[sdiy] quick capacitance question

[David G. Dixon]

> 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.

Ah, that's a new one for me.  I've just read a bit about it on the
internets.  Oh well, when I start laying out "proper" boards with a software
program (which will have to be pretty soon, since I'm starting to sell),
I'll start putting in guard traces.  In the meantime, I don't think I'll
have much of a problem.  The typical input bias current of LF444 is 10pA.
My sampling cap is 100nF, so the charge would fall about 0.36 V per hour
just from that.  I can't imagine that surface leakage current would be
anywhere near 10pA, especially if the board is clean...?