TI recommends connecting a combination of a 4.7-µF plus a 100-nF low-ESR ceramic decoupling capacitor to each AVCC and DVCC pin. Higher-value capacitors may be used but can impact supply rail ramp-up time. Decoupling capacitors must be placed as close as possible to the pins that they decouple (within a few millimeters). Additionally, separated grounds with a single-point connection are recommended for better noise isolation from digital-to-analog circuits on the board and are particularly recommended to achieve high analog accuracy.
Figure 7-1 shows the recommended decoupling of the power supply pins.