hi all,

i am new to analog design, i need to simulate and understand a fully differential op-amp. i tried to simulate a single op-amp before but i never simulate a differential output amp before. Can anyone tell me how to simulate the input common mode range of a fully differential op-amp? i tried to search document online but i couldnt get any as well.. thank you in advance  :)

Alicia,

  Just to expand on Jim's comments a little. The methodology I have used
for the measuring the input common mode range of a single-ended op-amp
in ADE is,

1) Set up a test bench to measure open-loop gain
2) Setup a parametric sweep of the input common-mode voltage
3) Simulate
4) Plot the dc, low frequency, open loop gain versus input common-mode voltage
5) Find the max value of the open loop gain, Avo_max
6) Then use the cross function to identify the input common-mode levels for
   Avo_max-3dB levels of the open loop gain

The big challenge is that there is no standard definition of input common-mode
range. My preference is for using the -3dB level to determine the range where
open loop gain is not degraded by the input  common-mode level.

  It might actually make a good topic for discussion, How should input
common-mode level be defined/measured?

One other suggestion is that the ideal_balun can be a useful block for creating
the input signal. Connect pin d to the differential input signal generator and pin c
to the common-mode input signal generator, connect pins p and n to your
circuit.

                                                                    Best Regards,

                                                                       Sheldon