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What is the minimum voltage change I can measure with a 6.5-digit multimeter?

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strahd_von_zarovich:
Hi everyone,

I am testing a very sensitive analog circuit, and I am unsure if I am measuring the values correctly.

The resolution of the multimeter is crucial for me. I need to measure changes of 100nV.

Can I do this with a 6.5-digit multimeter? My multimeter model is the Agilent 34411A. According to the datasheet, the resolution is given as 0.03 ppm in the 10V range and 100 nplc. So, I concluded that I can measure a voltage change of 30nV in 1V range. Is this correct?

Here is the relevant part of the datasheet:

Momchilo:
Hi,
not sure if you want to measure AC or DC. I guess DC. The last digit in the 1V range is 1µV. Nanovolts are not shown on the display. In the 100mV range the last digit is 100nV.

For DC the attached part of the datasheet is important. Those specifications are already with 100 NLPC. Your table is for additional error margins (RMS noise) with lower NLPCs.

If you want to measure 0.900003V, the multimeter is set to the 1V range.
With the 1 year calibration specification the tolerance would be: 0.900003V*0.000035+1V*0.000007=+/-38.500105µV
0.900003V-38.500105µV=0.899964499895V
0.900003V+38.500105µV=0.900041500105V

That means everything between 0.899964V and 0.900042V would be in specification if you want to measure 0.900003V without taking the temperature coefficient into account.

In conclusion you cannot measure 10nV in the 1V range. It is not even displayed. The last digit is 1µV but you have also a tolerance of +/-39µV.

nctnico:
The first question is whether the measurement is relative or absolute (resolution versus accuracy). Over a short time period, it should be possible to measure small -relative- changes using a high NPLC setting.

strahd_von_zarovich:
Thank you very much for your answer.

Yes, nanovolts are not displayed on the screen, but the data collected via PC shows much higher precision. I'm not sure how accurate these readings are.

My main goal is to measure changes rather than absolute accuracy.

For example, if I measure 0.100,000,000 V and then increase the source voltage by 50 nV, I want to measure 0.100,000,050 V. Is this possible?

As long as I can measure the change, I don't really care about the actual values.

Momchilo:
You cannot measure the increase of 50nV exactly. With luck, you will see that there is a rise.

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