How should I protect from overvoltage ADC?
You can use a non-inverting comparator op-amp such that the basic configuration for the circuit detects when the input signal, VIN is ABOVE or more positive than the reference voltage, VREF producing an output at VOUT which is HIGH as shown.


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With the help of a NPN you can pull the voltage applied to the ADC Pin down to zero by using this circuit " one of my designs "