uCurrent not acurate on nA range - Page 1
Forum section: electronics / testgear

author: cow
author_rank: Newbie
timestamp: June 01, 2015, 05:03:19 pm
content: So just received my uCurrent, I was very excited to measure how much my battery powered device really consumes in sleep mode. But before that I decided to test it on resistors. Here is my testing setup:1AA battery half full, happened to be about 1.064V great, makes calculations easier and set of 100k, 1M, 10M resistors.I created the test circuit and measured the current with my multimeter on uA range. I got expected results.Then I measured it using the uCurrent on uA range, got about the same results, so far so good.Then I switched to the nA range and when I measured current through 1M resistor I got about 770 nA, but it should be around 1000nA, when I measured the current through 10M resistor I got about 33nA, should be 100nA which is waaaaaaaay outside the specs.Is this expected behavior? Did I get a faulty unit? Is my methodology faulty? There was some oily liquid coming from the switches on uCurrent, I'm not talking about flux on the other side of the board, I'm talking about the tiny gaps in the switches, some liquid is oozing from it. Could this interfere with my measurements? Can you advice me how to trouble shoot this?

author: slurry
author_rank: Regular Contributor
timestamp: June 01, 2015, 05:10:06 pm
content: What about your cables and connectors? are they adequate and clean enough?it is very easy to loose some nanoamps along the way

author: pascal_sweden
author_rank: Super Contributor
timestamp: June 01, 2015, 05:35:24 pm
content: You should check with the designer of the uCurrent device

author: slurry
author_rank: Regular Contributor
timestamp: June 01, 2015, 05:41:49 pm
content: Quote from: pascal_sweden on June 01, 2015, 05:35:24 pmhmmm... i wonder if he is a member of this forum..?

author: pascal_sweden
author_rank: Super Contributor
timestamp: June 01, 2015, 05:51:10 pm
content: Apparently the OP's unit is not hunky-dory

author: MarkL
author_rank: Unknown
timestamp: June 01, 2015, 08:59:23 pm
content: You could try switching your DMM to read AC volts to see if something is oscillating or if you're picking up a lot of noise because of the high impedance of your test circuit. It should be close to zero if everything is fine.

author: tautech
author_rank: Super Contributor
timestamp: June 01, 2015, 09:34:12 pm
content: What?Battery is FLAT, ANY current draw will cause the battery volage to collapse.Try again with a decent battery.

author: wraper
author_rank: Unknown
timestamp: June 01, 2015, 09:51:44 pm
content: Quote from: tautech on June 01, 2015, 09:34:12 pmObviously not with 1 Megohm load.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 02, 2015, 01:16:19 am
content: Quote from: wraper on June 01, 2015, 09:51:44 pmYou would be surprised.

author: MatthewEveritt
author_rank: Unknown
timestamp: June 02, 2015, 09:44:11 am
content: Quote from: tautech on June 01, 2015, 09:34:12 pmI agree it's probably sensible to try with a better voltage source, but if that's the cause wouldn't you expect it to be better at 10M, not worse?

author: Psi
author_rank: Super Contributor
timestamp: June 02, 2015, 10:20:57 am
content: maybe you are running into the issue of dissimilar metals in your wiring creating a voltage and upsetting the current?

author: ElektronikLabor
author_rank: Regular Contributor
timestamp: June 02, 2015, 11:28:42 am
content: Did you measure the Voltage directly across the input of the µCurrent?The internal shunt is 10k and should result a voltage drop of 1.0mV, if the real current is 100nA; otherwise 0.330mA if the current is in fact 33nA.EDIT: I'm talking rubbish: what I said here does'n make sence2nd try: Did you measure the nA shunt resistor? It should be 10k.

author: KedasProbe
author_rank: Frequent Contributor
timestamp: June 02, 2015, 12:20:44 pm
content: Did you add any extra multimeter to the µcurrent device to check it? (and forget its influence?)

author: Kleinstein
author_rank: Super Contributor
timestamp: June 02, 2015, 01:35:39 pm
content: If the battery is a simple dry type (not alkaline), than the battery can well be the problem - it's really dead then. With an alkaline battery down at 1 V you may still draw 100 mA, so not a problem.

author: EEVblog
author_rank: Unknown
timestamp: June 02, 2015, 01:48:49 pm
content: People think measuring nA is trivial, it is not, many things can influence the measurement.Have you measured the 10k sense resistor on the nA range (with switch on another range), I'd be incredibly surprised if it's anything other than exactly 10k.

author: EEVblog
author_rank: Unknown
timestamp: June 02, 2015, 01:49:58 pm
content: A battery is a poor voltage source, particually afer being loaded, as the battery voltage will rise back up slowly after being loaded.

