ADC Voltage Reference

The reference voltage for the ADC (VREF) controls the conversion range of the ADC. Input voltages that exceed the selected VREF will be converted to the maximum result value of the ADC, for an ideal 10-bit ADC this is 0x3FF. VREF can be selected by writing the Reference Selection bits (REFSEL) in the Control C register (ADC.CTRLC) as either VDD, external reference VREFA, or an internal reference. VDD is connected to the ADC through a passive switch.

When using external reference voltage VREFA, ADCnREFSEL[0:2] in the corresponding VREF.CTRLn register should be configured to the selection closest, but above applied reference voltage. For external references higher than 4.3V, use ADCnREFSEL[0:2] = 0x3

The internal reference is generated from an internal bandgap reference through an internal amplifier, controlled by the Voltage Reference (VREF) peripheral.