[sdiy] quick capacitance question

[Ian Fritz]

At 05:30 PM 1/25/2010, David G. Dixon wrote:
>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?



The capacitance itself will not. But that's not relevant. The purpose of 
the guard ring is to prevent surface leakage currents -- the guard and the 
signal are at the same potential so no current can flow into the hi-Z node.

Ian