30.3.2.4.2 ADC Voltage Reference

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

When using external reference voltage V_REFA, 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.