DC 1 A current measurement linearity at ppm level? - Page 1
Forum section: electronics / general

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: October 27, 2018, 07:57:23 am
content: Has anyone looked at linearity of high current ranges on precision DMMs (3458A, 34470A, DMM7510)? I need to do tests where the current varies between 0.1 and 1 Amp over a few minutes and am wondering how much non-linearity one can expect from shunt self-heating?HP3458A specifies only 100 ppm short-term accuracy on 1 A range. The shunt resistor is 0.1 Ohm and has TC of 5 ppm. Why is the specification so poor? Is it just because of the internal resolution in the ACAL measurement of the shunt resistor value? So maybe the absolute accuracy is poor but linearity is much better?

author: TiN
author_rank: Super Contributor
timestamp: October 27, 2018, 08:23:00 am
content: Interesting questions. If you define step delay time, I can help you test linearity in 100mA to 1A range. Or -1A to 1A in 10mA steps maybe?

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 27, 2018, 10:20:31 am
content: I also was always wondering about these bad specifications of the 100mA, 1A ranges.In the CLIP, these range shunts consist of R212, a 1 Ohm, T.C. 50 ppm/K, and R213, a 0.1 Ohm, T.C. 5 ppm/K resistors.The T.C. w/o ACAL gives the apparent T.C. of these ranges, and that's about 28 ppm/K.That's already an obvious contradiction between the specification of these resistors and the ranges.During ACAL Ohm, that big T.C. might affect the 10:1 transfer process, which itself is obviously precise to < 10 ppm, as with the lower ranges, but it will add up into the 24h accuracy specification for 100mA and 1A.The strange DCI specifications might also be caused by the internal thermal voltages, which can't be auto-zeroed during the measurement.. So HP included these effects generously into the T.C. spec.If you look closely into the schematics, you'll see these strange solder joints/bridges JM 201..208. These might be used to compensate for equivalent solder junctions of the shunt resistors.That magic stuff kept me away from replacing these shunts by better ones.In the end, to verify the real behavior of your specific instrument, it would require a calibrated and variable current source being at least 10 times more uncertain than these two ranges, i.e. well below 10 ppm.. even the 57xxA calibrator @ 40..60ppm won't do the job, I fear.Frank

author: TiN
author_rank: Super Contributor
timestamp: October 27, 2018, 10:40:59 am
content: Yes, I actually triedreplacing shuntsbefore, and it actually made TC much worse for medium current ranges, even though replaced resistors itself are way more stable.57xx specifications are rather conservative, so there could be some good data to see. And since we can use multiple DMMs , it's possible to estimate difference between different units on same source. But self-heating and PCR of the involved shunts/source parts is important, that's why I want hear from maxwell3e10 on specific delay times and such.I also have few of these DCCT heads, which specified for current linearity <1ppm.But for 10-2000mA that would need some more work. I already have CSNG 2.5 ohm shunts for them though.

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 27, 2018, 01:45:37 pm
content: I saw up to 35ppm deviation between ACALs on the 3458A depending on what is happend closely before ACAL. If you run the meter with 1A and than run ACAL it is far away from an ACAL run without current before.The spec of the 8508A is worse, but the 1A (2A) range seems to be much better than the 3458A. We will see after the next calibration of the 8508A...You don't need to have a super accurated 1A source to test the meter. You only need a super stable one. A 57xxA should be good enough for such a test. One would short the input of the 3458A current input (use a resistor in series, to lower the residual current through the 3458A even further) and than let the calibrator and the monitoring shunt/DMM stabilize. After the stabilization open your short and monitor the behaviour of the meter under test. This can also be done with a jump from 100mA to 1A.I played with some Burster 1240-1 resistors, a Fluke 8508A and two 3458As. The 3458A had the best specs, but the worst behaviour. (Some 1240 were also bad)

