NXP provides some PCB layout design guidelines for the ADC modules in their LPC1100 and LPC1300 microcontrollers. These ADC modules, like many microcontrollers, use the power supply voltage as a reference so special care has to be taken to eliminate noise:
The microcontrollers in the LPC1100 and LPC1300 families are all equipped with a single external power supply. The digital (core) and analog parts (ADC/Vref) are all connected to the same external supply. Without precautions, noise produced by the core could influence the readout of the ADC via the internal Vref.