Hi guys , do you know how do you protect your PIC adc inputs against reverse polarity ? Ì'm feeding battery volrtage to my PIC16F877A and would like to protect it against operator error.
The best protection scheme is a series resistor and a pair of schottky diodes (such as BAT54S) one to each power rail. The resistor should be sized to not burn out the diodes and no affect the sample accuracy of the A/D. The Microchip data sheets give info on the accuracy issue.
The schottky keeps the voltage drop below the processors internal protection diode forward drops and therfore prevents latchup and destruction.
We have used this approach for many years with complete success. Typically we use a 10K, 1206 resistor although sometimes go smaller depending on how we are using the A/D, which comes down to the sample rate. The same circuit is also effective on digital inputs that are exposed to the external world.
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