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: October 27, 2018, 04:24:08 pm
content: Thanks for the feedback. We have done measurements with about 10 steps from 0.1 to 1 A with about 2 minutes on each step. The obvious test is to compare step-up and step-down measurements and they give opposite sign of non-linearity. Also tried random steps from low current to a particular point and then back or alternating back and forth between a high and a low value while keeping the average roughly constant. All these measurements give different results! The question is if it is from our DUT which includes magnetic materials that can have hysteresis, or from the meter. So far the meter we used is DMM7510 (but if HP3458A or another meter is clearly better, its a good excuse to get one).It would be interesting to find the time constant of whatever drift is happening. Perhaps it would have better linearity if one only measures for a couple of seconds at each point, we haven't tried that yet.Fundamentally, what is the source of the drift and large TC? Could be thermocouple effects, I suppose. For 1 A on 0.1 Ohm shunt, 1 ppm corresponds to 100 nV. With Seebeck coefficient on the order of 1 uV/K it wouldn't take much temperature gradient in the meter to mess things up. A shunt might have a power dissipation coefficient of about 10K/W, so one can see about 1 K changes. Also I am wondering if whatever alloy is used for shunt construction would have a particularly large Seebeck coefficient. Perhaps there is a compromise for a material with higher TC but lower thermal EMF. Also mounting and connection symmetry would be crucial.

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 27, 2018, 04:35:13 pm
content: I think 2min are too short for stabilization. I would try to make one bigger jump instead of the smaller ones to learn a bit more over the system.If your system is fast enough you can turn the polarity to overcome thermoelectric effects. Even if you can't use this in the end, it will help to learn were the sources of error are.Another approach could be to build 10x 100mA current sources. Every source with a switch to switch the current in your DUT or around. You can measure every source with very high relative precision (because it is always almost the same value) and the sum of the currents should be the "real" current. If you are really interested in ppm current linearity I wouldn't rely on any meter with specs which doesn't fullfill your needs.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: October 27, 2018, 06:32:33 pm
content: Quote from: e61_phil on October 27, 2018, 04:35:13 pmMaybe one way to do it is to setup a make-before-break reversal switch across the DMM, so one can switch the current from positive to negative through the DMM without disturbing the system too much.

author: TiN
author_rank: Super Contributor
timestamp: October 29, 2018, 08:55:06 pm
content: I guess this is a "teaser". Data still collecting, 3458A NPLC50 input direct to AUX output on calibrator, locked on 220mA range.Standard deviation on points about 40-50 nA.-110 to 110 mADC sweep first, then will do -1A to 1A sweep. Just one meter for now.

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 29, 2018, 08:59:56 pm
content: Especially for the -1A to 1A sweep I would always step between positive and negative for better stabilization.For example: 10mA, -10mA, 20mA, -20mA...If negative is interesting at all.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: October 29, 2018, 09:09:41 pm
content: Yes, its a question if one tries to minimize the thermal effects or just measure how big they can be. Also its not obvious which effects will be even with current and which will be odd. If its a thermocouple-type shift, it maybe odd.

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 29, 2018, 09:12:58 pm
content: I wouldn't sweep two unkown devices against each other. I think one can learn more with a current step on the DUT.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 29, 2018, 09:29:22 pm
content: For high accuracy current measurement at hight currents, I would consider an external shunt, that can be higher power and if needed could use power compensation. So have a heater coupled to the shunt and use it to keep the power / temperature constant.I know of 3 effects that contribute to shunt nonlinearity:1) Self heating and the TCR2) Self heating and thermal EMF3) magnetic effect from the magnetic field the test current produces itself. This tends to push the currents to go a path more to the outside of the resistor and thus cause an increasing resistance with higher current. I don't think this would be relevant at 1 A already. There could be magnetic effects in some alloys too.Shunt alloys are usually low thermal EMF, but of cause nothing is perfect.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: October 30, 2018, 02:38:26 am
content: What is a good shunt resistor to use? I've seen some people mention Burster 1240. Also on Newark site there are these Alpha resistors which have TC of only 2.5ppm/K, does anyone have experience with them?https://www.newark.com/alpha-electronics/pcyr1000b/res-metal-foil-0r1-0-1-750v-axial/dp/96Y7893

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 30, 2018, 06:03:11 am
content: A couple of weeks ago, I played with some Burster 1240. Most of them were far better than specified. But not all of them were very good.Thermal compensation can be quite difficult. One have to make sure, that the resitive element itself stays at a constant temperature. But there will be always thermal resistance between the sensor and the shunt resistance. The thermal flux through this thermal resistance will change with applied current. Therefore, the shunt resistance temperature will change, even if your heater is perfectly stable.

author: TiN
author_rank: Super Contributor
timestamp: October 30, 2018, 11:51:03 am
content: Well, even with zero effort, -1A to +1A sweep looks not too awful.This time I connected more DMMs, in a single loop (source Iout => 3458A I -> 3458A LO -> 3458B I -> 3458B LO -> FLK I -> FLK LO -> Source return).Here's slower run with NPLC 50 on HP meters:Rerun with NPLC 10 instead, and Fluke configured in 10A range.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: October 30, 2018, 11:58:20 am
content: Interesting that the two HP3458 behave so different. Are they from a different era or modified in some way?The third order non-linearity suggests a thermocouple effect.

