Precision measurement of current drift at 10A+ - Page 1
Forum section: electronics / general

author: Har
author_rank: Newbie
timestamp: May 03, 2019, 12:10:49 pm
content: Hi all,looking for a little advice about measuring current. I'm new to these kinds of precision measurements so hoping for pointers / pitfalls / schoolboy errors etc.I'm building a 0 to 20A current sink (electronic load) and would like to measure the stability of the current as precisely as possible. Idealy to 1ppm, but i'm aware thats a little tricky. I need to measure turn on stability, as well as drift over time up to several days. not so woried about absolute accuracy.so far i've got a 10mR shunt (Y09590R01000F0L, 10ppm/C) mounted on a heatsink+Fan, connected to a Keysight 34461A.here are my observations so far:1 - the self heating of the shunt seems fairly small, I measured a drift of about 1uV over 5 minutes with 5A going through it. (I let the electronic load warm up first then quickly switched the shunt in the circuit) so thats about 20ppm, but some of it will be because i've turned off the load for a second or 2 to put the shunt in the circuit.2 - there is however a strong dependence on ambient temperature of the shunt, a short spray from some canned air caused the reading to jump significantly.so Questions:1. whats the best way of maintaining a constant temperature on my shunt resistor? I have been looking at TEC modules and controllers, but they dont seem to say anything about time response - i'm guessing they take a couple of seconds to stabilise so i'm not sure its going to work. i'm also not sure how well this will deal with temperature changes on the non-cooled side of the resistor. should i just stick the resistor in a box with loads of polystyrene?2. what can i trust for the longer term? the keysight meter datasheet looks like it says it will drift 60ppm over 24 hours and 75ppm over several days even if the ambient temperature is kept constant (100mV measurement). do I need to buy the 3458A? is there something better?3. whats the best way of validating my measurement? is there a reference lab standard current source in the 10A range?4. what am i not aware of? what would you do? how good can i get it?thanks very much!

author: HighVoltage
author_rank: Super Contributor
timestamp: May 03, 2019, 12:29:35 pm
content: If you want to go in to ppm level current measurements, you need to have a much better current shunt.May I suggest a German madeIsabellenhütte RUG-Z-R001Shunthttps://www.isabellenhuette.de/fileadmin/user_upload/RUG-Z.PDF

author: Har
author_rank: Newbie
timestamp: May 03, 2019, 12:57:55 pm
content: Thanks HighVoltage,I've used this part in the design of the electronic load as it was the best i could find. I didn't choose it for the measurement because it seemed like the major challenges were the meter and measurement technique. the 3485A has 0.6ppm per day, so was thinking if i can controll the 10ppm/C shunt to 0.1C we're in about the same ballpark.however, point taken - i'm aiming for as good as i can get so it makes sense to use the RUG-Z so long as its not dis-proportionately better than the rest of the measurement.

author: HighVoltage
author_rank: Super Contributor
timestamp: May 03, 2019, 01:06:21 pm
content: Precision current measurements are much more difficult than I had anticipated, when I started.You might want to read through this thread:Low ohm precision resistor standard and testinghttps://www.eevblog.com/forum/metrology/low-ohm-precision-resistor-standard-and-testing/

author: Echo88
author_rank: Frequent Contributor
timestamp: May 03, 2019, 03:07:57 pm
content: May i suggest using a DCCT + suitably stable shunt at the 400mA-DCCT-output, if best stability is your objective?For example a 600A Ultrastab from ebay for about 100 bucks, 30 windings around it give 20Amax measurement range and then using a very good shunt on the 400mA-output-side like the VCS332 or VHP04-series.https://xdevs.com/review/ultrastab_review/http://www.vishaypg.com/docs/63240/vcs-vfp-csng.pdfThe DCCT itself is very linear and has high stability, while providing a way to use a lower wattage and therefore higher stability shunt.The DCCT-solution should maintain a very good stability compared to a 20A-shunt like the RUG-Z i suppose.User nandblog also did some drift-measurement of the rug-z-series somewhere here on the forum.

