On my circuit, a negative voltage (up to -1V) is occasionally applied to an analog input pin that normally takes positive voltages. I can either leave the negative voltage on the pin or use a diode and resistor to make it go away. But how bad is a negative voltage on that analog input pin. I remember reading a post saying that's bad but I don't see a reason. Those pn junctions in the input stage can't break down at just a few volts, right? Or is this going to affect the A/D converter somehow? Am I missing something important? Thanks.
The datasheet says that a voltage less than -0.5v on any pin may cause permanent damage. This may just be the Atmel legal department covering their asses but why not use a clamp diode just to be sure.
Any voltage less then around -5vdc will cause an internal protection clamping diode to start conducting current, and as the voltage lowers more there will be more current drawn until the diode is damaged.
The negative voltage won't harm the pin if you can externally limit the amount of current that can be drawn through the forward conducting diode. If you wire a series resistor from the pin to your voltage to be measured then there will be no harm when your measurement voltage goes negative. A 2K resistor should be OK.
Lefty
A 2k series resistor would be ok if the impedance of the device being measured is low, check to make sure that the resistor is not affecting the voltage being presented to the pin.
Those pn junctions in the input stage can't break down at just a few volts, right?
Wrong they can and do.
Thanks for all your replies. Your suggestions are great! In my case, the signal is really weak in power and I didn't even get a couple volts max. The current is about 1ma with 1k resistor. The end result, it worked!