Mark E. Skeels wrote:
> If I read Olin's post correctly, that technique was designed to
> minimize radiated RF, right?

That's one part of it.  Keeping the high frequency currents off the ground
plane prevents them from generating offset voltages there, which is noise
added to analog signals.  Remember that the ground plane impedence goes up
with frequency.  While it may be a dead short for practical purposes at DC,
you can't assume that's true at 100s of MHz.

> In the case of a mixed signal board I doubt this would work very well
> w/o considering other factors; as you point out, Russel, there are
> tradeoffs. In my case I am using the PIC's analog and PWM features at
> the same time.

This is a good example where you want to use the AVss and AVdd PIC pins
properly.  If using a local ground patch under the PIC, you still connect
the AVss pins to the main ground plane.  They don't carry much current and
therefore won't inject much noise on the main ground plane, and the A/D wil=
l
use the true zero volts of the main ground plane as its reference.
Similarly, AVdd is Vdd filtered by a chip inductor in series and a capacito=
r
to ground.


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