:53 am
content: Quote from: Dr. Frank on May 31, 2022, 07:26:21 pmAm I understanding you correctly that it's the variation in bias current, in particular with input voltage, that is causing the bias current correction as described byFluketo not work in your setup? I wonder if the Datron / Fluke series of 8.5 digit DMMs have better bias current stability.

author: magic
author_rank: Super Contributor
timestamp: June 01, 2022, 10:36:38 am
content: Quote from: Lesolee on April 28, 2020, 10:14:20 pmI could try to comment my calculations if there is interest. But there is nothing really deep or clever in them (that I can see), I just hammered the formulas mindlessly with algebraic transformations until they resembled the form I wanted to see. I have reviewed that stuff (again) and it still looks good to me.There is an error and a gotcha in your analysis. The final formula given for the series-to-parallel ratio of three resistors is:Rs/Rp = 9 - 6·δ²where δ is the matching tolerance.The error is that the ratio is less than 9. My own calculations indicate it must be greater than 9. I tried a few examples and the ratio was always greater than 9. I haven't analyzed where the mistake came from.The gotcha is that the proof relies on each resistor being a ratio of δ away from their average, rather than from the nominal value. For a skewed distribution like 9Ω, 9Ω, 11Ω the ratio is approx. 9.08, but naive application of the formula with δ=10% gives only 9.06 after correction for positive sign, or 8.94 as-is. That being said, it's not a huge difference and such distribution is likely the worst case that can possibly happen.For the record, my own conclusion was:9 ≤ Rs/Rp ≤ 9 + 12·δ²

author: Kleinstein
author_rank: Super Contributor
timestamp: June 01, 2022, 11:05:06 am
content: There are 2 types of error:One is from a mismatch in the resistors. In the parallel configuration the smaller resistor get more weight and thus a smaller than wanted resistance for the parallel configuration.The other is the added resistance of the switches. Without extra compensation this adds to the parallel confuguration and thus to small a ratio.So there are 2 effects, one going up and one going down.With some extra compensation in the wire length and choosing the right point to tap of the ratio the error can be reduced to some degree and the variations in the switch resistance can go in both directions. So the error can be both ways, depending on how stable the switch and cable resistance is. A point may be of there is a way to check the compensation. Testing the contract resistance is a good point and should be part of the self test.The effect of the 3458 input impedance (800 Gohms range from the leakage data) shows that parasitic leakage and parallel resistance could also be an issue, not the the switch on resistance, but possibly also the switch off resistance. In that respect the cabeling does not look so great - though hard to tell how good the isolation actually is.With the relatively large resistors the 752 seems to be made mainly for a 10 to 100 / 1000 V step and not really to go from 10 V down to 1 V and 0.1 V.Is there a corresponding lower resistance Hamon divider, that is better suited for the lower votlages ?

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 01, 2022, 12:21:57 pm
content: Quote from: alm on June 01, 2022, 08:41:53 amHello alm,thanks for this hint.Anyhow, this more qualitative article does not describe at all, what the decisive parameters for a given DMM are, which makes it usable as a 845A replacement, and how these parameters can be measured. Is it the bias current only, or are the impedance, isolation resistance, and so on also relevant?Additionally, FLUKE onlyhopes, that the bias of their 8588 would be in most cases around 5pA. Fluke also does not measure the bias over different voltages, or do I miss something? I guess, that each DMM will show varying bias currents over their input voltage.I think, I will add or cite a few documents, how other metrologists have characterized their long scale DMMs.The problem I see is not especially the change of this bias (over temperature for example), but simply any absolute value above 1pA.FLUKE themselves seem not to have calculated the errors like I did (see my xls table), which allows to estimate the induced error per range or mode, in a very clear and quantitative manner.So it's true, that the 100mV and the 1V ranges are affected the most, and to my opinion a DMM can definitely NOT be used for that purpose.I also feel confirmed by FLUKEs article, that the DMM solution suffers from those ground / isolation problems, e.g. this pump out effect and the noise problem they also mention, which the 845A probably does not have.@ Kleinstein: The 752A is as it is, so there is no use to discuss that it has too high an impedance in these low DCV ranges.I also don't know, how this new device from IET is engineered, nor do I know about any other commercial divider for that purpose.The 752A simply does it job perfectly, provided you use an 845A or similar low bias, high isolation instrument.Edit:I got a lot of insight about the 3458A DCV input behavior from Rado Lapuh: "Sampling with 3458A", especially about the bias current measurement.Two cited articles describe in detail, how to measure these input parameters.They can be downloaded somewhere else (via Research Gate) for free, because I think I'm not allowed to upload the files here:"Accurate determination of the input impedance of digital voltmeters", G. Rietveld"Determination of High-Resolution Digital Voltmeter Input Parameters", Ivan Lenicek, Damir Ilic, and Roman MalaricThey find similar values for the bias current of their 3458A (they use a different sign convention), but also do not determine the bias at different input voltages.

