The input selection to the ADC is controlled by the Positive and Negative Input Multiplexer (ADCn.MUXPOS and ADCn.MUXNEG) registers. If the ADC is running single-ended conversions, only MUXPOS is used, while both are used in differential conversions.
The reference voltage for the ADC (VREF) controls the conversion range of the ADC. VREF can be selected by writing the Reference Selection (REFSEL) bit field in the Control C (ADCn.CTRLC) register. Except for VDD, the internal reference voltages are generated from an internal band gap reference. VDD must be at least 0.5V higher than the selected internal reference voltage.
The input and reference selections are not buffered. Changing any of these while a conversion is ongoing will corrupt the output. To safely change input or reference when using Free-Running mode, disable Free-Running mode, and wait for the conversion to complete before making any changes. Enable Free-Running mode again before starting the next conversion.
After switching input or reference, the ADC requires time to settle. Refer to the Electrical Characteristics section for further details.