[sdiy] quick capacitance question

David G. Dixon dixon at interchange.ubc.ca
Tue Jan 26 01:30:43 CET 2010


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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