author: alm
author_rank: Super Contributor
timestamp: June 03, 2022, 10:17:41 pm
content: Quote from: Dr. Frank on June 01, 2022, 12:21:57 pmThe state that for the 8588A the bias current is "almost always less than 5 pA", so I imagine this is based on either inside knowledge of the 8588A design / production testing, or from characterizing a number of 8588As in their own labs. But it is very light on details indeed, like no procedure to validate this for your own meter.Thisis presumably an older version of the same article that contains slightly more details, like "The newest Fluke Calibration Reference Multimeters (the 8588A and 8558A) have significantly less bias current, at 20 pA maximum. This bias current is also adjusted to be effectively zero pA when they are originally tested. This virtually removes the bias current offset problem. Also, because the bias current changes very little over time, it remains near this level for an extended period into the future." and how to test for bias current.Have you tested or calculated the effect of using the offset correction as described in the article?Maybe this is worth a shot?QuoteQuote from: Dr. Frank on June 01, 2022, 12:21:57 pmWouldn't you able to correct it if offset current is constant and impedance as seen from the DMM is constant, or you can measure the effect on multiple ranges (1:10, 1:100)?Quote from: Dr. Frank on June 01, 2022, 12:21:57 pmI agree that the article basically states that the 845A is superior in terms of uncertainty introduced by all the factors you name, but that the DMM is more convenient in terms of automation.Quote from: Dr. Frank on June 01, 2022, 12:21:57 pmThanks for the references! Both papers reported values well under 5 pA. This is consistent with the Fluke figure, although for the 3458A.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 03, 2022, 10:44:23 pm
content: The 3458 input uses switching directly at the input with relatively little fitlering capacitance. So it is not a steady bias current, but also includes spikes in the current and relatively little (e.g. some 100 maybe even 10 pF) of capacitance could make a difference in the settling and charge injection. So worst case the capacitance and source impedance can make a difference to the input bias. It may be a good idea to check the bias with 2 different resistors and maybe a little extra capacitance.Besides the capacitane, a changing level of mains hum may also be a problem, effecting the input current.I don't know the details on the Fluke meter, but only have information on the predecessor Datron 1281. If the input part is similar, one can expect the input bias to be relatively constant and not effected much by the voltage, as most of the input amplifier is bootstrapped. This could explain why they ( Fluke ?) claim essentiall constant bias.The claim of the DA1281 to replace a null meter seems to be the DMM replacing the whole system of refrence, KV divider and nullmeter , not replacing just the null-meter and keeping the KV divider and extra reference.A more or less constant bias should have about opposite effect on the tests with both polarities. So the correction should be relatively easy and may just be averaging over those 2 tests. The extra input impedance may be the larger problem. The real DMMs will have a combination of a constant bias and impedance parallel to the input. The input impedance does not even have to be constant. The change in input current can be nonlinear in the votlage.

author: alm
author_rank: Super Contributor
timestamp: June 03, 2022, 11:28:59 pm
content: Attached tothis post(thesecond attachment) is a Fluke presentation that appears to was made before they made their own long-scale DMM, is more critical about the use of DMMs instead of null meters and goes into more detail about the DMM selection and measuring the stability of the observed offset.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 04, 2022, 07:34:26 am
content: It is a bit odd, that they assume a 40 K source impedance:AFAIK the 752 has a 120 K:120 K auxiliary bridge arm for the 1:1 self cal step. The main arm is 40 K and 40 K in this step. So the source impedance in the self cal setp is 20 K + 60 K = 80 K.Reversing the external 20 V supply (not just the bridge arm turn over for the adjustment) for the adjustment step should to a large part cancel out the offset / bias problem.The 40 K are approximate (though more like 36 K as 1:10) for the later use as a 1:100 divider. That would also be the case with significant votlage at the DMM input.

author: e61_phil
author_rank: Frequent Contributor
timestamp: June 06, 2022, 04:39:40 pm
content: Quote from: Kleinstein on June 03, 2022, 10:44:23 pmSome time ago I had a Datron 1071. That also had a bootstrapped input stage, but I never looked deeper into it. The calibration included to connect a 1Meg in parallel with a small capacitor and that was used by the meter to cancel out/reduce the input current. Unfortunately, I never measured the input current of that meter.