author: TiN
author_rank: Super Contributor
timestamp: October 30, 2018, 12:02:03 pm
content: Yes, one is modified, as outlined above. I will let you guess which one for now, sorry

author: HighVoltage
author_rank: Super Contributor
timestamp: October 30, 2018, 12:41:17 pm
content: I made a similar Test a few weeks ago to test a 1 Ohm Burster 1240Source: Keithley 2460Manual setting from +100 mA to 1A then again from -100mA to -1ALoop:A) Burster 1240-1 Ohm on 34470 (1)B) 34470A (2)C) DMM7510D) 3458AHere are the raw data.I am traveling, may be someone can put the data in to a graph

author: Kleinstein
author_rank: Super Contributor
timestamp: October 30, 2018, 12:45:14 pm
content: Quote from: maxwell3e10 on October 30, 2018, 11:58:20 amThermal EMF would be more like a second power INL contribution, as the error would scale with the power and thus square of current.Self-heating and TCR would cause a 3rd power contribution, as there is the current as an extra factor.

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 30, 2018, 01:31:41 pm
content: I took HighVoltages data and tried to plot the INL. I choosed the 1240-1 as reference, because I think it is the best device in this list. The results looks wrong. There is a big outlier on the 3458A and the 7510.@HighVoltage: Which 1240-1 do you use? And why are all values positive?

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: October 30, 2018, 01:40:33 pm
content: Great data, its exactly what I would have taken if I had all the equipment!I threw away outliers at 0.1A (maybe different range was used) and also one point each at 0.8 and 0.9 A. Here is the plot. It looks like 34470A is pretty bad and DMM7510 is perhaps slightly better than HP3458A.

author: HighVoltage
author_rank: Super Contributor
timestamp: October 30, 2018, 02:43:32 pm
content: Quote from: e61_phil on October 30, 2018, 01:31:41 pmThe two outliers are corrected, they were a typo. Enclosed is the corrected file.The Burster 1240 was my "1.0-A" SN: 187251I am back in the lab tomorrow (and we have a holiday), so I will repeat this testwith a second Burster 1240-1.If I remember right, the 100 mA was measured in the 100 mA rangeIn my new test I will take all measurements in the 1A range.

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 30, 2018, 02:51:35 pm
content: Quote from: HighVoltage on October 30, 2018, 02:43:32 pmThanks! If this is the case, I expect even better results with another 1240-1.

author: TiN
author_rank: Super Contributor
timestamp: October 30, 2018, 03:06:04 pm
content: HighVoltage's data with polynom fit:

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 30, 2018, 03:20:46 pm
content: Quote from: TiN on October 30, 2018, 03:06:04 pmPolynom? But at first order?Now we have five unknown devices. Is there a mathematical way to find the best one?

author: TiN
author_rank: Super Contributor
timestamp: October 30, 2018, 03:26:34 pm
content: Yes, code for line 3:Code:p3 = np.polyfit(real3,ideal,1)pv3 = np.polyval(p3,real3)diff3 = ideal-pv3diff_ppm3 = (diff3/1.0)*1000000.0Best unknown is still uknown. Without the calibrated reference, you can only compare unknowns, but which one is closer to truth?

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 30, 2018, 03:34:00 pm
content: Quote from: TiN on October 30, 2018, 03:26:34 pmQuote from: e61_phil on October 29, 2018, 09:12:58 pmReplace "two" by nAs one can see the graphs depend on what is fitted to what and that gives completely different results. Would be interesting to find the "best" of the options.I don't agree with the calibrated reference. You can learn a lot through a step response.

