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 value is 0x3FF.
VREF can be selected by writing the Reference Selection (REFSEL) bits in the Control C (ADCn.CTRLC) register as either VDD, external reference VREFA, or an internal reference from the VREF peripheral. VDD is connected to the ADC through a passive switch.
When using the external reference voltage VREFA, configure
ADCnREFSEL[0:2] in the corresponding VREF.CTRLn register to the value that is closest, but
above the applied reference voltage. For external references higher than 4.3V, use
ADCnREFSEL[0:2] = 0x3.
The internal reference is generated from an internal band gap reference through an internal amplifier, controlled by the Voltage Reference (VREF) peripheral.