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 07, 2022, 11:00:35 am
content: Motivated by Dr. Philipp to determine the HV linearity of my 5442A, and of the 3458A, I used the 752A as a high precision 10:1 and 100:1 divider for the 3458A, solely in its 10V range, to measure the exact output voltages of my 5442A, relative to its calibrated 10.00000V reference value, up to 1kV.At first, I measured the bias currents at various input voltages. These values changed compared to the first table above.I calculated the expected ratio errors for each data point. The highest error is 0.08ppm, so that already might affect high precision measurements.I calculated each 'exact' output voltage from the 5442A, by combining the precise divider ratio (0.2 and 0.5ppm uncertainty), the precise readings from the 3458A, and normalizing to 10V reference value, as the 3458A is not calibrated exactly to the 10V baseline of my reference group.Afterwards, I applied these 'exact' output voltages directly to my 3458A, to estimate its ACAL (ratio) uncertainty of its 100V and 1kV ranges, and to determine its High Voltage linearity.As can be seen from repetition of this process, the ACAL ratio transfers are mostly precise to 0.2..0.3ppm for the 100V range, and about 0.2 ... 0.5ppm for the 1kV range. That may change for successive ACALs.The HV linearity, by comparing 100V and 1000V readings in its 1kV range, is about -1.5 ..-2ppm, much better than the specified 12ppm.These measurements I also propose for TiNs CAL FEST.Now I had a closer look on the linearity of the 20V, 250V and 1000V range of my 5442A. Take notice of change in the bias measurements, and that for several low readings, one gets too high errors of -0.2ppm when using a DMM in this use case.One can directly see some quirks of the 5442A.The lowest value of each range deviates strongly.The linearity specification of 0.5ppm is violated in the 20V and 250V range, and in the 1kV range, it should not be as high as -1ppm for 1000V.This indicates a subtle error of my instrument. Subtle in the sense, that it just exceeds the specification, and was hardly detectable, only by use of the uncertainty of the 752A.I already knew, that my instrument was nOK for negative voltages, having a smaller gain factor there, and that negative voltages do not stabilize quickly, but instead need too long a time to reach their nominal value.That all did not restrict its use, as it was all inside the main specification.Now I had the sense current compensation in suspicion.These are internally up to 500µA for the + Sense, and up to 200µA for the - Sense. Like in the 732B reference, these currents are compensated internally to increase the accuracy of the output voltage.I set the 5442A to external sense and measured both sense currents by two handheld DMMs, i.e. the BM869 and the 121GW, resolving 1 .. 10nA each.It turned out, that the + Sense works correctly, i.e. quickly regulates to < 200nA. The - Sense shows exactly the anticipated error: it changes very slowly and stabilizes after a long time at several µA, depending on the output voltage and sign.So I will investigate on this assumed error mode around OpAmp U1 further.Frank

author: e61_phil
author_rank: Frequent Contributor
timestamp: June 12, 2022, 10:06:46 pm
content: Quote from: Dr. Frank on June 01, 2022, 12:21:57 pmHi all,Frank and I already started the discussion about that point. In my opinion the absolute value of the bias current is less important. It is very important, that this current is stable during the self-cal procedure.Fluke describes the procedure with the 8508A in the 5730A calibration manual. They nulled the meter with 0V applied to the input (0V not open!).The input current is a current (surprise, surprise) which means a constant current will develop a constant voltage above a constant impedance. In my opinion the Fluke procedure makes absolutely sense. Applying 0V (or apply a short) creates the same output impedance for the null detector as within the self-cal with low impedant 20V. And you know, that the bridge voltage must be 0V as long as you don't apply a voltage to the input. Therefore, that is the correct mode to zero your meter. That will only correctly cancel out the bias current for 0V. Which means if your bridge isn't perfectly balanced the reading of your nullmeter is influenced by the current difference between 0V and the actual bridge voltage. But that doesn't matter as long as your readings are still monotonic. You don't need to measure accurately a non-balanced voltage you just need to find 0V.I measured the input current of my Keithley 182 Nullmeter several times over the day and the current was very reproducible (K182_input_current.png). One can also see, that the slope is not very steep, which means the current is almost equal in the +/-1µV range of interest.To evaluate if the K182 ist suitable to self-cal a Fluke 752A (big thanks to High Voltage!!) I run the following self-cal procedure and verifed the zero after that with my Fluke 845AR.1. Set calibrator (connected to the input of the 752A) to 0V (in OPERATE!!)2. zero the K1823. Set calibrator to 20V4. adjust the 752A until the K182 reads zero (few 100nV)That worked perfectly with the 1:10 calibration, but I got ~8µV missbalance (measured with the 845) every time I tried to calibrate the 1:100 range. That would lead to almost 2ppm of ratio error.To figure out where the problem lies I measured the common mode current, which flows into the K182 as soon as it isn't sitting on calibrator LO anymore. To test this I used my Keithley 617 and connected both inputs of the K182 to the HI input of the K617. The interal voltage source of the K617 was placed between the GUARD/Shield of the K182 and the inputs.That measurement revelad more than 100pA at 10V. (K182_CM_current.png). That explains the 8µV over the balance arm. The curve looks very much like a simple resistance of ~70Gig. I also tested that with the Fluke 845AR and the current was well below 200fA at 10V. In my opinion this isolation resistance is way more important than the input bias current.To overcome this problem I changed the self-cal procedure slighlty1. short the input of the 752A2. apply 10V on the shorted input. (LO of the calibrator is internally connected to GUARD on the calibrator)3. null the K182 (this time the bridge voltage is again 0, but the meter sees the same CM as during the self-cal)4. remove the short and apply 20V on the input of the 752A5. adjust the 752AThis time the 845AR also agreed with the zero bridge voltage in 1:100 mode.Conclusion: You need not only a meter with low input bias, but also with very good insulation.As Frank already mentioned: The input current is also very important during measurements with the 752A. I also measured the input current of my 3458A (also very stable over many hours) and calculated the generated error, if one measures the 752A output in the 10V range.