I need to read a 0-6V buffered analog signal from a sensor with the ATXMega256A3BU ADC. With an external voltage reference I can measure voltage level up to about 3V. I'm going to realize a voltage divider but I need some help in finding the correct equivalent resistance of the voltage divider ( R1||R2 ) that is the voltage divider output inpedence. Atmel says that in the worst case the ADC input resistance Rchannel is 4.5k and the S&H capacitor Csample is 5 pF.
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.