required : sallen key filter unity gain model for PSpice

Discussion in 'The Projects Forum' started by KFUPMer, Dec 4, 2015.

  1. KFUPMer

    Thread Starter New Member

    Nov 4, 2015
    8
    0
    Hi guys ,


    I am trying here to design 4th order Lowpass Chebyshev filter

    and the given data :
    1/3dB Frequency: fo= 3KHz

    2/Pass Band Ripple = 1dB


    I have used this sites to design my filter :

    (http://www.daycounter.com/Filters/Sallen-Key-LP-Calculator.phtml)

    as u can see I have used unity gain

    If R= 2.5K Ω , then c1= 54.62nF and C2=11.1nF (The first stage)

    The second stage should be identical to the first stage .


    the circuit :

    upload_2015-12-4_16-29-11.png


    #Problems:
    1/after this long introduction , I want to simulate this circuit by using PSpice but I do not have a model for sallen key filter unity gain

    2/ Should the gain be exactly =1 in the simulation ???
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    What prevents you from just entering the component topology, the resistor and capacitor values, and a suitable opamp model into PSpice?

    Cascading two identical filter stages does not produce the correct 4th order response.

    Go to the MicroChip website and download FilterLab. It synthesizes active filters from specs, and will handle the two cascaded stages correctly....

    Here's what FilterLab comes up with:

    c3.gif
     
    Last edited: Dec 4, 2015
    Bordodynov likes this.
  3. KFUPMer

    Thread Starter New Member

    Nov 4, 2015
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    So , they should not be identical ???

    and this is my SPice file :
    Code (Text):
    1. project
    2.  
    3. Vs 8 0 AC 1 sin(0 1 1k)
    4.  
    5. R1 7 8 2500
    6. R2 7 2 2500
    7. R3 1 9 2500
    8. R4 9 5 2500
    9.  
    10. C1 1 7 54.62n
    11. C2 2 0 11.1n
    12. C3 4 9 54.62n
    13. C4 5 0 11.1n
    14.  
    15. X1 2 1 1 OPAMP1
    16. X2 5 4 4 OPAMP1
    17.  
    18. .SUBCKT OPAMP1  
    19. (what should I write here ?????)
    20. .ENDS
    21.  
    22. .AC dec 100 10 40meg
    23. .TRAN 0.01m 2m
    24. .PROBE
    25. .END
     
  4. KFUPMer

    Thread Starter New Member

    Nov 4, 2015
    8
    0

    By the way, is this an example or this is what my filter should be ????
     
  5. KFUPMer

    Thread Starter New Member

    Nov 4, 2015
    8
    0
    one last question , how can I enter the value of (Pass Band Ripple) in FilterLab ????
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Filter Menu, sub menu Design

    Do you know how to scale resistor and capacitor values if you do not like what FilterLab comes up with?
     
  7. KFUPMer

    Thread Starter New Member

    Nov 4, 2015
    8
    0

    Filter >Design > then what ?


    Actually I like FilterLab thanx ^_^
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Filter/Design/FilterSpecs/...

    Check all of the parameters that can be specified.
     
  9. KFUPMer

    Thread Starter New Member

    Nov 4, 2015
    8
    0
    sorry mike for my stupid question but there is nothing called Pass band ripple , but I found ( PassBand Attenuation its range from -3 to -1 ) and (StopBand Attenuation its range from -10 to -100)
     
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