Thanks for your Concern .
We didn't design the Real Op Amp yet , we are now working with a behavioural model for the fully differential op-amp built out of 2 vcvs . I believe it doesn't have the mechanism of CMFB , so , maybe that's the problem as the input common mode to the Op-amp isn't constant while the DAC is connected.
So , we'll take a few days to build the Real Op-Amp and get back to u if the problem still exists.
Thanks for your help ..
Just out of curiosity, are you also using RZ DAC.? Do you see any second harmonic in the spectrum?
Your test bench is not correct because there is no negative feedback through the DAC. How have you generated D1,D0 and D_1? It seems that they are independent of the opamp output. In a Delta Sigma, DAC provides the negative feedback around the loop, which will keep the virtual ground swing small.
Monsoon is correct, where is your feedback. You can simulate it open loop to see how it operates, but your integrator output will sooner or later go to the rails, then your input nodes will not be a VDD/2...
what are those resistors R2 and R3???
Also, Just to let you know, analog/mixed signal design is also an "art form",
you should be more around 2-4Xfs
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