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DVMs aren't benign
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Conrad Hoffman:
I was recently doing some voltage measurements in the 5-20 mV range and the results seemed a bit off. Just for a reality check, I hooked up a ratio transformer and several different meters, checking from 10 VAC on down. These were an HP 400E, an HP3478 and my ancient HP 3455. All the meters were hooked up in parallel and the signal was 1 kHz. The 400E seemed way off on the lower ranges. The other two were pretty good. When I unhooked the other meters from the 400E, it was dead on, better than the other two. I assume the DVMs put some sort of HF pulses on the line that the fairly high bandwidth 400E can see. Anybody else ever get tripped up by this sort of thing?
RandallMcRee:
I have noticed spurious signals from my DVMs when measuring noise using the 10000x LNA preamplifier.

It clearly shows a lot of noise with most meters. Take the meter off and the noise measurement goes back to normal (in the 1uV range). All of my DVMs cause some sort of problem though I have not tried to characterize it. I'm in the "ok, don't do that camp".

Also, when you turn your meter off, some meters seems to have a fairly low impedance and perturb the circuit. Turn it back on and its back to high impedance.
tooki:
Isn’t it far more likely that you’re dealing with loading effects from the input impedance of the other meters?
Conrad Hoffman:
My first thought is no, because the ratio transformer output impedance should be fairly low compared to the meters. That was the reason I went with the ratio transformer rather than a KVD. But, I don't have a number for it, so now I have to find out! Other then the 400E, which is 10 megohms input and 25 pF, the other meters are 1 megohm. Many/most DVMs with 10 meg inputs turn out to have 1 meg when used for AC.
David Hess:
It is called charge injection or more often charge pumping in connection with meters and obviously not often discussed or even acknowledged by the meter manufacturers.  (1)  It also shows up when trying to measure the input resistance of one digital multimeter with another digital multimeter.  Some, usually much older, digital multimeters have front end designs which do not produce this problem usually by including a precision analog high impedance buffer stage so there is some virtue to keeping an old meter around which has no charge pumping as a sanity check.

The charge pumping could come from several sources depending on the design.  The ones that immediately come to mind are chopper stabilization, automatic zeroing, sampling kickout.  Some meters allow disabling of automatic zeroing to reduce or eliminate charge pumping.

Chopper stabilized operational amplifiers have the same problem when the inputs of separate devices are connected together.  If they are operating at close to the same chopping frequency, then they synchronously demodulate the charge injection from the other producing a DC offset or low frequency AC signal at their output.

(1) HP/Agilent/Keysight is the only company I know to mention it and today they advertise some of their bench meters as having "low charge injection".  Does that mean that their other meters have high charge injection?
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