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 02, 2015, 02:02:51 pm
content: The description of the measurement circuitry is mediocre!Maybe the TO may offer a schematic or better: pictures for the different setups?Anyhow, if I assume right, and if the TO first has put the 1V in series with 100k, that would produce 10µA.If the DMM then read about 10µA, and so did the µCurrent adapter, the battery would not be the root cause:If 1M and 10M were put in series, then the 1µA and 100nA currents could not flatten the battery more, than a 10µ had done!Therefore, something else went wrong, maybe the TO added something in the current loop, which has additional resistance on the order of 300k for the 770nA, and 20M for the 33nAFrank

author: wraper
author_rank: Unknown
timestamp: June 02, 2015, 02:06:40 pm
content: Quote from: XFDDesign on June 02, 2015, 01:16:19 amThat voltage would drop by 70% to cause what OP sees?

author: MarkL
author_rank: Unknown
timestamp: June 02, 2015, 02:14:07 pm
content: I think the OP is introducing a lot of noise into the uCurrent and probably hitting one or both rails on the opamp. It's clipping the noise.That's why I wanted to see an AC voltage reading.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 02, 2015, 04:05:15 pm
content: Quote from: wraper on June 02, 2015, 02:06:40 pmWhile battery cells may be thought of as "voltage sources," they do in reality have finite source resistances. One particular way to model the issues involved, is to consider the fact that1. current is the flow of charge per unit time (dq/dt).2. with a perfect zero-flow, a static quantity of charge can produce a particular EMF3. If you measure static EMF and then work to flow an available charge, some amount of work is dissipated as heat.So suppose the battery is practically discharged, yet still reads 1.000V some how. If there is only a means for about 500nC/s of charge to flow, the presence of 1Mohm will get you a change in voltage reading down to 0.5V. Incidentally, this infers that the source impedance is on the order of 1Mohm anyway. Then consider the fact that the means of transmission is through dissimilar metals in the drycell, but as the thing ages, the electrodes get consumed. What might have been 1m^2 of contact surface may now be only 1mm^2, this too would be a form of source-resistance increase. There are many mechanisms which make a used battery a piss poor choice of standard.Another way to think about this, is someone buying a 9V battery and sticking it on a $4k Keithley 7.5 digit DMM and bitching that the Keithley is a POS because it measures the battery at 9.2105V and not 9.0000000V.

author: wraper
author_rank: Unknown
timestamp: June 02, 2015, 05:25:10 pm
content: Quote from: XFDDesign on June 02, 2015, 04:05:15 pmA lot of "smart" words. How this is relevant to this particular case? Please show me a battery which increases it's internal resistance by an order of magnitude with increasing the load resistance. But anyway please show me a real battery which shows 1V on multimeter (with its internal resistance) and then voltage sags by 30% under 1M load and by 70% under 10M load. I've seen batteries with very high Internal resistance BTW.So please read again and explain this behavior with internal resistance of the battery:Quote from: cow on June 01, 2015, 05:03:19 pm

author: djQUAN
author_rank: Frequent Contributor
timestamp: June 02, 2015, 05:38:06 pm
content: I haven't seen anyone mention it but.... why not wire the DMM or whichever ammeter you were using in series with the uCurrent?That way, they both measure the current at the same time. Ruling out differences between burden or other residual voltages that can affect the overall reading.

author: dom0
author_rank: Super Contributor
timestamp: June 02, 2015, 05:50:56 pm
content: Quote from: djQUAN on June 02, 2015, 05:38:06 pmMost DMMs are not accurate to <.1 µA

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 02, 2015, 07:44:26 pm
content: Quote from: wraper on June 02, 2015, 05:25:10 pmIf "smart words" are a bit much for you, then how about something remedial:The amount of charge left on the battery at his first test is unknown. Apparently you can tell us. Please do.The amount of charge left after his second test is now different and still unknown, again, you can tell us.Once we're on to the third test, what's left on a dead battery is still unknown and is different still. If the source impedance is high, after adeadbattery has given all it can, then it is perfectly reasonable that even 1Mohm will show up a 30% sag. So for all of your "please show me," tell you what, send me the battery and I'll do the measurements. You were the first to make an assertion, the onus is on you to back it. With your response, maybe you can show us all a perfectly linear battery which happens to never change it's source impedance as it dies. If not, sod off.

