[sdiy] quick capacitance question

[Jerry Gray-Eskue]

David,

The guard traces are more about killing EMF than capacitance.
They basically give you a 2 dimensional ground shield around your inputs.
If you do not have any disturbers (Digital, Clock, square wave, etc) near
your inputs, short leads may work just fine.

- Jerry

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of David G. Dixon
Sent: Monday, January 25, 2010 6:31 PM
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] quick capacitance question


I've been laying out an alternative version of the CGS Analog Shift Register
using DG409 in place of CD4052 and LF444 in place of four LF35x.  For
various reasons, it is very inconvenient for me to provide guard traces for
the sample and hold inputs, but I have managed to keep those input traces
very short.

This caused me a little bit of concern until I actually calculated the
capacitance between the traces.  The longest stretch of parallel trace I've
allowed is 0.6", the trace thickness (for one-ounce copper) is 0.0014", and
the trace separation distance is roughly 0.05".  Assuming an average
relative permittivity of 2, and given the vacuum permittivity of 8.85 pF/m,
this gives a whopping 7.6 fF (femtofarads), or 0.0076 pF.

So here's my question: am I correct in assuming that this tiny capacitance
will not cause a measurable sample drift?

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