51.6.3 Additional Features

The accuracy of the ADC is highest when the input common mode voltage (VCMIN) is close to VREF/2. To enable a full range of common mode voltages (rail-to-rail operation), the Rail-to-Rail bit in the Control C register (CTRLA.R2R) must be written to one. The Rail-to-Rail operation requires a sampling period of four cycles, and this is achieved by enabling offset compensation (SAMPCTRL.OFFCOMP = 1). The Rail-to-Rail operation should not be used when offset compensation is disabled.