author: HighVoltage
author_rank: Super Contributor
timestamp: May 03, 2019, 03:33:53 pm
content: It would be really nice to see a comparison between a RUG-Z and a Ultrastab DCCT for linearity and stability.Has anyone done such a test?

author: mzzj
author_rank: Super Contributor
timestamp: May 03, 2019, 03:37:04 pm
content: I was also thinking of 600A DCCT and several turns of primary winding.Danfysik DCCT's are quite low noise so you probably can get to around 1ppm resolution even with 10 turn primary, further reducing the secondary current and shunt resistor requirements.

author: mzzj
author_rank: Super Contributor
timestamp: May 03, 2019, 03:51:01 pm
content: Quote from: HighVoltage on May 03, 2019, 03:33:53 pm"It's complicated" in case of DCCT.They are somewhat sensitive to external magnetic fields and even if you gain more sensitivity by using multi-turn primary the sensitivity to external magnetic fields stays the same. (or becomes worse relative to max current depending which way you think of it)In lab setup where you can mount the DCCT to fixed position, keep unwanted stray magnetic fields out of the way and keep the high current cabling around it in fixed position you can control lots of the gain error and bias error.Danisense has bit more specifications than the old Danfysik datasheets:http://www.danisense.com/images/pdf/0-600A/DS50ID.pdf

author: Har
author_rank: Newbie
timestamp: May 03, 2019, 04:54:39 pm
content: Thanks all, lots to think about here.i dont mind about absolute accuracy as the thing will be tweaked to the right value when its in use. i care about it maintaining that value for as long as possible. i also need to know the switch on / warm up curve, (ideally so that i can compensate for it, but if not then atleast charachterise it and measure the repeatability)I had avoided the Ultrastab parts because of this thread:https://www.eevblog.com/forum/metrology/pcr-versus-tcr/in particular, Zeranin writes:"I have placed a Danfysik current sensor in series with the 16A current driver output, and was horrified to find that the measured current drifted by 200ppm (not a misprint) over the first 300 ms after a step current change, then overshoots, then converges to an approximately stable value. I contacted the manufacturer, who at first blamed my measurements, but after a month of email communication, were eventually able to independently verify that I was correct. To this day there is no mention of this imperfection in their data sheets though. The technical term that describes this effect is 'settling time'. "so it would be useless for my turn-on measurements.The RUG-Z on the otherhand appears to drift significantly over a long time (0.2%ppm per 2k hours). so maybe i need both to fully charachterise the load.I've just done another self heating / stability run on my measurement setup (pic attached). I let the load thermally stabilise then switched in the shunt+meter.this shows drift over 1hr of about 1uV (span 1.5uV includes the measurement noise). can I claim 20ppm over 1hr from this data or should i allow that things may have drifted much more in opposite directions?lets say i get a 10mR 1ppm/C RUG-Z, strap it to a huge heat sink and put it inside a metal box, and measure the voltage using a 3458A (0.6ppm). lets say the ambient temperature is +/- 1C, and the RUG-Z heatsinking is god enough to keep it within 1C of ambient. is this enough to know the current to 2.6ppm error over the course of 24 Hours?Thanks again

author: Echo88
author_rank: Frequent Contributor
timestamp: May 03, 2019, 11:27:06 pm
content: "Turn-On-Stability" is overshoots and the like i assume: switch in a normal simple shunt like the RUG-Z to measure the turn-on with sufficient accuracy and stability to judge the turn-on, after that you could use a DCCT for better long-term-stability compared to RUG-Z?I can be totally wrong here (metrologists please step in if i write bogus) but would calculate it the following way with your 10mR RUG-Z 1ppm/K:Errors:3458A 100mV range (120mV max) 2.5ppm + 3ppm (reading and range) in 24h-spec+3458A Temp-variation 0.15ppm + 1ppm (reading and range with ACAL and 1K variation)+12A²*10mR = 1.44W (12Amax to use 120mVmax-range) 1.44W*0.1K/W = 0.144K resistor-temp over ambient -> 0.15ppm-> Worst case error over 24h 5.5ppm + 1.15ppm + 0.15ppm -> ~7ppmhttps://www.isabellenhuette.de/fileadmin/user_upload/RUG-Z.PDFhttp://literature.cdn.keysight.com/litweb/pdf/5965-4971E.pdfWhere did you get the 0.6ppm-spec for the 3458A, thinking of the 10V-24h-spec?

