Isolated High Side High Voltage Current Sensing

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Hi,

I am seeing the AD210 is high common mode isolation amplifier. However it is huge is size for my application. If I have 2 power supply sides of the isolation amplifier isolated wrt each other coming  from my auxiliary supply , do I still need the high common mode tolerance capability for high side sensing? I have 900V battery and want to high side current sensing. Can I use some simple SOIC type isolation amplifier which has isolation >2kV but not high common mode capability.

For isolated amplifier why do we care with common mode?

Thanks,