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Stability Analysis (stb) OTA for active RC integrators

Paulo A

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.

Parents
  • Paulo A

    correcting...(I could NOT perform correctly the stb analysis).

  • Andrew Beckett
    in reply to Paulo A

    It would be rather helpful if you provided a picture of your testbench schematic (indicating where you've put the probe if any for the stb analysis) - and a clearer picture of the architecture of your circuit. It's a little hard to understand quite what you've done.

    Andrew.

Reply
  • Andrew Beckett
    in reply to Paulo A

    It would be rather helpful if you provided a picture of your testbench schematic (indicating where you've put the probe if any for the stb analysis) - and a clearer picture of the architecture of your circuit. It's a little hard to understand quite what you've done.

    Andrew.

Children
  • Paulo A
    in reply to Andrew Beckett

    Dear Andrew,

    Thank you for the quick reply. The testbench is attached. I am using one probe only.  It is a low-voltage circuit and all dc operating points are well defined. I am performing a DC analysis before stb to verify it.

  • Andrew Beckett
    in reply to Paulo A

    Bit hard to tell without more detail - this is something that would be far better dealt with through customer support. However, it looks to me as if the diffstbprobe is in the wrong place. I think it should be in the feedback loop (each half being in series with the capacitors in the feedback loop). However, the best thing would be to contact customer support so that we can look at the data in more detail.

    Regards,

    Andrew.

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