author: wraper
author_rank: Unknown
timestamp: June 02, 2015, 08:03:18 pm
content: Quote from: XFDDesign on June 02, 2015, 07:44:26 pmYeah, now take a battery with high internal resistance. Then you will notice than you won't be able to discharge it fast because of high internal resistance and very soon voltage will return to the previous level. Moreover something like 1M internal resistance won't happen with AA batteries. Though it can be present on very old lithium batteries like CR2032, they can hold most of the capacity at the same time regardless of this.

author: tautech
author_rank: Super Contributor
timestamp: June 02, 2015, 08:39:11 pm
content: Quote from: cow on June 01, 2015, 05:03:19 pmcow is quoting a load of bull.What AA battery @ 1.064 V is half full.Manganese, Alkaline, NiCad, NiMh.......they're ALL FLAT.....now can anybody get a measureable, repeatable result with that.New battery or a precisely set low drift PSU will at least take a MAJOR variable out of the equation.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: June 02, 2015, 08:51:30 pm
content: Would another dmm set to a high resistance range make a good current source?

author: SaintGimp
author_rank: Contributor
timestamp: March 01, 2016, 09:44:41 am
content: Quote from: MarkL on June 02, 2015, 02:14:07 pmI'm resurrecting an old thread here but I just wanted to say that MarkL is exactly 100% correct. I had precisely the same problem as the OP - uA range readings seemed to be ok, though a bit lower than I expected. nA range readings were less than half of what I expected and wandered around noticably. I spent quite a long time studying the schematic and measuring the behavior of the opamps directly. U1 seemed to be behaving ok even on the nA range but U4's output was just not right at all.I was just about to order new opamps and replace the "blown" U4 when I read MarkL's post and checked with an AC measurement rather than DC. 1.4V! Aha, that's the maximum possible output range. Smells like oscillation with clipping. Checked on the o'scope and yup, that's what was going on. So U4 wasn't blown, but what was causing it to oscillate so badly? Well, it's oscillating at 60hz. Oh, right. In the nA range all my cables are antennas. After much fiddling with cable configurations I came to the conclusion that (at least with my equipment and environment) that any significant length of cable that connects to Current+ is going to pick up huge amounts of noise which gets amplified 100x and clips the output. The only halfway-reliable way to measure in the nA range is to put my uCurrent on the low side of my device with as short of a wire between the device and the Current+ post as possible.It's (probably) easy to replicate this. Grab a 10M resistor and set your uCurrent to nA. Attach your resistor directly to the positive binding post on your power supply then run a patch cable from the resistor to the Current+ post on the uCurrent, and another patch cable from Current- back to power supply ground. Dial up 5V on the power supply. If you check DC voltage on the uCurrent voltage output you'll see something like 150mA to 200mA rather than the 500mA you should see. If you check AC voltage you'll see ~1.4V.Now simply swap the resistor and cable so instead of the resistor being attached to the power supply binding post, it's attached to the uCurrent Current+ binding post. You'll find that the DC voltage reading magically becomes correct at 500mA. That's because there's not much of a wire run between the load and the uCurrent to pick up noise.I wish there were some kind of "Idiot's Guide to the uCurrent" document that would walk us newbies through common pitfalls like this. There's a lot of nuggets of wisdom sprinkled throughout the forum but it's hard to sift them all out.

author: MarkL
author_rank: Unknown
timestamp: March 01, 2016, 05:59:55 pm
content: Thanks for the confirmation that was the problem. I had forgotten about this thread.It's easy for noise to creep in when so much amplification is being used. And as you found out, you'd never know it since the uCurrent output is usually just connected to a DMM.It could really benefit from an overload indicator to add confidence to the readings.

author: Muxr
author_rank: Super Contributor
timestamp: March 02, 2016, 12:01:05 am
content: Quote from: ManateeMafia on June 02, 2015, 08:51:30 pmThat's an interesting idea. Don't know the answer to that however.

author: Performa01
author_rank: Super Contributor
timestamp: March 02, 2016, 01:46:17 pm
content: Quote from: Muxr on March 02, 2016, 12:01:05 amThe answer is yes - for most DMMs, except for the very high end ones, which might used pulsed currents for resistance measurement.Lab bench DMMs usually don't output much less than 1µA in the 10MOhm range and don't behave like a current source on higher resistance ranges any more.Handheld DMMs on the other hand should be very usable for that, but the test currents might not be specified in the manual. Usually they internally use the mV range for resistance measurement, so a 20000 counts meter will have 200mV for full scale reading. For instance, this means just 100nA in the 2MOhm range and just 10nA in the 20MOhm range. Compliance voltage is very low obviously, but with a µCurrent, this shouldn't be a problem.