Regarding my earlier questions about calming down a 12 bit A/D converter,
here's a followup:

I reworked the layout of the circuit, particularly my grounding strategy
:-), and that helped. I'm now maintaining a separate analog power (same
supply, but I put a low-pass filter from the digital power to the analog
one), and a completely separate ground path that only joins the digital
one at the return point to the supply). There's still some jitter, but
it's down to 2-3 bits (from 5 or so), so not TOO bad. This is on a
breadboard, BTW, and I hope that once it's on a real board with nice wide
ground traces or a ground plane it might be even better.

I'm still getting significant jitter (>5 bits) on *some* input values
from *some* input sources (remember I have many possible plug-in input
sources, and their electrical characteristics are unknown for all
practical purposes), but I haven't yet got a "front end" between the
input and A/D, that's my next step. Any suggestions for a very low power,
single supply (5V), rail to rail in/out op amp?

Thanks again to all who offered their great advice. I am learning by
leaps and bounds, with the help of the experts, and it's truly a blast. I
haven't had this much fun on a job for years :-)

Dave Johnson