This ADC features multiple internal ADC reference voltage between 0.55V and VDD and external reference input VREFA. Different internal reference voltages are 0.55V, 1.1V, 1.5V, 2.5V, and 4.3V.
The reference voltage for the ADC controls the conversion range of the ADC. Input voltages that exceed the selected reference voltage will be converted to the maximum result value of the ADC. For an ideal 10-bit ADC, the maximum result value 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. The internal reference is generated from an internal bandgap reference through an internal amplifier, controlled by the Voltage Reference (VREF) peripheral.