Products > Test Equipment
DMM Input Capacitance
Sariel:
Why is a DMM with very low input capacitance favoured?
2N3055:
--- Quote from: mawyatt on April 23, 2024, 06:41:08 pm ---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,
--- End quote ---
I agree these are very low. It might be because LCR meter is mains powered.
I also want to see DE-5000, which is handheld like my UNI-T.
Floobydust made some good points.. :-+
KungFuJosh:
--- Quote from: 2N3055 on April 23, 2024, 07:27:13 pm ---
--- Quote from: mawyatt on April 23, 2024, 06:41:08 pm ---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,
--- End quote ---
I agree these are very low. It might be because LCR meter is mains powered.
I also want to see DE-5000, which is handheld like my UNI-T.
Floobydust made some good points.. :-+
--- End quote ---
I'll test with the DE5000 next. I'm curious, you think I should dig out my isolation transformer and power the LCR from that to see if it changes anything?
--- Quote from: mawyatt on April 23, 2024, 06:57:50 pm ---
--- 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
--- End quote ---
🤯 How is it that there's no electrocuted emojis on this site??
KungFuJosh:
Results with DE5000 (using kelvin clips) at 10kHz Cp, all else the same (ground ref in power outlet, Probe Master probes):
SDM3055X-E:
DCV:
Negative Terminal: 560.5pF
Positive Terminal: 66.26pF
ACV:
Negative Terminal: 559.9pF
Positive Terminal: 57.25pF
SDM3065X:
DCV:
Negative Terminal: 192.04pF
Positive Terminal: 52.36pF
ACV:
Negative Terminal: 190.88pF
Positive Terminal: 40.63pF
alm:
It makes sense that on a guarded multimeter (which I'd expect any halfway-decent bench meter to be), the impedance of the low terminal to ground is much lower than the high terminal. Generally the guard is connected to the low terminal except on meters with a separate guard terminal where you generally want to connect guard to the low terminal unless you are connecting it separately. And the guard is a big box (or at least a trace) between the input circuit and ground. So it should have a lower impedance to ground to do its job of shunting common mode current away to ground. In terms of impedance to ground or noise, you should treat your DMM a bit like a scope in that you connect the high lead to the high impedance point of the circuit and the low terminal to a low impedance point of the circuit. Connecting the low terminal to a high impedance point (like a high-impedance voltage divider) can lead to increased noise and unexpected circuit loading.
--- Quote from: KungFuJosh on April 23, 2024, 06:25:17 pm ---SDM3055X-E:
DCV:
Negative Terminal: 1.47615pF
Positive Terminal: 0.55323pF
...
--- End quote ---
These figures look off by at least two orders of magnitude. Figures like this would be good for an active FET scope probe, and unthinkable for a DMM. It may be either that the ground-referenced LCR meter was shorting it to ground, or maybe there was no connection between the Allen key and the DMM's ground.
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