Is the resistance of digital DMMs a concern under normal measurement conditions? - Page 1
Forum section: electronics / testgear

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: December 17, 2023, 03:39:01 am
content: Hello,Recently I was testing 2 DMMs for the purpose of reviewing them. I was measuring DC volts and I was way down in the uV ranges of the DMMs. One DMM gave a result, the other did not, despite being able to read that low. Both DMMs were rated at 10MOhms impedance. I was puzzled for a while until I realized that one DMM came with nickel plated probes and the other came with gold plated probes. I disconnected the gold plated probes and the DMM which wasn't giving a reading gave me the correct result.So here's my question, outside of such low voltage measurements, when else can/will I receive an incorrect measurement, or no measurement at all, when probing for voltage with a DMM? Is there a way to calculate when my results would be invalid due to a lack of input current?Thanks!EDIT: Please see here for a photograph and a bit more description.https://www.eevblog.com/forum/testgear/is-the-resistance-of-digital-dmms-a-concern-under-normal-measurement-conditions/msg5227776/#msg5227776

author: bdunham7
author_rank: Super Contributor
timestamp: December 17, 2023, 05:48:43 am
content: Which DMMs have µV ranges?? Are we talking about hundreds of microvolts or single digits? Can you describe the exact setup in more detail? I didn't really understand the part about disconnecting the gold-plated probes.In any case, sharp gold-plated probes are indeed often much better for reading small signals reliably. This usually shows up with resistance measurements, but it can be an issue with very small voltage measurements as well.

author: J-R
author_rank: Super Contributor
timestamp: December 17, 2023, 06:09:43 am
content: I am also curious what DMM and what leads, since this might be related to a jack issue, but beyond that I can tell you that the impact of lead resistance on voltage is almost always negligible due to the 10M Ohm DMM resistance.Get out a resistance substitution box and put it in series with your circuit and you will see what it takes to impact your measurements at each voltage.I just tried this out with 10uV and even 100k Ohms was hard to see.Are you at the very bottom of the DMM's measurement ability already?

author: J-R
author_rank: Super Contributor
timestamp: December 17, 2023, 09:03:42 am
content: Was thinking about this more and I could probably explain this better.When you connect your DMM to a voltage source, you're completing a circuit and current flows. From Ohm's Law we know this current is voltage divided by resistance, I=V/R.Now that you know the current, you can calculate the voltage between any two points in your circuit, again using Ohm's Law: V=IR.Do you want to know the voltage across the DMM? V=I x 10M Ohms.Do you want to know the voltage across one of the test leads? V=I x 1 Ohm if you have very bad test leads.Now you can see the reason the test leads really don't matter, because their resistance is a tiny fraction of the 10M Ohm resistance of the DMM. And this gets even more extreme with DMMs that have Gig Ohm resistances.I also thought about whether the voltage matters a bit more and while theoretically it might make a difference, from the numbers I crunched you'd need very high lead resistances, a very high count DMM, and be near full scale. If those conditions aren't met, you are just not going to be able to see the numbers on the DMM...

author: Kleinstein
author_rank: Super Contributor
timestamp: December 17, 2023, 09:10:52 am
content: The input resistance is usually not an issue with small voltages and a more normal moderate resolution. The case with a contact problem with the probes is more a probe thing.The 10 M can be an issue for high impedance signal sources, like in tube circuit, but here there are very few alternatives for higher voltages.The other point can be with very high resolution / accuracy demands (e.g. 6 digits and more), so that already moderate source resistance becomes visible.The 10 M are kind of the standard with DMMs, especially hand held.

author: Stray Electron
author_rank: Super Contributor
timestamp: December 17, 2023, 02:46:09 pm
content: Quote from: J-R on December 17, 2023, 06:09:43 amX2. I checked a lot of the meter leads that I have laying around (some cheap and some expensive brands) and the lead resistance is typically about 4 ohms each. That's insignificant when you're using a modern meter that has over a million ohms input resistant. But if you're trying to measure the resistance of devices like shunt resisters or the resistance of a coil or an electric motor then the 8 or so ohm total lead resistance if very significant so you MUST use a meter designed to do 4 wire measurements. With that technique, the lead resistance is a complete NON-issue. Generally speaking I use a meter with a 4 wire (aka Kelvin connection) for measuring anything under about 100 ohms, and also for anything under about 1k ohms if I need good accuracy.I strongly suggest that anyone that works of electronics have both a general purpose meter and 4 wire meter and to use the later for measuring low ohms or when accuracy is important. Good 4 wire meters are usually expensive but very GOOD ones like the Fluke brand can be found used for a reasonable price sincemanysellers don't realize the difference between a 4 wire meter and a normal 2 wire meter.

