We've never used the differential channel features in any of our AVR aps that use the ADC. I've got a sensor that gives a bipolar millivolt output, roughly in the range of +1VDC to -1VDC.
Perfect, I thought, to use on an AVR differential ADC channel. But my colleague pointed out that the AVR datasheet for the part (like Mega32, Mega169, etc.) says in the Absolute Maximum Ratings section:
Voltage on any Pin except RESET
with respect to Ground ................................-0.5V to VCC+0.5V
and that inputting a bipolar signal such as 5V Vcc, 5V ARef, & differential signal of +/- ARef/2 would violate the absolute maximum ratings.
Now I'm not too worried about frying the high-impedance ADC input with a millivolt signal, but can someone point me to some references on what signal levels can be stuffed directly into the AVR differential ADC channels? Does the signal need to be shifted into Gnd-to-ARef range first, and a "dummy" signal of Aref/2 be constructed for the negative ADC differential input?