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Help..Im stuck with designing a band pass filter with frequency range (20Hz - 500Hz)

Ahmad Jazlan
Prodigy 80 points

Hello everyone

I want to design a Sallen & Key Butterworth filter with frequency range 20Hz to 500Hz, but im not sure if filling in the right parameters. Could you all please give me some guidance.. I entered the following parameters into filterPro:

Gain A0 = 1                   V/V = 0dB

Centre Frequency F0 = 260Hz ; Because (500Hz-20Hz)/2 + 20 = 260

Allowable Passband Ripple Rp = 0dB (Butterworth Characteristics)

Passband Bandwidth BWp = 480Hz; Because 500Hz - 20Hz = 480

Stopband Bandwidth = 10000 (Im not sure how to select stopband bandwidth, I just keep trying 10,100,1000,10000,1000000..

Stopband Attenuation = -45 degrees

This is the band pass filter generated by filterPro. Is it really a band pass filter with frequency range 20Hz to 500Hz.

Thanks!

  • 0
    TI__Expert 5515 points

    Ahmad,

    It's a bandpass filter, but probably not what you were actually looking for. The Stopband Bandwidth can be a little confusing. Look at the diagram on the wizard when you're entering the specifications:



    The Stopband Bandwidth is the width between where the blue curve intersects the stopband attenutation - it's identified as BWs on this diagram. So in your case, you've said that is 10kHz wide. On the low end, that puts the -45dB point in negative frequency, while on the positive side, the -45dB attenuation would start at 5260Hz. FilterPro tries to meet both criteria, but obviously can't handle the negative frequency, so it makes something that works to meet the upper frequency limit (or at least approach it).

    Can you tell us what attenuation you need at what frequencies? In other words, what are you looking for at say, 10Hz? I'm assuming you expect -3dB at 20 and 500Hz. Tell us your design specs and we can show you how to get the filter you need.

  • 0 in reply to Rick Downs - WEBENCH Design Center
    TI__Genius 9560 points

    Dear;

      Recevently, I have also met one similar question, and I didn't know how to decide the BWs, according to the above descripiton, could you please tell me how to caculate the 5260Hz? For example, if I want to get one bandpass filter with the -3dB frequency range is 0.5HZ~150Hz, how to set the value of Bws and the Asb ?

     

    Best  regards

    Kailyn

  • 0 in reply to Kailyn Chen
    TI__Expert 3555 points

    Kailyn,

    Thanks for getting back to us. It has been a long time. For this filter, I suggest that you make BWp = 150 - 0.5 or 149.5 Hz. The values of Asb, BWs, and fo depend on your application, so let's get started.

    • What is the center frequency for your filter? (fo)
    • What are the frequencies (above and below fo) that you want to attentuate? This helps you determine BWs.
    • How much do you want to attenuate these freuqencies in terms of decibels (dB)? This helps you determine Asb.

    When you design your filter I suspect you will be tempted to use FilterPro. TI how has a new enhanced version to design your filters, located at http://www.ti.com/webenchfilters. Please use this tool in the future.