Hi,
The chip’s working voltage (VCC pin4 and pin 6) is 3.3v, and the ADC working voltage (AVcc pin 18) is 3.3V too. So the ADC reference voltage (AREF pin20) can not be higher than ADC working voltage. I have try to connect AREF to 5v, and find that range of the analog is 0~5V, however the AVcc, VCC pins become 5V.
Thanks!