Analog read different with external and usb power

Projects Programming
17

johnwasser:
The ATmega328p datasheet says

The OP is using a 1.2M:100k divider. I'm guessing this has an impedance greater than 10kOhms. Switching between analog inputs will require extra time for the sample/hold capacitor to charge. Reading from the same input a second time allows for more settling time and is helpful with higher impedance inputs.

I concur (So does my experience with this chip).

Regards.