Capacitor selection for band pass filter

Discussion in 'The Projects Forum' started by working_on_EMG, Jan 3, 2011.

  1. working_on_EMG

    Thread Starter New Member

    Dec 4, 2010
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    Hello everyone.

    Im currently constructing the 4th order sallen key low pass filter with 500Hz cut off frequency as shown in this attached figure. Im also constructing a 4th order sallen key high pass filter with 100 Hz cut off frequency. Im cascading the low pass filter and high pass filter to form a 8th order band pass filter with frequency range from 10Hz to 500Hz.

    Im not sure what type of film capacitors (polyester or polypropylene) should I use for the 3 nanofarad capacitors in the low pass filters and 33 nanofarad capacitors in the high pass filter. Or should i use ceramic or monolithic capacitors instead to implement this band pass filter? I am using Texas Instruments OPA277 precision operational amplifiers.

    In Malaysia where im at we have a branch of Farnell-Newark electronic parts supplier but no Mouser or digikey.
     
  2. working_on_EMG

    Thread Starter New Member

    Dec 4, 2010
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    Forgot the attachments. Here they are, the image on the left is the low pass filter and on the right is the high pass filter
     
    Last edited: Jan 3, 2011
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    A 4th order Sallen & Key is not the same a two identical cascaded 2nd order sections, which is what you have. You can get better frequency response by optimum pole positioning.
    See attachment.

    The lowpass filter can be optimized in a similar fashion.
    I used TI's FilterPro, which designed this filter.
     
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  4. working_on_EMG

    Thread Starter New Member

    Dec 4, 2010
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    Thank you sir for pointing out my mistake. In order to design this 4th order sallen key high pass filter with 20 Hz cut off frequency what values should we enter in the passband frequency, allowable passband ripple, stop band frequency and stopband attenuation?
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    If you choose a Butterworth instead of what you chose, you will still get better out of band rejection than you had with your original circuit. Butterworth has no passband ripple.
    I would probably choose the Sallen & Key topology, which is what both the circuits are that I posted (yours and mine).
     
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