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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: measuring a capasitance?
Message-ID: <17636@milton.u.washington.edu>
Date: 4 Mar 91 21:58:22 GMT
References: <1267@soleil.UUCP> <1991Mar4.044845.18652@syd.dms.CSIRO.AU> <1991Mar4.085406.25128@santra.uucp>
Organization: University of Washington, Seattle
Lines: 25

In article <1991Mar4.085406.25128@santra.uucp> 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?

	I don't know how good an idea it is, but think about this
possibility: look for the phase shift vs. frequency of the capacitor
as driven from an oscillator.  Choose some phase shift (like
45 degrees) that can easily be detected.  You will need, of course, 
good control of the oscillator's output impedance (use thermistors 
if you have to).
	Then sweep the frequency until you find the magic phase 
shift; measure that frequency.  The frequency measurement should
be trivial to the accuracy you want; the only other component that
need be temperature-stable is a single resistance (to provide the
output impedance of the oscillator).
	The advantage over a bridge (three critical components,
at least one adjustable) is in reduction of the number of
gizmos that have to be temperature-stable to 0.1%.  

	John Whitmore