Products > Test Equipment
DMM Input Capacitance
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KungFuJosh:
I have a convenient outlet on my bench, so I used a hex key inserted as my ground reference.

I tested with my ST2832 (setup image attached - Cs-Rs @ 100kHz) and I will likely also test with my DE5000.

SDM3055X-E:
DCV:
Negative Terminal: 1.47615pF
Positive Terminal: 0.55323pF

ACV:
Negative Terminal: 7.49180pF
Positive Terminal: 0.44843pF

SDM3065X:
DCV:
Negative Terminal: 1.60443pF
Positive Terminal: 0.38473pF

ACV:
Negative Terminal: 1.31416pF
Positive Terminal: 0.31178pF

I felt it was better to reference ground in the socket, rather than letting the chassis interfere. Also no inconsistencies between ground reference if switching devices ground ref point. And I suppose the Probe Master probes increase the capacitance too. 🤷

Thanks,
Josh
mawyatt:
Josh,

Those measurements seem very low. Try using your DE-5000 instead at 10KHz like we did. You can use short banana plugs & cables directly with DE-5000, and plug the negative DE-5000 end into your power supply Green ground terminal like 2N3055 did, or just connect to something that has earth ground.

No need to calibrate the DE-5000, we are just looking for relative capacitance, cable capacitance and such shouldn't matter.

Best,
KungFuJosh:
What's wrong with referencing earth at the outlet?
mawyatt:

--- Quote from: KungFuJosh on April 23, 2024, 06:55:22 pm ---What's wrong with referencing earth at the outlet?

--- End quote ---

Nothing if you get the Allen Wrench into correct terminal  ;D

Best,
floobydust:

--- Quote from: 2N3055 on April 23, 2024, 06:03:42 pm ---
--- Quote from: floobydust on April 23, 2024, 05:56:40 pm ---I wasn't sure about interpreting "when measuring bench DMM +- input terminal capacitance wrt to ground."
CM wrt to earth-ground it varies widely with handheld meters, depending on where they are, in your hand, on the floor, foil shield etc.
It's always imbalanced, (-) input is higher capacitance I believe because it goes all over the circuit board and not like (+) going to the divider resistor.

Bench multimeters you are basically measuring the power transformer winding capacitance or their Y-cap.

--- End quote ---

That is why i wrote:

sitting on top of isolated desk is 5pF both
siting on antistatic mat 15pF
sitting on grounded metal plate 19pF both.

And made a comment of not holding it in a hand.

Yes we know why benchtop meters are asymmetric in this regard.
Mike wanted to make this fact popularized by discussing it.

--- End quote ---

My interest is when the (differential) input or (common-mode) stray capacitance causes problems while making measurements out in the real world.

After a few decades of using many different multimeters and seeing a few malfunction over this capacitance/effect I know when it's a problem. But it's not discussed and people have difficulty understanding it. Very few multimeters have formal EMC immunity test certs or are subjected to EMI in a lab.

Another thing to mention is using a mains-powered LCR meter to measure capacitance to PE does not always work.
Either you get CM noise on the in-guard section from the isolator/SMPS, or it's swamped by a Y-cap, something to look out for.
With any mains-powered LCR meter or the mains-powered multimeter, people think their inputs are floating in outer space yet some gear has bad parasitics. You have to know your test equipment in this undocumented regard.
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