Can I connect Analog and Digital Voltage Source together?
Analog circuits are sensitive to variations and noise on their supply pins.  In general, you connect A_VDD (and A_GND, if there is one) to a quiet supply, while D_VDD (and D_GND) can be connected to something dirtier.

It should be in the data sheet.  Either connect A_VDD to a filtered version of D_VDD, derive A_VDD separately with its own dedicated regulator, or connect them together and accept that your ADC readings will be noisier than the datasheet specifies.