ADC minimum value is half of max
Did you read and do this...


  The analog offset of the AFE is configured in the AOFF field in the Channel Offset Compensation register
  (AFEC_COCR). The offset is only available in Single-ended mode. The field AOFF must be configured to 2048
  (mid scale of the DAC) when there is no offset error to compensate.To compensate for an offset error of n LSB
  (positive or negative), the field AOFF must be configured to 2048 + n.


Page 1276 of the spec sheet.