ADC input protection?
A simple pair of diodes is all you need.  One from the ADC input to \$V_{CC}\$, anode on \$V_{CC}\$.  One from the ADC input to GND, cathode to GND.



If \$V_{in}\$ goes above \$V_{CC}\$ then the excess will go through the upper diode.  If it falls below GND then it will flow through the lower diode.  The input to the ADC will be just the range between \$V_{CC}\$ and GND.

Most ADC inputs will have these diodes in them already - check the data sheet - but it can't hurt to have your own extra ones.

Shottky diodes would be best as they switch faster than normal silicon ones.