AVR-Chat

Gustavo,

Assuming that the chip is good, fig 98 in the data sheet (pg 202) shows
channels 3 and 4 going directly into the input mux so, if properly configured,
thay should act identically.

Changing the REF select bits or putting the inputs into differential or
pseudo-differential mode by setting the MUX3 and/or MUX4 bits can lead to
unexpected values.

Have you verified the ADMUX register content?

Bruce