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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
  • ringamplifier

    Hello!

    Andrew,

    Thanks for your professional reply!

    Now for my understanding, the Switched Capacitor Common Mode Feedback Cicuit (SC-CMFB), although operating in two different clock phases (the total capacitor connected in the loop is different in each phase), it makes the output common mode voltage to "jump" up and down around the desired value, to mimic the continues time one. The two different clock phases are relative to each other. So I think the "the time-averaged loop gain over the whole period" provided by Periodic Stability  analysis (pstb) is reasonable if we want to use pstb to assess the loop stability of the SC-CMFB.

    But for the Multiplying DAC (MDAC)  circuit I referred in Fig. 1, it operates in two different clock phases and each phase has different function (phi1 is auto-zeroing, feedback factor is nearly one; phi2 is amplification, feedback factor is determined by C1/(C1+C2) ) . These two phases are not "coherent" like that of SC-CMFB.  In order to make sure this circuit is stable, I need to make sure this circuit is stable in each phase. So that's why I want to find some method to assess the stability of particular phase. 

    However, the amplifier I used in the MDAC is a highly non-linear amplifier. So It's hard for me to specify a time to "represent" the amplifier in the transient simulation then use the method that you referred. So I want to use the pstb to analysis  "the time-averaged loop gain" within a particular phase.

    But as you said "The Periodic Stability analysis (pstb) does not give you the loop gain in each phase ", pstb may be not OK for this purpose.

    I really want your comments and suggestions !

    Best,

    Shen

Reply
  • ringamplifier

    Hello!

    Andrew,

    Thanks for your professional reply!

    Now for my understanding, the Switched Capacitor Common Mode Feedback Cicuit (SC-CMFB), although operating in two different clock phases (the total capacitor connected in the loop is different in each phase), it makes the output common mode voltage to "jump" up and down around the desired value, to mimic the continues time one. The two different clock phases are relative to each other. So I think the "the time-averaged loop gain over the whole period" provided by Periodic Stability  analysis (pstb) is reasonable if we want to use pstb to assess the loop stability of the SC-CMFB.

    But for the Multiplying DAC (MDAC)  circuit I referred in Fig. 1, it operates in two different clock phases and each phase has different function (phi1 is auto-zeroing, feedback factor is nearly one; phi2 is amplification, feedback factor is determined by C1/(C1+C2) ) . These two phases are not "coherent" like that of SC-CMFB.  In order to make sure this circuit is stable, I need to make sure this circuit is stable in each phase. So that's why I want to find some method to assess the stability of particular phase. 

    However, the amplifier I used in the MDAC is a highly non-linear amplifier. So It's hard for me to specify a time to "represent" the amplifier in the transient simulation then use the method that you referred. So I want to use the pstb to analysis  "the time-averaged loop gain" within a particular phase.

    But as you said "The Periodic Stability analysis (pstb) does not give you the loop gain in each phase ", pstb may be not OK for this purpose.

    I really want your comments and suggestions !

    Best,

    Shen

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