Hi Dhimas,
Why would you assume things when everything is written clearly in the Atmega16 datasheet.
AVCC
Analog Power Supply: This is the power supply voltage for analog
part
For a normal use this pin must be connected
AREF
Analog Reference: reference for analog converter . This is the
reference voltage of the A/D converter. As output, can be used by
external analog
To know the range of reference voltage that can be applied, please check the ADC section or absolute maximum ratings in the datasheet.