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Low Pass Active Filter Design Help

Discussion in 'Electronic Basics' started by [email protected], Feb 22, 2006.

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  1. Guest

    Hi everyone,
    Need some help with this problem. I need to design a low-pass active
    filter circuit such that its roll off attenuates frequency components
    at 14kHz or more by more than 17dB, Also it is desired to have the
    passband gain positive that doesnot deviate from 40dB by more than 3dB,
    The passband frequency needed is 280Hz and below.

    Hope that someone can help me ASAP . Thanks.....
     
  2. Download and install a copy of FilterPro, a free design aid from Texas
    Instruments.
    http://focus.ti.com/docs/toolsw/folders/print/filterpro.html
     
  3. sannuti

    sannuti Guest

  4. Do I correctly understand that you want 40dB of gain (+/-3dB) from DC to
    280Hz, and 23dB or less of gain above 14kHz? A simple RC lowpass filter
    rolls off at 6dB/octave, and there are more than 5 octaves between 280Hz and
    14kHz, so you don't need anything fancy. For instance:


    ___ |\
    --|___|--o-------|+\
    10k | | >---o--
    --- .---|-/ |
    33nF --- | |/ |
    | | ___ |
    GND o---|___|--'
    | 100k
    .-.
    | |
    | |1k
    '-'
    |
    GND

    This is within 1.5dB of 40dB gain from DC to above 280Hz; and at 14kHz, it's
    down by about 29dB. It's not really an "active filter" at all - it's just a
    passive RC lowpass, followed by an amp stage with a gain of slightly more
    than 40dB.

    One thing you have to watch out for is that you must use an opamp with
    enough bandwidth that it still has 40dB of gain at 14kHz - that is,
    gain-bandwidth product of at least 1.4MHz, preferably a lot more.

    But if I misunderstood your question and you want to go from 40dB of gain at
    280Hz down to -17dB of gain at 14kHz - that is, a total difference of 57dB -
    then you do need an active filter. With 5.5 octaves to work with, a
    two-pole filter (12dB/octave) will just manage, but you might be happier
    with three poles.

    Don Lancaster's "Active Filter Cookbook" is handy for things like this.
     
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