Spikes at Loop Gain Bode Plot-need kind explanation

Discussion in 'The Projects Forum' started by prietess, Jul 23, 2014.

  1. prietess

    Thread Starter New Member

    Nov 30, 2010
    Hello everyone, good evening (my timezone is evening now),

    I am working on a school project on a differential chopper amplifier (simplified schematic attached).

    When I simulated my feedback loop gain, I saw two spikes (sort of spikes, please kindly check picture) at 10kHz and 100kHz, which I cannot understand why.


    My switching frequency is 5kHz and my input signal frequency is 200Hz.
    I did PAC simulation with spectre and I got the result as attached for my loop gain (I calculated loop gain as db20 of loop output/loop input).

    Does someone know why I have such response at 10kHz and 100kHz please?
    I tried removing the choppers and simulated AC simulation and of course I do not have the spikes at 10kHz and 100kHz (shown in picture).
    And also, can I say that my loop is stable from the loop gain plot?

    I am really stuck, and I cannot understand why do I have such things at that particular frequency 10kHz and 100kHz.
    Thank you very much for your kind answer and kind suggestion!
  2. prietess

    Thread Starter New Member

    Nov 30, 2010
    No one???
    PLease need help T.T
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
    I don't know much about choppers, but the plot looks like you have plenty of phase margin left at 0 db crossing. The bumps almost look like your program is not taking enough samples to average out the curve. Can you change the chopper frequencies and see where the 'bumps' go?

    Some reading.

    Last edited: Jul 24, 2014
  4. prietess

    Thread Starter New Member

    Nov 30, 2010
    Hi hi!!
    Thanks for your kind reply at my thread!

    Ya the bumps look like lack of points, but I already tried adding points for every decade of PAC sim, but I still see almost same plot, I got something at that 10kHz and 100kHz.

    I indeed tried to change the input frequency of my signal and found out that the bumps are still at same place.

    btw, I dont have permission to access the link you gave above.