Electronics > Metrology

Nanovolt design challenge - build and show your own nV-meter in 256 days [DONE]

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coppercone2:

--- Quote from: dietert1 on January 12, 2022, 10:17:26 pm ---Don't forget that 10 nA input current times 1 ohm probe cables = 10 nV.

Regards, Dieter

--- End quote ---

1 Ohm probe cables?? that should only be the case for the most strict cryogenic setups (i remember something about weird not so conductive metals being used at these temperatures).. something is not right if your probe is 1 ohm unless its a insane setup.

Kleinstein:
1 Ohms for the cable, switches (to get 2 inputs) and the signal source are not that unreasonable. The usual thermocouple probes are in the 10 ohms range, somtimes more.

Another part that can add some resistance is the protection of the input agains overvoltage and possible EMI (though AFAIK not directly part of the requirements in the challange).

Several nA of input current are definitely a problem for practical use, though it may still be fit for the conditions of the challange.

coppercone2:
oh, for thermocouples that changes the situation completely. I guess you would need a compensation junction external to the meter right? I did not think to hook up a thermocouple to one. I never saw the internals of a nanovoltmeter from HP that has a thermocouple button on it.. do they put a reference junction inside?

And you are right for a nanovolt meter you can put protection on it, unlike a electrometer, since you are measuring low impedance sources.

I was imagining switches like a resistance standard for a good nanovolt meter, silver triple bladed... and I would go for heavy wire chokes here, not a minimum size SMD inductor)

KT88:
A nanovoltmeter is a slight overkill for TC applications (several uV/K). But even then the offset could be calibrated out to the most part.
My concern would be that it might damage or at least disturb a Weston Cell if someone happens to still use it.

Kleinstein:
The low voltage on thermocouples are still a challange to normal DMMs and so you would want one with good sensitivity. So it would make some sense to have cold junction compensation on a sensitive voltmeter - though this mainly makes sense with an dedicated input, with a proper plug. There are other sensors that use thermocoules to measure differences directly.

There are other sources that are not that low in resistance. Due to the Johnson noise, there is however less need to care about source resistance in the higher kOhms though. A meter should still have brought range of uses, even if overkill in some ranges.

The difficult part is the protection - this tends to add more like 10s of ohms. Even a low current (e.g. 100 mA) fuse may be a few ohms.

One can compensate for the average input current. This is done with some meters, like the HP34420, some fluke and datron meters with BJT based input.
However it is very hard to compensate for drift and noise of the input current.

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