author: shapirus
author_rank: Super Contributor
timestamp: December 17, 2023, 03:09:24 pm
content: Quote from: bdunham7 on December 17, 2023, 05:48:43 amThis is Brymen 869s.Won't be very usable in the single-digit range, as the last digit is jumpy (although much less so with shorter and thicker leads), but tens of µV should be fine. Accuracy is specified as "0.02% reading digits + 2d", and I believe the "2d" applies to the 5-digit mode, so tens of µV. However the 6th digit, where accuracy is not specified, can still be used to monitor changes of measured values over time. It would be interesting to test it against a µV reference, but I don't have anything like that.

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2023, 03:20:38 pm
content: Quote from: shapirus on December 17, 2023, 03:09:24 pmPut it on a battery, to enable you to see the effects of short-term noise (thermal, popcorn, etc) in the meter's reference.With care, that could also allow you to assess some aspects of medium-term drift/noise.

author: shapirus
author_rank: Super Contributor
timestamp: December 17, 2023, 03:33:55 pm
content: Quote from: tggzzz on December 17, 2023, 03:20:38 pmWhat do you mean on a battery? It's already powered by a battery. Something else?

author: coromonadalix
author_rank: Super Contributor
timestamp: December 17, 2023, 03:45:58 pm
content: again with the 500k count loll its a gimmick, more resolution is not precision

author: shapirus
author_rank: Super Contributor
timestamp: December 17, 2023, 03:54:33 pm
content: Quote from: coromonadalix on December 17, 2023, 03:45:58 pmthey don't claim anywhere that it's about precision.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 17, 2023, 04:04:25 pm
content: There is no extra difficulty for the scale factor for the lowest range. The difficulty is in the offset part, so getting really a zero reading for a short. This starts with thermal EMF from the test leads. So holding the probes with the hands in usually a no go if one cares about µV. Thermal fluctuations can also be a relevant noise source at the low end.The extra resolution could still be useful for low voltages, like thermocouples or short finding from residual voltage at ground points, even if it does not improve the accuracies in the more normal ranges.For checking the meters reference and stability of the scale factor it is usually better to use a higher range (e.g. 2 V) to avoid those low voltage offset problems.A battery if undisturbed can be a low noise and short time reasonable stable voltage source for some tests. However batteries are still sensitive to temperature and mechnical stress that can cause the voltage to make jumps.One such test with a battery could be the turn over test: measure a battery voltage with both polarities of the meter. The difference is from the meters offset and INL error.With a meter with 10 M input resistance even that can be enough load to a battery to get it slowly drift (usually down). So a test may want to check a few cycles (pos, neg and zero).The noise when measuring the battery voltage over some time is a mix of noise and drift of the battery and the meters internal reference. When handled with care the battery is usually at least lower noise.Normally the lead resistance should be way lower than 4 ohms, usually more like 0.1 - 0.2 ohm or so as the reading for a short with 2 wire ohm. Probes with 4 ohms would likely blow when used with a higher current. If the contacts are that bad (e.g. chrom coated) it would be a reason to through them out or grind down the tips.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 17, 2023, 04:08:13 pm
content: Quote from: shapirus on December 17, 2023, 03:09:24 pmI've used that 500k count mode for trending drift. I wouldn't say it's a gimmick but useful in the right hands. Following shows mine attached to a reference.https://youtu.be/dnnPDajITqM?t=141

author: shapirus
author_rank: Super Contributor
timestamp: December 17, 2023, 04:24:20 pm
content: Quote from: Kleinstein on December 17, 2023, 04:04:25 pmIt displays values changing in the -0.004..-0.007 mV range when the inputs are shorted right at the jacks with a short piece of (thin) copper wire. Stable -0.00 mV in the 50k-count display mode.Of course I wouldn't care to measure absolute µV values with a DMM like this, but it can work just fine to monitor a slowly changing value.

author: Fungus
author_rank: Super Contributor
timestamp: December 17, 2023, 05:45:40 pm
content: I think the gold leads is a red herring. Try swapping the leads around and see.The resistance of DMMs is a concern in proportion to the impedance of the voltage source.Some of them have high impedance modes for measuring delicate circuits.Quote from: bdunham7 on December 17, 2023, 05:48:43 amLots of them. Even some cheap Anengs.Quote from: bdunham7 on December 17, 2023, 05:48:43 amI assume the meters were connected in parallel.

