Re: simulation!
you can calculate the gain by the following method.
just give a sweep of dc (vref + or - 20mv) and the other one keep it as vref.
in the dc voltage give a value of ac amplitude of 1v .
Then go to analysis and choose ac and give 1 to 100M in the window where you need to do the simulation.
Then run simulation.
in ADE go to Results ----) Direct Plot ----) AC20dB---) select the signal you want to see the gain and press escape.
in the waveform window you can see the gain of the OTA.