The default configuration will enable the respective source when the ADC0 or the AC0 are requesting a reference voltage.

The voltage references for the ADC0 and AC0 can be forced ON by writing a ‘1’ to the respective Reference Force Enable bits (ADC0REFEN and AC0REFEN) in the Control B (VREF.CTRLB) register. This serves to reduce the start-up time of the respective peripheral.

The default reference voltage for the AC0 is 1.024V but can be configured by writing to the AC0 Reference Select (AC0REFSEL) bit field in the Control A (VREF.CTRLA) register.

The reference voltage for the ADC0 is configured by selecting the reference source within the ADC0 registers. Refer to the ADC documentation for details.