Hi! Sorry for the lag. I'm getting over a lousy cold and the things put off thereby.
The Nano is powered by the USB and the pot by the +5V (really about 4.5V) supply on the Nano. The cap goes directly from PC7 to Gnd. Well, the columns(?) on the breadboard, of course. (The
pot was originally powered by the +5V buss powered by the nano +5V but even when I de-powered the breadboard and ran a wire directly to the pot, the noise remained.)
ADC ref is Vcc. I can see the full 0-1027 range from 0V-Vcc.
The 1 uF cap size was to see what would happen. I'm still rather NEWB! to the whole bypass cap thing. Also it seemed about the right choice for the 100kHz - 10MHz regime I expected for the noise.
My scope (C. Zeitnitz's cool soundcard oszilliscope http://www.zeitnitz.de/Christian/scope
) doesn't show anything that I can interpret. I mostly use it as a logic analyzer/timing checker.
Thanks for the interest. I'll get back to something more systematic in a day or two.