Re: Pin input current from external source.
The circuit is quite low impedance < 1k so a little reverse current isnt too big a problem.
The circuit had a wrong resistor hence such a big voltage on the input and that has been fixed now.
I have also designed a USB scope which again uses PIC32mx A2D's.
I havent had a problem with that but the input can only go -5v negative and not over +3.3v positive.
There is a 1k series resistor which limits the current into the A2D.
However I will add Shottky's in the future just to be safe.