author: Har
author_rank: Newbie
timestamp: May 07, 2019, 10:19:09 am
content: Sorry Echo88, i took 0.6ppm from the datasheet front page. should have looked deeper into the document.yes okay, so "ppm"s add linearly, and we have ~ 7ppm over 24 hours, or 8+1.15+0.15= ~9.5ppm for 90 days.what does the note 4 mean when it says > 4 minutes? is it saying that the input signal needs to stabilise for 4 minutes before the 24hr spec is met?by "turn-on-stability" what i mean is that i want to measure the drift during turn on, and correct for it. the way i see it, there are 3 separate effects which are on 3 separate timescales - please correct me if i've misunderstood something.starting with a square edge on the demand signal:1. circuit settling time (< 1s). - time for the capacitors int he circuit to charge to steady state, and for any I terms in the PID loop to take control. this is pretty quick and dependent on the bandwidth of the control circuitry, though can take exponentially long for the last few ppm.2. Thermal stabilisation (1s to minutes). - mainly drifts due to the self heating of the parts and circuit.3. Environmental effects (on going). temperature variations in the ambient, presumably also electrical and magnetic interferance, humidity, air pressure, quantum gremlins etc.most talk about stability seems to focus only on stage 3 (note the foot note in the 3458A about 4 hours warm up), but for my application stage 2 is important as well.Its a slightly different kettle of fish to stage 3, since its somewhat repeatable if you know the history of the circuit. this means that even if you can't make it small enough to ignore, you can correct for it by adjusting the input demand. this does however mean you need to be able to measure it accurately.are there any other gotchas? will thermal emf be an issue with the RUG-Z setup i described?~~~~~~as an aside, since the 10V spec is so much better than the 100mV spec, is it worth gaining up the signal and measureing it on a different range? some AD/LTC parts claim offset drift as low as 2nV/C, so with a gain of 100 we get a 10V signal with a drift of 200nV / 10V = 0.02ppm/C? the real drift comes from the resistors then, but we can get matched arrays to 0.2ppm/C. This additional drift is more than made up for by the additional stability of the 10V range. if its that simple, why didn't HP put it in the units?Thanks

author: David Hess
author_rank: Super Contributor
timestamp: May 07, 2019, 08:15:20 pm
content: The current shunt is going to limit stability.I do not think there is anything to be done about transient response errors from temperature changes; you will just have to accept short term variation.Since the temperature will always be higher than ambient, instead of using a TEC, I have had good results using a PID controlled variable speed fan to regulate the temperature. The trick then becomes measuring the temperature in the best spot for stability. Placing the temperature sensor on the opposite side of the current shunt (Kelvin temperature measurement?) from the heat sink is a good place to start.Another idea I have thought about but not tried is to inject a high frequency current into the shunt and measure the voltage change after synchronous demodulation to measure the shunt resistance so that it can be compensated out. Some bench multimeters use this method to make in-circuit current measurements.

