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I have two high-q band-pass filters with TI op-amp OPA2376 (http://www.ti.com/product/opa2376) in series of each other. Here is one of them:

enter image description here

My objective is to center the band-pass around 40kHz. On the input, I have some strong noise around 15kHz. Therefore, I want to have a good gain slope that filters such noise (everything below 25kHz is really cut-off) and still gains a lot my signal (everything above 35kHz is gained up).

On the other (upper) side of the band-pass, I have more flexibility. I know that the two stop points are not equally spaced above and below the center frequency but they will look equally spaced if plotted on a log graph. Once again, the upper band is about to be safe so the cut-out frequency can be higher if needed.

What values would you recommend?

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  • \$\begingroup\$ Do you have to reject anything above 40kHz or can you maximize your rejection of the 15k stuff by using just high pass filters? You'll get much more rejection of the lower frequencies if you use high pass filters. BTW thanks for the previous answer acceptance! \$\endgroup\$
    – Andy aka
    Commented Oct 18, 2014 at 20:20
  • \$\begingroup\$ Reconfigure one of them as a notch filter (search Twin-T notch filter) tuned to 15kHz. \$\endgroup\$
    – user16324
    Commented Oct 18, 2014 at 21:00

2 Answers 2

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This website helped me to answer my question: http://sim.okawa-denshi.jp/en/OPtazyuBakeisan.htm

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"I want to have a good gain slope that filters such noise (everything below 25kHz is really cut-off)"

Your circuit shows a classical 2nd order bandpass which has below the center frequency a "gain slope" like a 1st order highpass (20 dB/dec). I don`t know if this is "good" enough for you. On the other hand, please be aware that it is not possible to "cut-off" any frequencies. You only can attenuate the unwanted frequency region according to the order of the filter.

The classical step sequence for designing any filter starts with a damping scheme which shows minimum attenuation values for the unwanted frequency regions. From these requirements the necessary filter order is derived.

Question: What is the purpose of the second bandpass stage you have mentioned?

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