ATMEL Xmega ADC Input impedance
The ADC characteristics can be found in the datasheet



There is a specific application note from Atmel for Xmega ADC Using the Atmel AVR XMEGA ADC.
If you refer to section 1.8 there is an explanation of how the output impedance of the circuit that feeds the ADC affects the conversion speed:



The lower the sampling frequency the higher output impedance you can use for the  divider. If you want to use a universal value that can work for a wide range of sampling frequencies then you can go for about 1k or so.