# Design a Sallen-Key lowpass filter

#### kentman234

Joined Dec 7, 2018
3
Hello,

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.

#### bertus

Joined Apr 5, 2008
21,652
Hello,

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

Bertus

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#### kentman234

Joined Dec 7, 2018
3
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.

#### bertus

Joined Apr 5, 2008
21,652
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

#### AnalogKid

Joined Aug 1, 2013
9,471
I think the normalized frequency for 20 dB is 24/8=3 Hz is that right?
Yes.
So the 2nd order is enough.
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

#### Audioguru

Joined Dec 20, 2007
11,249
I agree that a 3rd-order filter is needed.
Do you want a Butterworth response? Do you want a gain of 1?

#### kentman234

Joined Dec 7, 2018
3
I agree that a 3rd-order filter is needed.
Do you want a Butterworth response? Do you want a gain of 1?
Yes, a Butterworth response. And yes a gain of 1.

#### crutschow

Joined Mar 14, 2008
28,152
Here's a calculator for a 3-pole LP filter using only one op amp.