I hadn't looked hard, but hadn't spotted that 'ingenious and innovate' positioning of the only guaranteed source of high-frequency noise, the crystal, w.r.t. the ADC inputs. A useful observation. Thank you.
I had noticed Atmel had changed the pin-out diagram from "you can see every pin function on one pin out diagram", which I really like, to just the name of the I/O register.
e.g. in doc8135, about the ATxmega128D4, ATxmega64D4, ATxmega32D4, ATxmega16D4 figure 2-1
I had felt the diagram change was a change for the worse because it made it harder to work with the chip. I wonder what other annoyances are masked by the bland port-only pin-out diagram?
Thank you.