Analog read different with external and usb power

I think that the poster was thinking about reading from different analog pins.

Only (according to the datasheet) for a high impedance source where the sample and hold circuit may not have time to settle.

Contatore Geiger: in attesa dei tubi... Ovvero: Geigerino, il mio contatore Geiger con Arduino - Pagina 18 - New Radioactivity Forum
Contatore Geiger: in attesa dei tubi... Ovvero: Geigerino, il mio contatore Geiger con Arduino - Pagina 18 - New Radioactivity Forum

Not a good cite. Only the datasheet should be believed.

Mark