Hi Sir ,
If the input voltage is over voltage and what is the output status?
I think the internal diode is turn on and output will present to 0.2~0.7 V (Vd) is right?
If you have an over-voltage condition at the input of the ADG1204 you are possibly violating the Absolute Maximum Ratings for the part (see datasheet page 7).
If you are trying to protect against an overvoltage you should consider a Multiplexer from our Overvoltage protection & Detection range. These devices can stand off voltages of up to +/-55 V on the inputs.
For more information on these parts and on Overvoltages Please see the following article: