On Tue, 23 Mar 1999, K.V.Madanagopal wrote:

> Hi!
> Are there commerical suppliers of capacitance sense ckts (transresistance
> amps) which can sense 10's of fF's (or tenths or nA's)
>
> Thanks
> Madan
>
>
>

Dont know about commercial suppliers, but I'm familiar with the
problem. Sensing such small capacitances using a commercially
produced, conventionally-packaged ICs (such as a FET op amp) as a
charge amplifier is difficult because the stray shunt capacitances
on the package pins is 3 orders of magnitude larger than the
capacitance being sensed. Mounting packaged parts on a circuit board
introduces even more capacitance, as does locating the amplifier
remote from the sensor.

We have combined MEMS-fabricated sensing elements with a custom VLSI
chip to form complex sensors. The custom IC incorporates arrays of
charge detectors, each of which is capable of resolving less than 1fF.
Perhaps incorporating a custom IC right into your sensing structure
may be useful.


Michael Mladejovsky, Phd.