author: jbb
author_rank: Super Contributor
timestamp: May 14, 2019, 04:10:15 am
content: The injected current idea is quite cool. I wonder how much is required and how much will leak out into the EUT.I guess that with a few nV/rt(Hz) noise in the op amp and demodulator, a 1 Hz BW gives a few nV noise. Does that mean a few mV of primary voltage are required to detect 1 ppm above the noise? And therefore a few hundred mA excitation signal? Or squeeze the BW down well below 1 Hz and accept slow updates?I was also wondering about a different approach a while ago. Does it make any sense to use 2 shunts as follows?- have a ‘crude’ 10mOhm shunt which is normally in circuit and subject to self heating- switch the ‘precise’ 10mOhm in occasionally for a precision measurement without lots of self heating

author: David Hess
author_rank: Super Contributor
timestamp: May 15, 2019, 12:10:44 pm
content: Quote from: jbb on May 14, 2019, 04:10:15 amNoise is actually much lower so the excitation can be very small. Since the excitation is known, synchronous demodulation can be used to produce a very narrow bandwidth.QuoteIf you were going to switch the shunt, then you could just as easily reverse it producing an AC measurement. Now you can use the same trick above and synchronously demodulate the output. With the much lower noise, a much lower shunt resistance can be used so there is less self heating.

author: splin
author_rank: Frequent Contributor
timestamp: May 15, 2019, 04:18:50 pm
content: An alternative is to heat the shunt; the absolute temperature isn't important as you can said you can compensate for a fixed offset. Ideally you need to operate the shunt at constant power to keep its temperature constant. You could use an AC current through the shunt and filter out the AC voltage from the measurement. The AC current is reduced proportionately as the measured current increases.It may be easier to use a seperate DC heater as tightly coupled to the shunt as possible. To minimise the shunt temperature change under load, keep the shunt + heater power constant. IE. if the heater has the same resistance as the shunt, for 5A max measurents then drive the heater so that Ishunt + Iheater = 5A. Obviously you need to let temperature stabilise before using the shunt. Operate the whole in a temperature controlled oven if the room tempersture varies too much over the duration of the measurements.This has a big advantage over trying to measure the shunt temperature because it is much faster due to the thermal mass of the shunt/packacking/heat-sink - the shunt temperature will rise long before you see the temperature rise at the surface of the shunt's encapsulation.It won't be perfect as the heat flows will be slightly different from the heater compared to from the shunt element itself, but it shouldn't be too hard to measure. You could try modelling the heat flows with one of the free packages such as THERM:https://windows.lbl.gov/software/thermThe interface is a bit quirky and the free version has some serious limitations but it is still pretty good.For the best precision you could make your own shunt using bifilar wound insulated resistance wire, one of the windings being the heater.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 15, 2019, 08:30:45 pm
content: Quote from: jbb on May 14, 2019, 04:10:15 amThe BW for the measurement is kind of fixed by the thermal time constants. One should not average more, as one can than no longer compensate for temperature / power changes.One can however do things about the few nV/sqrt(Hz). Special AC amplifiers can be well below nV/sqrt(Hz), especially if a transformer is used and one only wants to correct against a second reference shunt for a few 10 mAs. Things could still be tricky, like using 2 shunts in parallel and changing the current sharing only.

author: tggzzz
author_rank: Super Contributor
timestamp: May 16, 2019, 01:11:10 pm
content: Strange machine with a "repair opportunity", whichmightconceivably be relevant.Looks like it might contain a 7-decade KVD.https://www.ebay.co.uk/itm/Guildline-9975-Current-Comparator-Resistance-Bridge-Galvanometer-115-230V-50VA/173903985031

