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

Mike Jenkins (jenkins@lsil.com)
Mon, 13 Apr 1998 12:43:51 -0700

Greg,

If you don't mind, I'd like to recommend an alternative method for
measuring pin capacitance. L-C meters may be fine, (potentially)
accurate instruments, but, as you've seen, they're not much good
for telling you what's going on.

My preferred method uses a TDR with a scope capable of waveform
math (subtraction and integration).

____________________ ___
| * | ___________
without | * | ___________()--- +
cap -->| * <-- with cap unit Zo |
| * step C ===
_________|* _|_ _|_
///

Without the cap, the TDR shows the classical open-circuit doubling
reflection. With the cap added, the reflection adopts an RC time
constant. The time constant is Zo * C.

Of course, real waveforms don't look like this -- ringing and other
ugly stuff. However, the Zo*C time constant is still the integral
of the difference between the "without" and "with" waveforms. This
is exact, even with arbitrary incident step waveforms and even with
inductance in series with the cap. And the TDR trace lets you see
the effect of such inductance. The accuracy is as good as your
equipment, probably limited by fixturing (including knowledge of Zo).

I'll get off the soapbox, now.

Mike

ps: If you're interested, the proof involves the LaPlace initial and
final value theorems.

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