Does the AD8605 have a protection against voltages below 0V ?
I can see in the datasheet (p 16) that there is a protection against overvoltage. It says that if the voltage is 0.5V higher than Vcc supply, a current limiting resistor should be added to limit the input current below 5mA. I suppose that this 0.5V means there is an internal clamping diode to Vcc, but I don't see Vcc+0.5V specified in the absolute maximum ratings. Why is that?
Then, I don't see anything in the datasheet regarding protection against voltages below 0V. Is there a clamping diode to ground?
In fault condition, my AD8605 can see a voltage as high as -/+65V with a 250K series resistor. Can the AD8605 withstand it?
This is a general rule on protecting the analog inputs in overvoltage conditions.
Most of the amplifier has an ESD diodes that turns on when overvoltage occur and limiting the current passing to that diodes by around 5mA, we can avoid damaging this protection diode.
We do specify that the input voltage should be limited to GND to VS and differential voltage to 6V on the Abs max section. If you are going to exceed on that specification then you need to consider putting series resistor to limit the current going into the device to avoid breaking the parts.
I agree that this part should survive at +/-65V as long as there is a 250k series resistor. Just be aware though that this series resistor gives additional offset voltage (Ibias x Rseries) as well as thermal noise that you need to be accounted on your design.