Reverse polarity protection for analog input pin

Question

I am designing a PCB with a PIC 16 microcontroller. The purpose of the PCB is to measure the voltage of a button cell battery, before the battery is placed in its device (assume this battery testing is involved in a mass production line, so its used very much, and very quick). Now I am using a sort of clothspin to clamp the battery on the top and bottom side (anode and cathode). But the mistake to reverse the polarity of the battery is very easy to make. For that reason I want to include a reverse polarity protection, so that if the battery is placed wrong (negative voltage on the analog input) it doesn't fry my microcontroller. What is the best option I can use? A diode won't do the trick in my opinion since that makes the voltage that I want to read drop, but I also don't want to make a very difficult circuit for this RPP.

This is the circuit. The Battery wire is going to the analog INPUT. The resistor values are not right yet. They will be something like 100, 1000, 1000 ohm.

Answer 1

The pin has built in protection diodes to its supplies. So the usual method is to use a resistance between what your measuring and the micro to limit the current. Ideally look up what the adc input impedance needs to be. Pick something slightly less than that and check if less than say 1mA can flow through those protection diodes to keep thing safe over hundreds of encounters.

The other method would be a p-mosfet in series with your positive battery terminal. If the polarity is wrong it open circuits the battery so your circuit only sees the negative spike for a moment. You still need to protect against it like above. But it means leaving it in backwards will not strain your circuit for a longer period of time.