Datasheet

333XMEGA A4U [DATASHEET]
Atmel-8387G-AVR-ATxmega16A4U-34A4U-64A4U-128A4U-Datasheet_03/2014
38.4 ATxmega128A4U
38.4.1 rev. A
 ADC may have missing codes in SE unsigned mode at low temp and low Vcc
1. ADC may have missing codes in SE unsigned mode at low temp and low Vcc
The ADC may have missing codes i single ended (SE) unsigned mode below 0C when Vcc is below 1.8V.
Problem fix/Workaround
Use the ADC in SE signed mode.