I found that even short spikes above the analog supply voltage immediately causes severe damages to the chip.
Also protect the inputs from negative voltages. A schottky diode applied to the last OpAmp takes care of that (in the feed back path).

This may easily occur, when powering the analog circuits (e.g. Op Amps) and the ADuC from different supplies.

The best cure against frying the chip - put a 1KOhm resistor and a 0.1 uF ceramic capacitor in front of each A/D input -the closer, the better.

This is quite different to AD's suggestions. When using the above filter/protector you might get some strange samples at e.g. 6V input, BUT the chip survives.

This is supported by a 10 Ohm resistor/filter applied to the analog supply line. Use two caps: 0.1uF and one 10uF, the 0.1uF very close to the chip.

It is also important, to connect both ground systems
as close as possible at the chip.

Strictly following these rules, you will never loose a chip again- proved by some dozend application running even under very harsh conditions.

Hope, it helps