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Fluke 87-V HighRes values are different...?

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The other day when I was checking my 5uA DC current standard with a Fluke 87-V I noticed something really strange. When I compared the high-resolution values with the standard resolution values things didn't add up. I know that going into HighRes mode on this DMM does not make it more accurate, it just gives you more resolution which is fine with me. But what I did not expect to see was that the values did not agree when rounded. I then ran an experiment where I connected some of my DMM's in series with a Keithley 236 (set to a current source). The table in the attached image file shows my results. As you can see the high-resolution values have an offset of about 0.15uA when compared to the Keysight 34461A readings. However, in standard mode (one decimal less) the Fluke 87-V values are spot on...What could be causing this discrepancy? Are the Fluke 87-V calibrated separately in the standard mode and high-resolution mode? Thanks in advance!

The calibration steps don't include any for the Hi-Res modes, and there is no change in the input resistance/burden voltage, so it must be an artefact of the ADC implementation. Mine behaves similarly when sourcing 5-199uA from my Agilent U1401B.

Worth pointing out that it is in spec, and that the LSD uncertainty count is x10 in Hi-Res mode, and that the nA that the LSD represent are well down into the noise for this meter.

User manual:
Calibration manual:

Thank you! Yes, I am not concerned about being in or out of spec. It was just so confusing to see that while it is not more accurate in HighRes mode, it is actually LESS accurate than in normal mode... Good to know! Still love this meter!

From a user's perspective it would seem logical that the HighRes mode are the internal values and that they get properly rounded before being shown in normal mode. But that does not seem to be the case here which caught me by surprise. Thanks again for sharing your insights!!

Interestingly, if you manually set the 6000uA range, then go to Hi-Res mode, it gives a different reading to the 600uA range to the tune of 200-300nA for a 10uA nominal reading, when technically it should be identical.
It does it in both polarities.

I really think it must be how the firmware configures/uses the ADC.

Interesting! Will have to try that too. As you can see in my images, I used auto-range on all DMM's. Thanks for sharing!


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