Fully differential Opamp CMFB - Strange behavior in closed loop

[Chr's]

Hi All,
I'm still on TT polyphase filter with current input and I saw a strange behavior in this structure when I'd simulated the CP1.

My Opamp is a classical 2 stages with a CMFB on activ load. In Open loop the gain is 58dB for a UGF at 27MHz and Phase margin is 45 degree.

The stange behavior is the following:
In the global strucure, when I'm applying a current sine on I and Q ways, the voltage point generated on Iinp and Iinn are not on opposite phase, they are in pahse Oo' !!!
When I add a vcvs with unity gain on each outp (outp and outn), the signals I and Q are in opposite phase.
What could be the issue ?
Thanks in advance for your help.

 

[shar2013]

Hi,
This behavior sounds like you have more than one DC valid OP. If you are running an AC simulation try setting initial conditions. Or you should run transient with a ramp up on your voltage supplies and check the OP. If you do have an extra state valid for DC OP you should add a start circuit.