In my project to measure analog voltage or current, I'm using ADG1402 analog switch to connect or not a shunt resistor used to measure input current in a system with a ADC. My input is differential and protect against differential surge using a tranzorb, overcurrent by using a PTC and schottky diodes are present between ADG1402 inputs and power supplies (Vdd=+15V and Vss=-15V). When I apply a 500V surge in common mode (input signal and ground), ADG1402 is damaged by this surge and my system crashs. After ADG1402 is removed, system works well.
My question is what is causing this damage with respect to ADG1402 ?