Strange pz-analysis

Discussion in 'Cadence' started by Stefan Joeres, Jan 23, 2006.

  1. Hi altogether,

    I just tried to implement a 4th order lowpass filter function in cadence
    like this :

    V(vout) <+ laplace_zp(V(vin),{-1,0,-1,0,-1,0,-1,0}
    {0.718,0.396,0.583,0.133,0.583,-0.133,0.718,-0.396});


    So - I'd expect the pole zero analysis to plot me exactly the same ...
    but ...
    Poles (Hz)
    Real Imaginary Qfactor
    1 9.27873e-02 +/- 2.11676e-02 **RHP -5.12846e-01
    2 1.14273e-01 +/- 6.30254e-02 **RHP -5.71005e-01
    Zeros (Hz)
    at V(outp,0)/V4
    Real Imaginary Qfactor
    1 -1.59121e-01 +/- 3.35491e-05 5.00000e-01
    2 -1.59189e-01 +/- 3.35671e-05 5.00000e-01

    The ac simulation didn't plot the expected curve either ... am I missing
    something important ?

    Why ??

    Regards,

    Stefan
     
    Stefan Joeres, Jan 23, 2006
    #1
  2. It doesn't surprise me that the pz analysis doesn't extract the poles and zeros
    of a behavioural laplace function - in essence this is a frequency dependent
    component, and it will only be evaluated at a particular frequency. I think I
    tried using this approach to see whether pz was working properly during the beta
    phase, and R&D told me something similar...

    However, I'd expect the ac response to be OK. I've not checked your example to
    see if there's a good reason why not (I'm a bit pushed for time at the moment to
    do the experiments).

    Regards,

    Andrew.
     
    Andrew Beckett, Jan 24, 2006
    #2
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