Op Amp help needed

Discussion in 'The Projects Forum' started by Pika, Jul 27, 2010.

  1. Pika

    Thread Starter New Member

    Jul 27, 2010
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  2. Audioguru

    New Member

    Dec 20, 2007
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    A normal resting human hearbeat is about 1Hz (60 beats per minute).
    4.8Hz would be from the heart of a running mouse at 288 beats per minute.

    The filters are extremely simple using only one resistor and one capacitor each. Then their slopes are very gradual.
    A 1Hz filter has an attenuation of almost nothing at 1Hz (-3dB). They use a 4.8Hz filter calculation so that 1Hz is attenuated 10dB and lower frequencies are attenuated more.
     
    sage.radachowsky likes this.
  3. sage.radachowsky

    Member

    May 11, 2010
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    AudioGuru, I know almost nothing about double pole filters. Do you think you could explain just a little about how that works in this circuit? Thanks in advance for any wisdom.
     
  4. Pika

    Thread Starter New Member

    Jul 27, 2010
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    To make sure I understand, some of the 1 hz signals will be filter out as well as the lower frequencies? Also may I ask why do they have two cutoff frequencies?

    Thanks again!
     
  5. Ghar

    Active Member

    Mar 8, 2010
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    Because of the op-amp input the two RC sections don't interact at all.
    The input filter is just an RC high pass filter while the op-amp feedback gives you a high frequency gain.
     
    Last edited: Jul 27, 2010
  6. Audioguru

    New Member

    Dec 20, 2007
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    Look at the graph in the document. 1Hz is attenuated -10dB which is an amplitude of about 0.3 times.
    The graph shows the response is flat down to 30Hz, is down to -3db at 5Hz, is down to -10dB at 1Hz and is down -40dB (1/100th the amplitude) at 0.2Hz.
     
    Last edited: Jul 28, 2010
  7. Ghar

    Active Member

    Mar 8, 2010
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    Just as a clarification that plot is normalized to the high frequency gain of 151 or 43.6 dB.
    That is, the plot is gain minus 43.6 dB.
    That's what they mean they say "the plot doesn't include the gain of the amplifier"

    The op amp portion at low frequency is a voltage follower while at high frequency it's a gain of 43 dB.
    The input RC portion is a high pass filter, which attenuates the low frequency signals.

    They amplify the signal of interest using the op amp and attenuate the unwanted signal using the input RC. Because they normalize the plot it makes it look like extra attenuation.
     
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