How to protect ADC input against the negative voltage?
Different requirements will require different methods of protecting an ADC input against out of range input voltages. Some may compromise accuracy, some may not fully protect the ADC.
One simple way is to buffer the ADC input with a rail to rail op-amp running from the ADC rails. Although this passes the problem to the op-amp, we can often configure the op-amp gain resistors to tolerate large input voltages.
Another way is to use a shunt diode to ground. This will allow a small negative voltage through to the ADC, which is usually tolerable, especially if the input current is limited. The input resistor R1 limits current into the protection diode. The next resistor R2 limits current into the ADC input when D1 is shunting current. D2 provides protection against large positive voltages.
R1, R2 values should be adjusted for your particular situation. High enough to limit current to safe levels, low enough to preserve ADC accuracy against input current requirements.
If tighter clamping than 0.7 is required, schottky diodes can be used, or a transistor can be used as shown. This will limit the negative excursion to around 0.2v below ground, or 0.1v if it's biassed to 0.6v though with an increasing leakage current as the clamp voltage gets closer to ground.
Use a diode in series with the ADC. Place the diode in such a way that it will appear to be reverse biased to a negative voltage.
The potential drop will be maximum across the diode in this case, and hence, your ADC will be protected.
Or else, you could just pass your analog input through a full wave rectifier and lose the negative voltage before it even reaches the ADC.
Note:There will be some potential drop across the diode, around 0.7 volts for standard diodes. Use diodes only if you're sure that a drop of 0.7 volts in input will not cause a relatively bad output.