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How to use PSS+PSTB or PSS+PAC to simulation the loop gain of the amplifier in the MDAC in each clock phases

ringamplifier

Hello, everyone!

I am currently designing  a amplifier for the multiplying dac (MDAC) for the pipeline ADC. The circuit diagram is show in Fig. 1. This circuit operates  in two clock phases: phi1 and phi2. In phi1, the amplifier is auto-zeroing and its offset will be sampled in capacitor Cc. Vin is sampled in C1 and C2. In phi2, the amplifier will do the amplification. Clearly, the feedback factor of the amplifier in phi1 and phi2 is different. So I want to simulation the loop gain in phi1 and phi2 separately and also want to see the effect of different feedback factors.

Since this circuit is a discrete time circuit, it seems that I should use PSS analysis to find its operation point and do the small signal analysis to analysis the loop gain. 

I also found a slide in the internet to teach how to use the PSS+PSTB simulation to analysis the loop gain of switched capacitor CMFB. The link is   

lumerink.com/.../Loop%20Stability%20Analysis.pdf

However, I have a question about the method using in this slide. For better description, pls see Fig. 2 (actually page 28 of the slide). The SC CMFB also operates in two clock phases. The feedback capacitor of the CMFB circuit is different is each phase. So the question is how can the PSS analysis distinguish two clock phases and PSTB simulation results is corresponding to which clock phase?

So I want to make a clear statement of my question:

How to use PSS+PSTB or PSS+PAC to simulation the loop gain in each clock phase

Fig. 1

Fig. 2

Parents
  • Andrew Beckett

    The Periodic Stability analysis (pstb) does not give you the loop gain in each phase. It gives you the time-averaged loop gain over the whole period, which is what you really want. If you want the loop gain in a particular phase, you could run a transient, set up a stb analysis, and then specify the acnames and actimes parameters of the tran analysis to measure the loop gain at the times where each loop is closed. However, that is probably not what you want... pstb is more likely to be useful.

    Similarly, Periodic AC (pac) gives you the time-averaged small-signal response to a small signal input over the period.

    A couple of corrections from the document you referenced (there may be others, I just scanned it quickly):

    • The iprobe does not "break" the loop. The loop remains closed at all times, which is the point of these analyses. It is the perturbation/measurement point for the loop gain
    • You should no longer use cmdmprobe to measure differential and common-mode stability. Instead there is a superior component, diffstbprobe which copes with the loops not being balanced (i.e. if there is common-mode to differential-mode leakage or vice versa) and also allows the choice of whether you are measuring common mode or differential mode to be made on the analysis form, rather than on the component itself.

    Regards,

    Andrew.

Reply
  • Andrew Beckett

    The Periodic Stability analysis (pstb) does not give you the loop gain in each phase. It gives you the time-averaged loop gain over the whole period, which is what you really want. If you want the loop gain in a particular phase, you could run a transient, set up a stb analysis, and then specify the acnames and actimes parameters of the tran analysis to measure the loop gain at the times where each loop is closed. However, that is probably not what you want... pstb is more likely to be useful.

    Similarly, Periodic AC (pac) gives you the time-averaged small-signal response to a small signal input over the period.

    A couple of corrections from the document you referenced (there may be others, I just scanned it quickly):

    • The iprobe does not "break" the loop. The loop remains closed at all times, which is the point of these analyses. It is the perturbation/measurement point for the loop gain
    • You should no longer use cmdmprobe to measure differential and common-mode stability. Instead there is a superior component, diffstbprobe which copes with the loops not being balanced (i.e. if there is common-mode to differential-mode leakage or vice versa) and also allows the choice of whether you are measuring common mode or differential mode to be made on the analysis form, rather than on the component itself.

    Regards,

    Andrew.

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