Strange behavior of Closed Loop Full differential Amplifier

[Ali263]

I am too lazy to read whole discussion.
What is you ac analysis setup and gain expression measurement?
For me it looks like "invalid user" case (in testbench not circuit)

dB20 ((VoutP - Vout_M)/(VinP - Vin_M))

dB20 of differential output divided by differential input? Is it wrong?

 

[Dominik Przyborowski]

dB20 ((VoutP - Vout_M)/(VinP - Vin_M))

What is in parenthesis?
Provide exact expression, please.

Some useful hints for future:
1. Common practice is to use unit input (ac=1), to not worrying about division.
2. For SE to diff conversion (and vice versa) is good to use ideal_balun element.