Reference

The ADC can use different reference voltages VREF, controlled in the REFSEL field of the CH[n].CONFIG register.

These are:

Internal reference, VREF = 0.9 V
External reference, VREF provided by the EXTREF pin

Note: The external reference voltage should be close the internal reference voltage. Preferably no more than 5% deviation from the internal reference voltage, VREF. Using a lower reference voltage will lead to increased leakage, and can lead to undefined behaviour.

The SAADC is preceded by a gain stage which has a programmable gain. The voltage range seen at the input of the gain stage is:

VRangeDifferential = ± VREF/GAIN

VRangeSingleEnded = ± 0.5*VREF/GAIN

The AIN0-AIN7 inputs cannot exceed VDD, or be lower than VSS. The input ranges are also limited by the REFERENCE and GAIN used. The condition

 [V(P) - V(N) ] * GAIN/REFERENCE <= 1

must always hold true for valid measurements, otherwise the ADC will saturate and report the max value determined by the RESOLUTION.