I believe that simulating the open loop gain of the operational amplifier from the open loop needs large care to compensate for the offset voltage otherwise simulation will fail. However, for fully differential amplifier the CMFB circuit is assuring that ampliifer is working in the saturation region, therefore I am testing the open loop gain and AC char from open loop setup, is that ok ?
I believe that simulating the open loop gain of the operational amplifier from the open loop needs large care to compensate for the offset voltage otherwise simulation will fail.
I don't believe so. Most people are simulating open loop gain in an intelligent setup, using some kind of DC feedback that doesn't affect the AC gain. That's not so simple in hardware but very easy in simulation.
yes you are right by using the DC feedback like probe this will make the simulation very easy, however I don't know why using the diffprobe with my fully differential amplifiers is not giving me the accurate result, therefore I shifted to simulate the AC performance using AC simulation but from open loop, no feedback is applied