Electronics > Metrology

Low AC voltage measurement issues

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pwnell:
I am trying to measure an AC 1kHz signal’s RMS voltage using a Keysight 34465A bench DMM.  However it measures 000.000 0mVAC.

I have ac filter to >3Hz. Manual 100mV range. Using bnc cable. I am generating a 1kHz 17.666mV signal using my signal generator. That I measured using the Keysight. I feed that into a simple 46.64k + 463 ohm resistor voltage divider network to get out 0.175mVAC RMS. Yet when I measure there, I get 0 as per above. I know there is voltage as the output of the voltage amp this signal feeds into, reflects the amplified signal.

What am I doing wrong? Surely this DMM can measure 175 micro volt?

alm:
Why would you expect the DMM to measure 175 uV RMS AC? The datasheet has this footnote for the ACV specifications:

--- Quote ---Specifications are for sine wave input > 0.3% of range and > 1 mVrms
--- End quote ---
Which is much better than for its predecessor, the 34401A:

--- Quote ---For sine wave input > 5% of range. For inputs from 1% to 5% of range and < 50 kHz, add 0.1% of range additional error
--- End quote ---

Joe Geller wrote an article about the limitations of the 34401A for measuring low level AC (I'm linking to an archived version since he took his website offline).

Any meter using an analog RMS converter will have such a limitation due to the noise/offset of the converter, which, assuming it's uncorrelated with the input signal, adds as the root of squares. They normally won't read zero with shorted inputs (though presumably in the 34401A there's a firmware hack to round small values down to 0). The ones that digitize the AC signal, like the 34465A, do better, but still have offsets and noise.

pwnell:
Thanks for the response. That paper makes it sound that with the improved 34410A model they were able to measure down to 100uV. I am trying 175uV with a better meter.

When using the digitizer I can clearly see the sinusoidal wave, and using the stats, calculating the rms from Vpp gives a result of approx. 367uV which is wrong (input is pure sine wave).  Is there any way I can make that measurement with my meter?

bdunham7:

--- Quote from: pwnell on November 28, 2021, 09:01:39 pm ---What am I doing wrong? Surely this DMM can measure 175 micro volt?

--- End quote ---

I think it is not specified to measure 175uVAC.  Interestingly, an ancient, obsolete, cheap HP 403B will measure it just fine!

It is interesting that they choose to show you 000.000mVAC rather than some noise.  Not very sporting, IMO.

Andreas:
Hello,

attached a linearity comparison between 34401A K2000 and my oscilloscope for AC voltages <1mV in 100 mV range.

with best regards

Andreas

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