High Pass Filter

Discussion in 'The Projects Forum' started by Nanophotonics, Mar 9, 2010.

  1. Nanophotonics

    Thread Starter Active Member

    Apr 2, 2009
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    Hello,

    I'll be designing an inverting high pass filter. Is the MFB the only inverting topology?

    Thanks.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Have you tried Linear Technology's FilterCad? It comes with their free LTSpice.
     
  3. Nanophotonics

    Thread Starter Active Member

    Apr 2, 2009
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    No I haven't tried it. I'm using OrCAD Capture to simulate the circuit. I know how to calculate the desired values and I intend to use the Sallen-Key topology. However, I would like to know the effect of inverting high pass filters on a signal compared to non-inverting ones in terms of dB. Thanks.
     
  4. Nanophotonics

    Thread Starter Active Member

    Apr 2, 2009
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    Is it possible that the output voltage of a single filter go from positive to negative?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Filters are usually designed so that the output signal is the same as the input, sans the filtered frequencies.

    Why don't you post the circuit you are working on, explain your input signal, and your desired output signal?
     
  6. Nanophotonics

    Thread Starter Active Member

    Apr 2, 2009
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    I haven't designed the circuit yet. I'm still reading the theories behind filter designs. I'm basically trying to understand the idea of transfer functions and how cascading/adding filtering stages affect the overall response of the system, a bit of signal processing. Thanks for helping.
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    Can you describe the frequency response of the filter you need? freq? filter order?

    hgmjr
     
  8. Nanophotonics

    Thread Starter Active Member

    Apr 2, 2009
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    The filter should basically have a flat response (Butterworth I suppose is best in this case), 6th order, cut-off frequency of 7 kHz. Would it make any difference if I use an inverting filter instead of a non-inverting one? All of the filters I've seen are non-inverting with positive feedback. Only MFB seems to be inverting. But I'm not sure about the impact it might have on the input signal, keeping in mind that we are within the audio frequency range.
     
  9. hgmjr

    Moderator

    Jan 28, 2005
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    I would not be overly concerned with whether the filter inverts or not. If that is an issue you can always add an inverting stage.

    Have you considered using three second-order filters in series to achieve your end response?


    hgmjr
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Do you know this application note of TI?
    It is about filter design.

    Greetings,
    Bertus
     
  11. Nanophotonics

    Thread Starter Active Member

    Apr 2, 2009
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    Last edited: Mar 14, 2010
  12. Nanophotonics

    Thread Starter Active Member

    Apr 2, 2009
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    OK, suppose I have a series RLC circuit. Taking the output voltage to be across the capacitor, I have a particular transfer function. In this case, it's a second-order transfer function. Now, how can I convert the transfer function to 1 throughout the audio range given that the resonant frequency is around 10 kHz. Any clue? Multiplication in the frequency domain?
     
    Last edited: Mar 14, 2010
  13. hgmjr

    Moderator

    Jan 28, 2005
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    This question is a bit confusing. It sounds like you are dropping your original 6th-order filter request in favor a second order filter.

    Is that your intent?

    hgmjr
     
  14. Nanophotonics

    Thread Starter Active Member

    Apr 2, 2009
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    I might need the 6th order, as long as I can get this response flat, I believe it will depend on the roll-off rate of the transfer function. Anyway, I'm a bit uncertain here, cascading and summing have different effects. I'm confused about how to flatten out the response.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    Look up m-derived filters. Their rejection sucks (about 20 db), but man do they have a tight knee with flat responses.

    I spent a day tuning a bandpass at work, they can be a stone b...ear to tune.
     
  16. Nanophotonics

    Thread Starter Active Member

    Apr 2, 2009
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    Oh yeah, filters can be quite a headache to tune.
     
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