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From: tomb@hplsla.HP.COM (Tom Bruhns)
Newsgroups: sci.electronics
Subject: Re: measuring a capasitance?
Message-ID: <5170125@hplsla.HP.COM>
Date: 26 Mar 91 21:10:54 GMT
References: <1991Mar4.085406.25128@santra.uucp>
Organization: HP Lake Stevens, WA
Lines: 23

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?

In my previous posting, I forgot to ask about parasitic elements in this
capacitance.  Are you trying to measure the capacitance in the presence
of a (presumably variable) series or shunt resistance?  If so, that may
temper how you want to go about it.  For example, the shunt resistance
in a tuned circuit would have little effect on the point of resonance, but
in the "compare the currents" method I suggested, could be an insurmountable
difficulty (because the excitation frequency is generally low enough that
the capacitive reactance is itself rather high; that's fairly important to
porper operation of that scheme).  On the other hand, a series resistance 
(so long as it is very much lower in impedance magnitude than the capacitance)
would have very, very little effect on the net impedance (and therefore the 
measurement) because the net impedance is a sqrt of sum of squares thing.  
In response to the ideas, any better description of just what you are doing?  
(Don't want to be nosey, just want to be able to offer more enlightened 
suggestions).