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Precise Offset Compensation Ohm Measurements and Validation of DMMs

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babysitter:
Very interesting.  Looking at the ocomp measurement waveforms with an oscilloscope before the "valid" measurement might hint at troubles if the voltage over the DUT is not stable. Also, the classic "component tester" setup with very slow excitation frequency (arbgen or programmable PSU) might indicate dielectric relaxation.

I wonder if the "ham radio standard microwave oven test" might be used for qualifying isolation materials - put your plastic in a microwave oven together with a glass of water, turn it on and see if your material under test gets hot - then, something is trying to follow the field.



lukier:
Thank you for this detailed investigation.

Recently I got 3457A that has OCOMP and I've read the manual, journal, schematics, but I wouldn't suspect parasitics affecting the measurements. Very interesting indeed.

BTW I'm not 100% sure, but I think 3457A has true 100 NPLC (unlike 3458A 10 NPLC + digital). With AZ and OCOMP it takes ages to get a reading :)

Kleinstein:
The microwave ofen test will test dielectric absorbtion in the GHz range. The problem here is dielectric absorbtion in the Hz region. So 9 decades slower. You would have to do the microwave test at a much higher temperature to speed things up (e.g. 200 K higher) - thus to hot for most plastics.

The more sensibel way would be to measure the voltage over the resistor, after diskonnecting a voltage (e.g. 1.5 V or more if self heating and resultiing thermal EMF of the resistor is not a big problem). If there is significant current from dielectric absorbtion this should show up as it's rather close to what happens in the ohms measurement, in the compensation phase.

So ideally the meter should not just wait a predefined time (even something like 5 s might not be enough) but do a measuremt over time and warn if there is to much "drift"  / residual current from relaxations.

Unless you use very long or shielded cables there should not be so much capacitance to get in the 100s of pF range.
As shielding might be a good idea one might have to think about driven shields when doing very accurate higher ohms measurements - not just GOhms resistors but allready if Gohms in parallel might be a problem.

Vgkid:

--- Quote from: lukier on January 02, 2016, 06:55:26 pm ---BTW I'm not 100% sure, but I think 3457A has true 100 NPLC (unlike 3458A 10 NPLC + digital). With AZ and OCOMP it takes ages to get a reading :)

--- End quote ---
Sounds similiar to my 3456A, so that is most likely correct, I havent checked the output for that.

Andreas:
Hello Frank,

that is probably also the right explanation for the problems that I had with the AC-Multiplexer on my T.C. resistor measurements.
Above 1kOhm (e.g. 12k)  I got unreliable results (but I also have EMI capacitors in the range of some nF which have to be charged every time).
The self adjustment time after switching the MUX of 80ms before the measurement actually starts was not sufficient to give reliable measurements.
Although more than 20 Tau are within those 80 ms.

With best regards

Andreas

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