author: TiN
author_rank: Super Contributor
timestamp: October 31, 2018, 04:32:13 am
content: Quote from: e61_phil on October 30, 2018, 03:34:00 pmDidn't say we can't learn anything yet. I just mean we don't know real current values, only can estimate the stability of unknowns over the currents span.I have completed some additional tests, using 4-terminal 1 ohm resistor as a shunt, with 3458A as detector on 100mV range.Sweeps are 100mA, to keep thermals reasonable, as we don't have oil bath here.I have usedthis resistor, build during livestreamfor first test. It is simpleVPR221 1 Ohm 4-wire TO-220 BMFin a cast metal box, with Low Thermal posts.Loop:a) 3458B on 100mA rangeb) F8508A, on 190mA rangec) K2002 on 200mA rangePlot using calibrator programmed value as ideal reference.Now same data, but using reading from 1ohm/3458A shunt setup as ideal reference.And repeated sweep but with IET SRL-1 standard resistors instead. Calibrator as ideal reference.Please note, SRL-1 sweep is only -100 mA to 0 mA, so it's only half of the graph above.Now same data, but using reading from SRL-1 1ohm/3458A shunt setup as ideal reference.So far I can only see that 8508 and calibrator are tracking the best, while 3458B being the worst.DIY 1 ohm also tracks well, better than SRL-1 in these coarse tests.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: October 31, 2018, 07:34:19 am
content: I ordered 0.5 Ohm Vishay Foil Resistors ‎Y14670R50000B9L‎ and Y16900R50000D9L‎ from Digikey. They claim 0.05ppm/K TC and 4ppm/W power coefficient, so in principle should be linear to 2 ppm. If the two resistors give similar results, perhaps one can trust it since they have slightly different construction.

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 31, 2018, 08:21:23 am
content: Quote from: maxwell3e10 on October 31, 2018, 07:34:19 amThat's only correct, if you were able to stabilize / fix the surface temperature of the resistor.So you have to mount it on a heat sink, and force-cool that to 25°C, see note (1) in the datasheet (of similar devices).You might as well heat it to an appropriate temperature above R.T. and stabilize / ovenize that temperature.That's exactly the remark, e61_phil has made, about the internal heat-resistance.Worst case T.C is 2ppm/K, so at 4ppm/W, this internal thermal resistance might be about 2K/W.If you just place the resistor on a heat sink, and let it warm up freely, then you'll get a much higher thermal drift, about 20 ppm, if the heat sink warms up from 25°C to 35°C (estimated) at 0.5 W P.D.Frank

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: October 31, 2018, 09:24:21 am
content: A big heat sink should have a thermal resistance of about 2 K/W. If one can believe that TC is typically less than 1 ppm near room temperature, than overall rise of the heat sink temperature should not have a big effect. Its mostly the internal temperature rise, which should be covered already by the power coefficient. But we'll see what happens in reality.

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 31, 2018, 09:34:45 am
content: If costs aren't the biggest issue you could ask wekomm. I talked to a retired PTB guy some months ago and he told me wekomm has developed very stable 1R resistor. He said with this resistor they were able to measure 1A with an overall uncertainty of 5ppm.

author: HighVoltage
author_rank: Super Contributor
timestamp: October 31, 2018, 11:57:20 am
content: Today I tested this again.This time with a different Burster 1240-1, SN: 610912Fixed current range of 1A on all DCI instrumentsSource: Keithley 2460All Instruments had warmed up for 4 hours.After changing the current, I waited 30 sec, before taking the readings.Loop:A) Burster 1240-1 Ohm on 34470 (1), 1V fixed range, 100 NPLC + Smoothing Filter ONB) 34470A (2), 1A fixed range, 100 NPLC, + Smoothing Filter ONC) DMM7510, 1A fixed range, 5 NPLC, + Smoothing Filter OND) 3458A, 100 NPLCMay be you guys can do your magic with the graphing again.

author: tszaboo
author_rank: Super Contributor
timestamp: October 31, 2018, 12:49:50 pm
content: Quote from: Kleinstein on October 30, 2018, 12:45:14 pmThermal EMF matters, because the sense leads need to be the same temperature to not create errors. Due to the physical layout it is usually not the case, also, often times sense resistors are not symmetrical in construction. It can be observed if you have a precision current source, and reverse the current on the DUT, the same current leads to different reading, while testing the same analog frontend, the offset is much smaller.Keysight in their newer meter uses this as shunt:https://www.isabellenhuette.de/fileadmin/user_upload/SMV.PDFI tested some samples (10) from Isabellehütte, against a 3458A and other high precision shunts, and they showed excellent stability, much better than datasheet spec.Measuring 1A isnoteasy, as I found (I had an error budget for a current source and sink of 300ppm over temperature, time humidity etc, few thousand was made from this). Most current shunts are low ohmic value and generate only a few mv or they have too much tempco for anything precision. If you have big budget, or it is a one off, there is always the RUG-Z, they make it up to 100Ohm, down to 1ppm, and with 250W rating self heating is not an issue. Just be careful with long term stability.It is always easy to improve current measurement of a multimeter with an external shunt. In a 1000 EUR meter, they have a budget like 3 EUR for the shunt, if you need more precision, do it externally.