author: Har
author_rank: Newbie
timestamp: June 20, 2019, 10:48:02 am
content: 1 month on, i have a RUG-Z 10mR 1ppm/C, an HP3458A on order with the 002 option, and a big heat sink 0.6K/W.so self heating of the shunt should be of order 1W, and we have 0.1k/W + 0.6k/W + thermal paste maybe 1k/W total = 1K temperature increase = 1ppm drift.ambient temperature should be regulated to 1C, so we have total resistance change of about 2ppm. this is well below the 7 - 10ppm we get from the meter.injected current is a nice idea, but it seems like added complexity and i dont fancy the idea of measureing resistance to 1ppm using an AC coupled signal. i've also got the option of using a TEC to control the resistor case to 0.05C, so we then only have to worry about the 0.1k/W of the resistor body. for a total dR of 0.15ppm, but this wont work with before the TEC PID loop settles.that said, i'm still thinking about adding a zero drift op-amp to up the 100mV to 10V so i can use the best specified range of the 3458A. its a gain of 100, so with eg OPA189 i get 500nV/C drift (typical) on the output = 0.5ppm/C (plus any bias current drift but thats measured in pA), plus the gain resistor drift, but i can get 0.2ppm/C from LT4500 if i can find a way to make a gain of 100 from the available values.lets say i do a really bad job and end up adding 4ppm to the measurement with my circuit, this still brings my meter accuracy down to 0.55ppm (10V range) from 5.5ppm (100mV range), so i've gained 1ppm net. what have i missed here?Part 2 - LEM ultrastab.i'm worried about the long term shelf-drift of the RUG-Z parts, so i'm also going to get one of these LEM current transducers (IT-65S). however, getting the burden resistor seems almost as difficult as getting the current shunt. LEM tech support advised that i should aim for 10V or so burden voltage resulting in about 100R, otherwise the DCCT has to drop that voltage internally and self heats, but this brings me back to about 1W of power at 100mA secondary. now i could find a 100R RUG-Z, but isn't that going to have all the same problems as the 10mR one? thereby making the introduction of the LEM pointless.what burden resistors are normally used with the ultrastab things?thanks.

author: HighVoltage
author_rank: Super Contributor
timestamp: June 20, 2019, 11:54:40 am
content: Quote from: Har on June 20, 2019, 10:48:02 amI am not sure what is normally used but I have been using a German made Burster calibration resistor (Model 1240) with my LEM.Looking forward to you measurements results.

author: Echo88
author_rank: Frequent Contributor
timestamp: June 20, 2019, 01:33:59 pm
content: IT-65S:Secondary current at 10Aprim: 10A / 600 (conversion factor) = 16.66mA.Necessary secondary shunt to get 10V at 10Aprimary: 10V/16.66mAsecondary=600RI assume the LEM can work with the 600R at 10Aprimary (deduced from the steeper geeting graph on page 5, which doesnt go down to 10Aprim), producing the 10V.The following is assumed:A 100R-resistor has usually much better long term drift, less PCR and TC than very low value resistors despite being used at 1W load.Suitable types from Vishay:http://www.vishaypg.com/docs/63005/vhp3vhp4.pdfAside from that:Here is a nice paper which might be relevant:https://doc.xdevs.com/doc/_Metrology/wang2014.pdfAnd taken from the references in said article:https://ieeexplore.ieee.org/document/536703With a more practical approach and nice resistor/Manganin-details.

author: tszaboo
author_rank: Super Contributor
timestamp: June 20, 2019, 01:51:14 pm
content: Quote from: Har on June 20, 2019, 10:48:02 amRightly so. They specify 0.2% for 2000h at nominal load, but even at smaller loads it doesnt go exponentially down. I had these loaded with max 5W and they did significant drift over a year. It was used as a calibration resistor (transfer standard), used frequently for short time, moved around, and had more than 100ppm drift. It warranted a recalibration of other equipment. But this was within spec of Isabellenhütte.But they are still probably one of the best high current resistors on the market.And about the amplifiers, dont forget the thermal EMF. That can easily be a huge error in your measurement. You will have thermal EMF in your amplifier design, and on the lead of the shunt. I've corrected a (PCB) design, which had 10s of PPMs of errors on the current shunt, just because the area around the shunt wasnt symmetric.

author: jbb
author_rank: Super Contributor
timestamp: June 21, 2019, 07:54:23 pm
content: On LEM burden voltage: you can use a lower value precision resistor for the I->V conversion, and then simply add more series resistance separately.

