I need to withstand full input voltage with power off. i.e. input V still there when unpowered. I am using ad8422
You will destroy the part if you do this. It's like feeding the part while it's asleep. Yes, you will wake the part at a certain time but eventually you will kill it and as a result, the part will be useless.
1. What input voltage will you use? If it is +/-40V, then the AD8422 can withstand it as long as there is a minimum of +/-2.3V or single 4.6V supply voltage. It can also withstand above the Abs Max input voltage but external protection is required (see page 21 of the datashet) and a supply voltage with the spec table is also required.
2. Why do you need to have an input voltage but without a supply?
3. What exactly is your application, if you don't mind sharing it?
we are building a multi-channel battery tester. It is a large unit with many channels and test procedures running. It could be (and will be) powered down, by the customer, while still connected to a battery it is monitoring.We can ask them not to do that but they will. It is required that the circuitry handle this.There will be a series stack of these test batteries totaling up to 1300v.(Hard to believe people want a 1300v battery, but I guess they do) The individual channel's voltage will be +20v at the board input but divided such that the AD8422 sees < 5v. Each of these channels must be isolated to allow the large common mode voltage (up to 1300v) to be removed. I was wanting to use the Adum6402 series but I dont think it will cover that high of a isolation working voltage. But I stray from the point. Anyway I must protect the front end.