Design a Sallen-Key lowpass filter

Discussion in 'Analog & Mixed-Signal Design' started by kentman234, Dec 7, 2018.

  1. kentman234

    Thread Starter New Member

    Dec 7, 2018
    3
    0
    Hello,

    I have this task:

    Design a Sallen-Key lowpass Butterworth filter with a 3 dB bandwidth of 8 kHz and 20 dB attentuation at 24 kHz.

    Can you give me some hints?

    Thank you.
     
  2. bertus

    Administrator

    Apr 5, 2008
    18,688
    3,638
    Hello,

    It looks like you are looking for a 2nd order filter.

    Butterwoth_filter_responce.png

    Have a look at the attached PDF for more info.

    Bertus
     
  3. kentman234

    Thread Starter New Member

    Dec 7, 2018
    3
    0
    Hi,

    yes. I think the normalized frequency for 20 dB is 24/8=3 Hz is that right?

    So the 2nd order is enough. And what about calculating the elements?

    I have the solution here. It says R=28.13 kohm, C1=1nF, C2=500pF.
     
  4. bertus

    Administrator

    Apr 5, 2008
    18,688
    3,638
    Hello,

    Without a schematic I can not tell you if the values are correct.
    Have a look at chapter : 16.3.2.1 Sallen-Key Topology of the given PDF.

    Bertus
     
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  5. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    7,080
    2,001
    Yes.
    No. According to the plot in post #2, the attenuation at 3 Hz with a 2-pole filter is approx. 17 dB. You will need a 3-pole filter to meet or exceed 20 dB.

    ak
     
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  6. Audioguru

    Expert

    Dec 20, 2007
    10,606
    1,182
    I agree that a 3rd-order filter is needed.
    Do you want a Butterworth response? Do you want a gain of 1?
     
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  7. kentman234

    Thread Starter New Member

    Dec 7, 2018
    3
    0
    Yes, a Butterworth response. And yes a gain of 1.
     
  8. danadak

    Well-Known Member

    Mar 10, 2018
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  9. crutschow

    Expert

    Mar 14, 2008
    20,261
    5,734
    Here's a calculator for a 3-pole LP filter using only one op amp.
     
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