author: Har
author_rank: Newbie
timestamp: July 24, 2019, 03:07:16 pm
content: Hi All, Thanks for staying with me so far.i've put the LEM thing on hold for now, because i'm just trying to tackle one thing at a time. (i the LEM transducer, but the VHPx burden resistors are on a huge lead time and i can only seem to find grey market ones of non ideal values.) i will get back to it if the project deadline allows.in the meantime, i've got the RUG-Z and the HP34568A, but there seems to be something a-miss with my measurement technique.perhaps somebody can shed some light?the first thing i did was to connect it up loosely to test the measurement method (using a couple of foot long banana plug to croc-clip cables). I noticed it was drifting all over the place compared with what i expect, so i replaced these cables with twisted wires and a dual banana plug. it behaved much better, so i put the bad wires back in and saw the same kind of wobbling. pictures and graph attached.traces 1 and 3 (blue) are the croc clip -> banana plug arrangements (bad_wires.jpg)trace 2 (grey) is the dual banana and soldered to the RUG-Z (good_wires.jpg)so, can this be explained by the thermal EMF of the wires?as i understood it, so long as the paths were symetrical and the equivalent junctions between dissimilar metals were kept at the same temperature, then all was okay? the room is fairly stable temperature wise, the resistor itself heats up less than 1 degree, the DUT heats up by a few degrees, but its blown away from the resistor by a fan.if it is Thermal EMF, how can i trust my "good" wiring? the middle run seems much slower drift than the other 2, but its still shifting by 1uV (10ppm) over about 15 minutes. the temperature of the base of the RUG-Z as measured by a PT1000 changed by about 0.8 degrees over the second run, so i would expect the RUG-Z shunt to drift by less than 1ppm. the 3458A is specified as 1.2ppm of reading + 1ppm of range /C, so thats about 3.2ppm total error for the measurement if the meter shifted by 1C. ( a generous estimate since the resistor which is actually self heating shifted less than that)so the drift in run 2 could be my DUT*, or a subtler version of whatever caused the haywire measurements in run 1 and 3, or something else?*i dont think it is my DUT, but if i knew for sure i wouldn't need to do this test!do the RUG-Z resistors need burn-in time? its had about 2 hrs of 10A so far over the course of the afternoon, but aside from that its unused.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 24, 2019, 04:36:18 pm
content: You improved your sense wires nicely.Next I would improve the force wires.- Use pure copper lugs instead of these cheap automotive lugs.- Try to use high quality cable like LiFY copper cable- Use copper screws or high quality screwsHow does the other end look like of your DUT and connections?Can you share your DUT setup and purpose?What current source do you have for the 10A?My RUG-Z did not need any burn in time at 10A.

author: Har
author_rank: Newbie
timestamp: July 24, 2019, 05:38:20 pm
content: Quote from: HighVoltage on July 24, 2019, 04:36:18 pmThanks! what is it about the force wires which will affect my measurement? i thought the point of 4-wire connections was to remove the force wires influence?i will try to find some better connectionsQuote from: HighVoltage on July 24, 2019, 04:36:18 pmthe DUT is a current sink, which is basically made of a DAC, a op-amp, a fet, and a resistor (another RUG-Z).its purpose is to drive a stable current pulse (can you call it a pulse when its on time is measured in hours?) through a load.my test set up is something like the attached diagram.as far as the actual physical connections, the FET drain is on a PCB, there is a front panel with some 4mm banana sockets and then the wires to the RUG-Z resistor as shown (with another set of banana plug-sockets near the resistor) in the previous posts.the V+ from the power supply is actually routed through the unit, so both sides of the resistor force wires are symetrical untill it gets to the PCB, where one goes to the drain of the fet, and the other crosses the pcb and returns to the bench supply along with the power ground for the DUTI can share more, what specifically are you looking for?

