Isolation amplifier and common mode

[anindya]

Can anyone explain/give reference as to how an isolation amplifier can increase the common mode range of an instrumentation amplifier. I will give an example in a TC signal conditioning circuit an INA114 instrumentation amplifier is used as a pre amplifier before AD210 an isolation amplifier. The signal source / TC is assumed to be floating input to the IN amp. Normally the common mode range of Inamp is (power supply-2) Volts approximately. It is said that by using the isolation amplifier the common mode range is extended to the isolation barrier level. I have an intuitive understanding but i am looking for a detailed answer

 

[FvM]

INA114 common mode range is in fact limited considerably below it's supply voltages. The isolated input circuit, including the INA114 supply ground must be connected to the signal ground to make it work.

 

[anindya]

INA114 common mode range is in fact limited considerably below it's supply voltages. The isolated input circuit, including the INA114 supply ground must be connected to the signal ground to make it work.

Agreed when i connect the two, a ground loop is formed giving rise to a common mode voltage at the amplifier. If the common mode is large it will saturate the inamp or even worse destroy it. My question is when we use isolation amplifier how is this common mode range increased ? Is it due to the fact that no current can flow unless there is a breakdown of the isolation barrier ?

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INA114 common mode range is in fact limited considerably below it's supply voltages. The isolated input circuit, including the INA114 supply ground must be connected to the signal ground to make it work.

Agreed when i connect the two, a ground loop is formed giving rise to a common mode voltage at the amplifier. If the common mode is large it will saturate the inamp or even worse destroy it. My question is when we use isolation amplifier how is this common mode range increased ? Is it due to the fact that no current can flow unless there is a breakdown of the isolation barrier ?