Hello,
I am designing a CT sigma-delta modulator with active-RC integrators. I have designed my OTAs and performed manual analysis and compensation of the CMFB loop (I could perform correctly the stb analysis).
However, I would like to use the stb analysis to check it. Most tutorials I have verified on the internet uses stb analysis for operational amplifiers with resistor feedback instead of capacitive feedback.
In my case, I have a fully-differential OTA, and I am using the diffstbprobe. Let's consider my first active RC integrator. It has an input resistor of Rin=100kohm and an 8 pF capacitor. The load of its integrator is another active RC integrator with R=500kohms and C=1 pF. Thus, the load of my first active RC integrator is 500 kohms||8 pF.
I have performed the AC analysis (open loop) with a load of 500 kohms||8 pF and found: DC gain of 43 dB, GBW of 13 MHz and PM of 89 degrees.
For stb analysis, I have followed the tutorial: "Loop Stability Analysis: Differential Opamp Simulation" from Vishal Saxena & Zhu Kehan. However, instead of a feedback resistor I used an 8 pF capacitor, and a load composed by a 500 kOhms resistor. Also, the input resistor of the active RC integrator is equal to Rin.
The results of stb simulations for the differential gain are attached. It seems the TF of high-pass filter. Please, can someone give me some tips on what I am doing wrong?
Should I use the stb analysis for circuits with feedback capacitors?
Kind regards.