author: e61_phil
author_rank: Frequent Contributor
timestamp: July 24, 2019, 06:25:50 pm
content: Why do you expect, that your current source is more stable than your reading on the 3458A?Is the resistor in the current source the same as the shunt on the 3458A?If you already trust your current source, you could try to reverse the current polarity on your shunt frequently to cancel out thermal voltages. (That should also work without a trusted current source)

author: TiN
author_rank: Super Contributor
timestamp: July 25, 2019, 01:31:37 am
content: I also wouldn't suggest placing heatsink on top of DMM front end, try to keep them further apart? I'd avoid use of bananas altogether, just shove clean copper ends into 3458A posts and lock them tight. One way to estimate thermal EMF error - perform measurements in two polarities at 3458A, so you can see how much is contributed by the TEMF (difference between readings), given that your particular 3458 is stable, not drifty ADC and have typical linearity.

author: Har
author_rank: Newbie
timestamp: July 25, 2019, 10:43:34 am
content: Quote from: e61_phil on July 24, 2019, 06:25:50 pmits a good question, and i might be wrong (as i said i have no experience with precision stuff untill this project). I thought I had some results which indirectly measured the DUT stability, but some of the data looks a bit off on closer inspection. i will try to re capture the graphs and present my case.the DUT uses the same brand resistor as the shunt (RUG-Z) but a different valuetheres been a few different suggestions about swithing polarity, or switching between 2 different shunts, or sending AC through the load and using it to measure the resistance. i've generally avoided this because i feel like any switching thing will de-stabilise the measurement. can you buy modules which will do this or would i have to make my own? are you talking about switching the actual current direction or just the sense wire connections to the DMM?Quote from: TiN on July 25, 2019, 01:31:37 ami guess that removing the plugs is to reduce the number of metal to metal contacts?I will try experimenting with the placement of the heat sink, though i had assumed that the priority was short wires. I dont really understand what the biggest sources of error are in this measurement, so am struggling to optimise the setup.

author: e61_phil
author_rank: Frequent Contributor
timestamp: July 25, 2019, 11:01:57 am
content: To cancel the thermal voltages in the measurement I wouldn't touch the sense wires and reverse the polarity of the current through your shunt instead. Take care that the polarity reversal doesn't open the circuit and interfere with the current regulation of the source. Depending on how low impedand your circuit already is, it could make sense to increase the impedance a little bit to decrease the effect of shorting the shunt while polarity reversal.These are only ideas, I haven't done such things at high currents.Some time ago I measured a 60µR shunt with 1A (the highest current I could deliever with good stability). I used my Keithely 182 Nanovoltmeter and a Burster 1R Shunt as reference. That worked quite well with polarity reversal. You want to measure the current and not resistance, but that should work in the same way.I also measured a RUG-Z (2mR) with current reversal and used a 3458A and a 8508A to measure current and voltage while a Agilent Source meter delievers +/- 1A. That gives a standard deviation of ~14ppm due to the not ultra stable current from the SMU. These days I would use a better current source to measure that (Fluke 5730A for example). Want I want to say is: It might get hard to be much better than that what you already achieve.

author: JohnG
author_rank: Frequent Contributor
timestamp: July 25, 2019, 05:07:18 pm
content: Perhaps this document from Ohm-Labs might be useful:http://www.ohm-labs.com/pdfs/Shunt%20Calibration.pdf.I have also heard of errors due to Peltier effect (inverse Seebeck) effect. Each end of the shunt has two dissimilar junctions. If we call them left and right, and say the current flows from left to right, the right junction has both the current and the junction reversed from the left. If only one of the current or junction was reversed at the right-hand junction, then the two junctions would either pump heat into or out of the shunt, depending on the materials of the shunt and the terminations. However, since both the current and the junction is reversed, the two junctions will force heat to flow either left or right through the shunt element, depending on the direction of the current flow. This can drive a temperature difference between the two ends of the shunt. Unfortunately, I can't find a reference, although I am sure I have seen one at one point or another.I have no idea how strong this effect may be.Hope this helps,John