polyam
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Hi,
I am designing a CT delta sigma modulator. I just want to focus on the first integrator and current steering DAC. I have a 3-bit quantizer and my first integrator is an active-RC integrator. You can see my first integrator in the picture. I’ve implemented a 3-bit ideal current steering DAC in a way that you see in the picture. When I use this ideal DAC, the transient signal, at the nodes 1 and 2, I showed in the integrator, looks kind of weird to me. My supply voltage is 1.2 V and the CM level is 0.6 V. But the level of signal it not located at a level I expected. When I replace the ideal DAC with a transistor level one, the CM level goes to a level that it should be. I have to mentioned that both ideal and real DACs give me a correct result in terms of output spectrum!!! This is really important for me because when I am trying to test my op-amp individually (transistor level) with the ideal DAC, my system does not work and it is not because of the op-amp. I think it is because of the ideal DAC. I am guessing this because when I use both op-amp and DAC at the transistor level I don’t have any problem and my system works well. Can anyone help me to understand what the cause is and how to modify it?
Many thanks
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