Electronics > Metrology

Null voltmeter

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ecclectrics:
Ok, thermal EMF of the pot - what all needs to be taken into consideration!

One other question I have in particular to you: in some other threads on the Null-Voltmeter from Conrad Hoffman (e.g. here https://www.eevblog.com/forum/projects/conrad-hoffman-null-detector/msg2893432/#msg2893432) you suggested smaller values of the caps for the input filter, which were at 0.47µ there. Here they are with 1µ even bigger. I wonder what the pros or cons are for having smaller (i.e. 47n .. 100n) or larger (i.e. 0.47µ ... 1µ) caps in the input filter?
I have actually cobbled together a version of the Hoffman circuit with some jellybean parts, which was a valuable lesson on 1/f noise. With the opamp I used it was several µV and severely limited the possibility to get the offset really low. Now the autozero opamps seem to have it in the order of several 100nV depending on the type, but is still an issue if I want to get the offset lower than that. So I think a lot of filtering will be necessary for me, and I am looking for the best way to do it without messing up something else.

Kleinstein:
The capacitors at the input give a filter response, where the source impedance adds to the resistor in the circuit. With a high impedance source this can become sluggish and dielectric absorbtion can make it even more sluggish than the RC time constant suggests, if there was a much larger voltage before.
With an analog display one may want the analog filter fuction, especially for the highest gain range. Thus the filter at the amplifier that changes with gain.
For use with a digital display it would be better to do the final filtering digital with a more FIR type filter and thus a finite response time instead of exponential decay type. Digital filtering is also easy to make flexible.

The filter capacitors also are part of the ESD protection to reduce the peak voltage in addition to the diodes.

Vtile:
Has the parasitic resistances ( cabling ) talked about. This is a subject most of the times overlooked when the null detection is spoken with wheatstone bridge example in academia and totally forgotten the wiring resistance from equation given in these examples (which is much more complex with wire resistances) this of course is mostly for bridge setups using null methods.

guenthert:

--- Quote from: Vtile on August 08, 2024, 08:14:19 pm ---Has the parasitic resistances ( cabling ) talked about. This is a subject most of the times overlooked when the null detection is spoken with wheatstone bridge example in academia and totally forgotten the wiring resistance from equation given in these examples (which is much more complex with wire resistances) this of course is mostly for bridge setups using null methods.

--- End quote ---
   I presume you mean the leakage of the insulation of those cables, as the resistance of the conductor is very low (expect it to be less than 100mOhm) compared to the input resistance (100kOhm in case of HP419A, 1MOhm in Keithley 155 and Fluke 845A) and irrelevant at null.

   I'd think the leakage of the cabling is implicitly included in the leakage of the case.  Many bridge circuits have the detector at ground potential in order to minimize leakage effects (however then requiring very good isolation of the generator, see e.g. ESI 801).

acstd90:
Have tried the AVM. Maybe someone else may have had better luck than myself but they seem to have problems with Noise, Drift and the battery is always dead (self discharge)
Mine holds down a spot Under the workbench.

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