RE: [SI-LIST] : measuring CMOS Cin with HP4275A

Andrew Ingraham (Andrew.Ingraham@digital.com)
Fri, 10 Apr 1998 16:17:25 -0400

I don't know how these particular HP bridges work ... but generally,
bridges work by nulling some voltage. If you throw noise into the
setup, it could make it impossible to find the point that has zero
signal. Thus, a bridge might be rather sensitive to noise from a ground
loop, or from harmonic distortion, etc. That's my guess.

On the other hand, I tend to doubt that DC leakage itself would affect
the bridge. It is probably an AC bridge with an AC voltmeter. However,
the real part of the admittance at the operating frequency of the
bridge, might affect its ability to find a good null.

Does the HP manual give any recommendations for measuring components
that are already grounded and not floating?

Maybe you need to isolate the LCR meter from earth with a transformer.

Regards,
Andy