Bandpass & notch filter probelm

Discussion in 'The Projects Forum' started by teen devil, Oct 5, 2013.

  1. teen devil

    Thread Starter Active Member

    Apr 6, 2009
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    hi guys ,

    i am trying to make a 2nd order sallen key band pass filter from 0.05hz to 500hz .. when i am trying to simulate it my high pass is working fine and blocking the frequency at 0.05hz but my high pass filter isn't blocking at 500hz . sample goes for my notch filter i am created a 50hz notch filter in bode plot there is perfect 50 block but when i am giving the 50hz its not blocking . i am attaching both screeshots ..

    will waiting for your help guys .. or if anyone have a 2nd rder filter schematic please share
     
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  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Deosn't the chip need a power supply for correct simulation?
     
  3. teen devil

    Thread Starter Active Member

    Apr 6, 2009
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    here it is i changed the ic to AD705 also 12 v supply is given in BPF here's the result
     
    Last edited: Oct 5, 2013
  4. teen devil

    Thread Starter Active Member

    Apr 6, 2009
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    here are the simulation results
     
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  5. ronv

    AAC Fanatic!

    Nov 12, 2008
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    You need both plus and minus 12 volt supplies. Or you need to bias the input to Vcc/2 and ac couple it.
     
  6. teen devil

    Thread Starter Active Member

    Apr 6, 2009
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    i've changed my approach i came across Texas instrument document about filters there i read about a different configuration about sallen key band pass filter ( screen shot attached) i ran the simulation on it using all the necessary voltage supplies getting a good amplification ..

    now its stopping frequecy lower than 0.05Hz but still my Low pass filter isn't working properly it should be at 500Hz but not working .. am i doing the right thing .?

    Thanks for your help guys i am new in this field.
     
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  7. crutschow

    Expert

    Mar 14, 2008
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    Try 29.4k for R1 and 15nF for C2.

    Reduce the upper frequency range of your Bode plot to 1kHz for better resolution.
     
  8. LvW

    Active Member

    Jun 13, 2013
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    Why do you think the low pass "isn`t working properly"?
    What do you expect from a second-order bandpass?
    By the way - show a more detailed BODE plot between 1mHz and 50 kHz (instead of Ghz).
     
  9. teen devil

    Thread Starter Active Member

    Apr 6, 2009
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    here u go changed the values what u values suggested according to formula of 1/2*pi*R*C low pass frequency should be 360 HZ but still its not blocking
     
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  10. teen devil

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    Apr 6, 2009
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    cause i think it should attenuates or rejects any frequency more than 500hz which i set it with the resistor and capacitor values but it is allowing freq more than 500Hz to passu can see in the screenshot abv
     
  11. LvW

    Active Member

    Jun 13, 2013
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    Please, be aware that there is no filter circuit that is able to "block" a frequency range. Your circuit shows the classical and normal behaviour of a second-order bandpass.
    Please note that all frequencies beyond the upper corner frequency (500 Hz)
    are attenuated by the filter circuit with a damping factor of 20 dB/dec only.
    In detail: A frequency of 5 kHz is attenuated by a factor of 10 (20 dB) only.
     
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  12. teen devil

    Thread Starter Active Member

    Apr 6, 2009
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    oh i didn't knew tht .. thnx alot . so my 2nd order bandpass filter is working fine :)
     
  13. teen devil

    Thread Starter Active Member

    Apr 6, 2009
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    one last thing after designing my filters i tried connecting AD620 instrumentation amplifier to make ECG amplifier .. i have attached screenshot too . can you tell me why i am not getting good amplification . i think i am doing something wrong with AD620 .. i read the datasheet and attached RG = 100 ohms for gain of 500 but its still not working .

    i would really appreciate the help
     
  14. teen devil

    Thread Starter Active Member

    Apr 6, 2009
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    the ECG amp screenshot
     
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  15. crutschow

    Expert

    Mar 14, 2008
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    The AD620 inputs require a DC return path to ground (as all amps do), which it does not have in your circuit (the XFG1 output is floating).
     
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  16. teen devil

    Thread Starter Active Member

    Apr 6, 2009
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    sorry i am really at loss here .. i am new electronics fields can u please explain this in detail .. i tried connecting ground to -ve input of op-amp but it didn't work ..... i'll be really thankful to you if u explain the DC return path you are saying i am missing
     
  17. teen devil

    Thread Starter Active Member

    Apr 6, 2009
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    still waiting :/
     
  18. JoeJester

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    Apr 26, 2005
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    Have you looked at the datasheet for the AD620? Specifically page 14 of 20 in revision H.
     
  19. teen devil

    Thread Starter Active Member

    Apr 6, 2009
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    I tried what u said i configured Inst. Amplifier with the right leg drive circuit as per datasheet . finally i started to get a good amplification tht's in screenshot 1 .. but when i am connecting my bandpass filter with the notch filter the voltage is dropping again to nV .. u can see in screenshot 2
     
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  20. JoeJester

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