31.3.3.5 Input and Reference Selection

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 (V_REF) controls the conversion range of the ADC. V_REF can be selected by writing the Reference Selection (REFSEL) bit field in the Control C (ADCn.CTRLC) register. Except for V_DD, the internal reference voltages are generated from an internal band gap reference. V_DD 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.