AVR-Chat

On Tue, Nov 18, 2008 at 7:11 AM, Kathy Quinlan <kaqdialup@iinet.net.au> wrote:
> NO, the internal diodes are not designed for this level of protection,
> depending on your external resistor values, you could exceed the maximum
> sink current of the device.

Definitely correct.  Although the internal protection diodes exist,
you are never supposed to use them.
A series resistor to limit current, schottky diodes to VCC and ground,
and another resistor into the ADC input pin is about the minimum I'd
use.
For real protection, a buffer op-amp would be better.

A lot depends on how much current this device can source, but I
suspect it's pretty low.

I did a battery management system recently that had to work with 6V -
48V input of any polarity, AND be "taser-proof" on the input leads.
That was fun.