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Is the resistance of digital DMMs a concern under normal measurement conditions?

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Veteran68:

--- Quote from: Stray Electron on December 17, 2023, 02:46:09 pm ---   X2.  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.

--- End quote ---

I'm going to assume that's a typo?  :o

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.

csuhi17:
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.

tggzzz:

--- Quote from: shapirus on December 17, 2023, 03:33:55 pm ---
--- Quote from: tggzzz on December 17, 2023, 03:20:38 pm ---Put it on a battery, to enable you to see the effects of short-term noise (thermal, popcorn, etc) in the meter's reference.

--- End quote ---
What do you mean on a battery? It's already powered by a battery. Something else?

--- End quote ---

A 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.

shapirus:

--- Quote from: tggzzz on December 17, 2023, 07:38:38 pm ---A 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.

--- End quote ---
That'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.

tggzzz:

--- Quote from: shapirus on December 17, 2023, 08:23:37 pm ---
--- Quote from: tggzzz on December 17, 2023, 07:38:38 pm ---A 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.

--- End quote ---
That'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.

--- End quote ---

I 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.

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