My apologies if this has been asked and answered before, I just can't seem to find it searching the forums or Google.
I am using an ATMEGA328 ADC to convert an amplified photodiode signal run through a low pass filter. My conversion values at power up when measuring are consistent, and then consistently drift by decreasing slowly over 5 minutes to 90% of the original value (a 10% decline). Then they stabilize. I am using AVCC with a 0.1uF external cap.
I am trying to figure out why the ADC would change over time. Possibly as a result of heat? I am using a 9 volt alkaline battery with a MIC5205 VLDO voltage regulator. Could it be a power supply issue? The behavior is consistent if I power off, wait a few minutes then power on again.
Any help would be much appreciated.