# Active Filter design with FilterPro

Discussion in 'The Projects Forum' started by working_on_EMG, Dec 30, 2010.

1. ### working_on_EMG Thread Starter New Member

Dec 4, 2010
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0
Hello everyone.

I want to design a second order sallen key butterworth high pass filter with 20 Hz cut off frequency with texas instruments filterpro active design software.

In step 2: Filter Specifications, what values should I fill for passband frequency, allowable passband ripple and stopband frequency? When I input 20Hz into the passband frequency box, a red star appears and asks me to fill enter a value between 0 to 20 for the stopband frequency. Is there a relationship between passband frequency and stopband frequency?

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Last edited: Dec 30, 2010
2. ### marshallf3 Well-Known Member

Jul 26, 2010
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You've got the passband and stopband values reversed. If you're doing what I think you're doing put 20K in as pass and 20 as stop.

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3. ### working_on_EMG Thread Starter New Member

Dec 4, 2010
12
0
Thank you sir. I have used another more straighforward version of the sallen key active butterworth high pass filter calculator at: http://www.pronine.ca/acthpf.htm. I entered the value of cut off frequency to be 20Hz and capacitor C1=C2 = 1.5 microfarads. The calculator returns the value of R1 = 3.75 and R2 = 7.50.

Sir what type of capacitors would you recommend in order to design a good 2nd order sallen key active butterworth high pass filter? Will precision op amps such as OPA277 or audio op amps such as OPA2604 be a good choice for implementing active filters?

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4. ### marshallf3 Well-Known Member

Jul 26, 2010
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Just plain film caps would be fine and since 1.5 uF is kind of an odd value you may need to parallel a 1 uF + 0.47 uF

You don't need to go wild on the op amp either, half of a TL082 or TL072 will work perfectly unless you're shooting for distortion levels way below what anyone could possibly hear.

http://www.mouser.com should have everything you need if you're in the US, regardless you really need to add your location to your profile so we know where you're at thus it's easier to suggest a supplier.

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