Products > Test Equipment

DMM Input Capacitance

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mawyatt:
This was precipitated from a measurement here:

https://www.eevblog.com/forum/testgear/sds800x-hd-actual-use-cases/

Curious what others find when measuring bench DMM +- input terminal capacitance wrt to ground.

Best,

KungFuJosh:
What's your test setup for this? How are you referencing ground?

Thanks,
Josh

2N3055:
Meter used Uni-T 612 LCR meter.
100kHz, C mode
Negative terminal connected to PSU ground terminal (it was just convenient)
Banana cables, calibrated in approximate position of measurement.
Grounded metal plate was simply top of other grounded instrument.

Absolute values are far from being accurate, but relative values should be good for relative comparisons.

Rigol DM3068 (T+ T- are positive and negative input terminals)

+ T 30pF
 - T 180pf

This is one spec where high end handheld will be "more" differential than benchtop meter for small AC signals...

Metrix MTX3293 :

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

Brymen is a bit worse, but still better than benchtop and also if kept isolated (by distance to ground and NOT holding it in hand) absolute values are order of magnitude better:

BM869S :
+T 4,8pf    -T 6,4pf
+T 10,4pF  -T 14,4pF
+T 12pF    -T 16pF

Best,

mawyatt:
@ Josh,

Just used a handheld LCR meter (DE-5000) @ 10KHz to stay away from mains frequency influence. Connect handheld LCR meter from DMM + or - terminal to chassis ground (earth mains ground in our setup).

Best,

mawyatt:
@ 2N3055,

Important point you made, measurements are relative not absolute...thanks for pointing that out, we should have done so in the 1st post!!

The SDM3065X is the worst of our bench DMMs with ~500pF from - terminal and ~93pF from + terminal at 10KHz.

Ironically the old HP and AG 34401A are best we have wrt this!!

Another interesting observation while doing this capacitance test, note the difference in direct DMM +- input capacitance when in DCV and ACV modes!!

Best,

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