[quote user="Marco2002"]The VCC pin of the OpAmp is directly connected to a generic pin of the microcontroller, which turns ON the OpAmp when needed.[/quote]
You shall connect amp to AVCC rail instead, run tests. You think about extreme decoupling strategies of AVCC, DVCC, filtering AVCC from digital noise and ... DANG!! - connect amplifier to output pin that comes out of noisy uC digital fabric.