I think I got this right but I better check with the experts.
The ATXMEGA16A4U-AU seems to be the smallest Xmega with both ADC and DAC. Also USB and possibly correctly working ADC and DAC.
The ATXMEGA16A4-AU seems the same but no USB and for some reason more expensive?
I would have gone for the non USB type as I don't need it but not if it is more expensive. :roll: