Protect ADC input from over-voltage with Schottky diodes
You can do something like this: 







simulate this circuit &ndash; Schematic created using CircuitLab

D2 is a shunt regulator capable of sinking about 100mA. R2/R3 set the regulator voltage to 3.3V nominal. R1 provides the bias current the regulator needs without loading the inputs (it needs 1mA minimum to regulate). 

As shown you can protect multiple inputs with a single shunt regulator. The series resistors limit the op-amp output current and may not be required if the op-amp has a guaranteed maximum short-circuit current that is sufficiently low. 

As shown it will protect as many as 5 inputs by adding diodes and resistors. 

The voltage drop due to Schottky leakage and the series resistors should be negligible, but check the worst-case conditions (maximum temperature).