On Fri, 26 Apr 2002, Dave Dilatush wrote:

> Aaron wrote...
>
> >First thanks to all that helped so far. Especially Brian Taylor for that
> >great idea.  I actually fabricated a little pressure transducer today on
> >pcb worked great!! (approx 35pf 28-30pf on applied pressure)
> >Used a cut up chip bag for the diaphram :P


Delta C = 2pF. That is enormous! :)

>
> I've never seen capacitance transducers sold alone, though I
> admit I haven't looked very hard.  Some of the industrial
> pressure transmitter manufacturers (Rosemount, Inc., for example)
> use their own proprietary differential capacitance transducer in
> their products; but these are specially built.

Qprox sells capacitance sensing IC's, but not the whole transducer

http://www.qprox.com/

> One of the problems with capacitance transducers is that if you
> need high resolution, repeatability and stability, they require a
> significant amount of analog circuitry to convert the capacitance
> change into a DC signal that can be measured with an A/D
> converter.  This tends to make them a challenge to use.

Working for Synaptics I can speak first hand about this! Our TouchPad
capacitance ASIC's are capable of resolving sub picofarad changes in
capacitance. While I obviously can't divulge the details on how, I can say
that there is a significant amount of analog circuitry involved. I can
also say that analog circuitry and the accompanying A/D are inadequate.
In addition, the on-chip microcontroller performs a significant amount of
signal processing. Resolving finger position to 1/1000th inch requires
quite a bit more circuitry than a 555 and an 'F84!

Scott

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