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| Is the resistance of digital DMMs a concern under normal measurement conditions? |
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| ballsystemlord:
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 |
| bdunham7:
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. |
| J-R:
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? |
| J-R:
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... |
| Kleinstein:
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. |
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