author: Veteran68
author_rank: Frequent Contributor
timestamp: December 17, 2023, 06:00:26 pm
content: Quote from: Stray Electron on December 17, 2023, 02:46:09 pmI'm going to assume that's a typo?If not those are the worst leads I've ever heard of. Going through a sample of mine, even the el-cheapo DT830 leads I've tested are in the 0.3-0.4 ohm range once you get past the crappy coating on the probe tips. Better leads like Brymen and ProbeMaster measure in the 0.02-0.1 range, and some meters can't read that low and report 0 ohms.

author: csuhi17
author_rank: Frequent Contributor
timestamp: December 17, 2023, 06:37:45 pm
content: I have a feeling that the question is wrong.I think he confused uVolt with uAmper.it would be good to know the type of DMM.I don't understand what he mean by "digital DMM" and whether it's handheld or desktop.The question is, which unit does the DMM display, mV or uV.I think 00.001mV is not equivalent to 001uV.

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2023, 07:38:38 pm
content: Quote from: shapirus on December 17, 2023, 03:33:55 pmA DMM's reading is the input voltage relative to the DMM's internal reference, Vin/Vref. Noise/drift in the internal reference often determines a decent meter's performance.Measure a batteries' voltage with the meter. That should have a low noise, unlike a typical PSU.From that you may be able to assess some aspects of the meter's internal reference.

author: shapirus
author_rank: Super Contributor
timestamp: December 17, 2023, 08:23:37 pm
content: Quote from: tggzzz on December 17, 2023, 07:38:38 pmThat's an interesting thing to observe. 5V range, single cell of a LiPo battery, 500k counts mode. Most of the time it sits in the same spot, frequently (like up to twice a second -- I guess that's the refresh rate of the meter in this mode) alternating between 3.83956 and 3.83957, but from time to time it reaches 3.83959, and, much less frequently, 3.83954, with the least significant digit again alternating between N and N+1 at any given spot. This lower frequency modulation has a period of say 20-30 sec, sometimes it tends to stop at the extremes for some time and then go back to the middle point.I wonder how much of this can be attributed to the various factors involved: some chemical processes in the battery that's been lying around for a year and then got suddenly loaded with a 10MOhm load, EMI received by the leads (I'm using the regular probes), inherent noise of the internal reference itself.All in all, the least significant digit in the 500k counts mode is only half-useful and requires the user to understand the limitations. Well, just like it is with everything.

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2023, 08:33:36 pm
content: Quote from: shapirus on December 17, 2023, 08:23:37 pmI think that is a 50uV variation. I don't see a chemical process causing that, nor thermal voltages. I suspect either EMI or popcorn noise in the internal reference.Twisting the leads together might reduce some types of EMI.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 17, 2023, 08:46:23 pm
content: The Li cells have a voltage changing quite a bit with the charge state. It is not that clear if this may not include jumps.I still also suspect more the reference in the DMM. Low power reference have quite some noise and RTS / popcorn noise is quite common.

author: floobydust
author_rank: Super Contributor
timestamp: December 17, 2023, 09:07:18 pm
content: Pay attention to where your wireless gear is located. Cellphone, WiFi router, iPad, cordless phone etc. all make interference in the lab. Before you blame your equipment.You can try clamp-on ferrite beads or move the leads or equipment, add a small disc cap across the input jack, to see if that changes the birdies. Chirps, whistles I get from my PC speakers if my cellphone is downloading a text message, and there is the regular packet heartbeat as well. Very hard to filter out.I find oxidation on cheap chinesium plated probes creates all kinds of poor connections, rectification, half-cell potentials - totally unsuitable for low signal work. It seems to be some chromium, steel mix at the tips and banana jacks. Terrible plating.If you leave the laboratory and use a multimeter out in the wild where there is dirt, moisture, corrosion - the high input resistance is no good and you will get ghost readings. This is why LowZ voltage functions are very useful and a basic need when troubleshooting.

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: December 17, 2023, 09:07:28 pm
content: And I'm unable to reproduce what I was seeing. I have no idea. I thought maybe it was a problem with input resistance.Here's the original photograph taken after I pulled one of the leads.What you're looking at is a 12v AGM cell, a 10MOhm resistor and a 3ohm resistor. The meters were connected in parallel.PS: The battery voltage was/is 12.5XXXv.