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From: tomb@hplsla.HP.COM (Tom Bruhns)
Newsgroups: sci.electronics
Subject: Re: measuring a capasitance?
Message-ID: <5170124@hplsla.HP.COM>
Date: 25 Mar 91 18:59:23 GMT
References: <1991Mar4.085406.25128@santra.uucp>
Organization: HP Lake Stevens, WA
Lines: 39

s32852b@puukko.hut.fi (Petri 'Pete' Karha) writes:
>I got a really tricky thing to do and I would appreciate some help. We
>must design a device that can measure a varying capasitance the value of
>which goes from 0 to 100 pF. Well that's not tricky, just a bridge but
>it should have about 0.1% accuracy and it should work in a wide
>temperature range (about -20-+70 C). Any good ideas?

Well, I'm surprised the only other response you got (that I've seen
here) was a discussion about measurements based on phase shifts.
It seems to me (without many calculations to back it up) that 
measurement of resonant frequency would be a good alternative.  You
could build a very compact oscillator in an oven, one frequency-
determining component of which is the capacitance you wish to measure.
Measuring frequency is usually pretty easy, and the biggest trick 
would be to keep the oscillator in calibration.  "Autocal" features
might be a way to do that.  I assume you mean about .1 pF accuracy,
not .1% of the capacitance; that would be an impossible task.  Indeed,
you will find the capactiance of connecting wires will change easily
by that amount due to humidity changes when you are down in the
single-digit pF range.

Another thought:  quadrupole mass spectrometers typically rely on
extremely stable capacitors to monitor the RF voltage applied to the
quadrupole structure.  If the capacitance you wish to measure can
stand moderate voltages, you might be able to turn that idea around and
compare the current through your capacitance to the current through
a stable reference capacitor; such a ratiometric measurement might
be capable of 0.1pf reliably.  You want to use a moderate voltage so 
accurate rectification is easy.  Actually you are dealing with a
large enough capacitance that you could use a low enough frequency
excitation that rectification might be pretty easy just using an
active (op-amp based) rectifier; mass spec works in the megahertz
region, where it's tough to get the required accuracy.  As a point
of comparison, the mass specs I worked on used 3 pF capacitors to
sample up to a couple kilovolts, and wanted to control that voltage
to a few millivolts stability.  Your problem is at least an order
of magnitude easier; again, temperature control may be required
(an NPO reference cap may have 30 ppm/degree tempco; +/-45 degrees
could put you out of spec...)