33.4.2.6 Input and Reference Selection

The input selection to the ADC is controlled by the Positive and Negative Input Multiplexer bit fields in the Input Control register (INPUTCTRL.MUXPOS and INPUTCTRL.MUXNEG). For single-ended conversions, only MUXPOS is used, while both MUXPOS and MUXNEG are used for differential conversions.

The reference voltage (V_REF) determines the conversion range of the ADC. It can be selected by configuring the Reference Selection bit field in the Control C register (CTRLC.REFSEL).

Except for V_DD and internal ground, the internal reference voltages are generated from an internal band gap reference.

Table 33-5. ADC Input Range and Resolution
Mode	Resolution	Negative Input Voltage (V_INN)	Positive Input Voltage (V_INP) for Minimum Result	Input Voltage (V_INP) for 0 Result	Input Voltage (V_INP) for Maximum Result	LSb Resolution
Single-ended	8	GND	GND		V_REF	V_REF/256
	10					V_REF/1024
	12					V_REF/4096
	13	V_REF			2 * V_REF	V_REF/4096
Differential	8	MUXNEG	V_INN - V_REF	V_INN	V_INN + V_REF	V_REF/128
	10					V_REF/512
	12					V_REF/2048
	13					V_REF/4096

Changing the input or reference selections while a conversion is ongoing will corrupt the output. To avoid this, detect an ongoing conversion by reading the ADC Busy bit in the Status register (STATUS.ADCBUSY). Writing STOP to the Start Conversion bit field in the Command register (COMMAND.START) will stop an ongoing conversion.

After switching the input or reference, the ADC requires time to settle. Refer to the Electrical Characteristics chapter for further details.