author: HighVoltage
author_rank: Super Contributor
timestamp: October 31, 2018, 02:33:21 pm
content: I had the constant 100 mA running for over 1 hour and recorded the current.Here are the result.

author: tszaboo
author_rank: Super Contributor
timestamp: October 31, 2018, 02:48:45 pm
content: Quote from: NANDBlog on October 31, 2018, 12:49:50 pm

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: October 31, 2018, 03:28:02 pm
content: Here is the plot of the latest HighVoltage's data. It looks similar to before. DMM7510 has the smallest deviation, about 5-10 ppm. This time I dropped all points at -0.1 A because they were outliers.For the time dependence, it would be interesting to measure at full 1 A for maximum thermal effect. Maybe measure all the meters at the same time to compare the transient.

author: TiN
author_rank: Super Contributor
timestamp: October 31, 2018, 06:31:54 pm
content: Before going to next step, I ran also lowish current sweep, 19mA, in same setup with SRL-1 in loop.As expected, 1 ohm + DVM give very bad result due to voltage noise.a) 3458B show no problems on this span (using it's 100mA range), about +/-2ppm INLb) 8508 stayed well under +/-0.5ppm vs source.c) K2002 was within +/-3ppm vs source, using it's 20mA range.

author: TiN
author_rank: Super Contributor
timestamp: November 01, 2018, 04:44:10 am
content: If you guys tired of my colorful wiggly lines, let me know, I'll stop.First try with DCCT (green line). 3 turns for primary, 10 ohm VHP-4 as burden resistor on secondary.Setup photo, excuse the mess:

author: Pipelie
author_rank: Regular Contributor
timestamp: November 01, 2018, 06:40:54 am
content: interesting， the curve shape of SRL-1 is kind of similar to the DCCT's.I wonder which one is more close to the “truth”the linearity SPEC of LEM IT-600 is less than 1ppm as good as I remember.

author: HighVoltage
author_rank: Super Contributor
timestamp: November 01, 2018, 08:22:56 am
content: Quote from: TiN on November 01, 2018, 04:44:10 amTotal opposite, just keep going.Amazing stability of the Fluke 8508A.BTW, is that your 8508A, did I miss your post on it ?

author: MiDi
author_rank: Frequent Contributor
timestamp: November 01, 2018, 08:30:37 am
content: Quote from: HighVoltage on November 01, 2018, 08:22:56 amJust my thought

author: Pipelie
author_rank: Regular Contributor
timestamp: November 01, 2018, 08:38:52 am
content: I don't think so.He wishes but it's rental.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: November 03, 2018, 03:11:03 pm
content: Here are the data for two 0.5 Ohm Vishay Foil Resistors ‎Y14670R50000B9L‎ and Y16900R50000D9L‎, plotted against each other.They were mounted on two large heatsinks sitting in the lab and simply wired to BNC connectors for current and voltage sense.One is measured by DMM7510, the other by Keysight 34470A. We took four sweeps stepping the current up and down, with 1 minute dwell at each value. It looks pretty good, at a level of 1-2 ppm, at least up to 0.6 A.

author: e61_phil
author_rank: Frequent Contributor
timestamp: November 03, 2018, 04:44:49 pm
content: Thanks!BNC is not the best connector for DC precison.I would also measure the step response. That will give good insights and one can rule out many external effects (source INL and so on)

author: Kleinstein
author_rank: Super Contributor
timestamp: November 03, 2018, 05:51:29 pm
content: At the 1 ppm level one may also have to check if the INL of the DMM used to measure the voltage is good enough. In the low range, it is not just the ADC in the DMM, but also the amplifier that can contribute. INL in the 100 mV/200mV range can be worse than the prime 10 V range - though it's also more difficult to measure because of noise and thermal EMF.For understanding the thermal contribution to INL, it might be worth to measure the thermal resistance (temperature rise on current) and the TC of the external shunt. For the temperature rise it is of cause only the case that can be measured, but that would at least take the heat sink out of the unknown.As the thermal effect gets larger with more current (expect I³ for the TC related effect), one could extend the test to slightly larger current than one would normally us the shunt for a precision measurement. For the 0.5 Ohms resistors from Maxwell the 0.6 A test current is already such a case - this would be a 300 mV burden and thus more than normally used for high accuracy in this current range. At half the current it would be only 1/8 the error to be expected (for heating and TC).