Designing a filter

[Robby23]

Hey everyone I need help designing a filter. I need to design a lowpass filter using an op amp that will decrease a 650Khz signal by a factor of 0.001 of its original value. But I need the filter to pass the frequencies from 100-200 Hz.

I thought of using the transfer function T(w)= 1/ (1+jWRC)

W-frequency in rad/sec

then find a resistor and capacitor that make that 65 at 650Khz. But I need to corner frequency to be above 200 Hz and the its not if I do that. Can anyone help.

I think I need to set my corner frequency to 200 Hz then go from there. But im not sure. Thanks!

 

[ericgibbs]

Hey everyone I need help designing a filter. I need to design a lowpass filter using an op amp that will decrease a 550Khz signal by a factor of 0.0001 of its original value. But I need the filter to pass the frequencies from 0.05- 150 Hz.

I thought of using the transfer function T(w)= 1/ (1+jWRC)

W-frequency in rad/sec

then find a resistor and capacitor that make that 55 at 550Khz. But I need to corner frequency to be above 150 Hz and the its not if I do that. Can anyone help.

I think I need to set my corner frequency to 150 Hz then go from there. But im not sure. Thanks!

hi,
I suggest you download this free Filter designer from TI.
Active Filter Design Application - FILTERPRO - TI Software Folder

 

[audioguru]

0.0001 is -80dB. Each order of a Butterworth lowpass filter has a slope of 20dB/decade.
From 200Hz to 550kHz there are 3.5 decades and a single order RC filter will have a reduction of only 70dB which is not as much as you want.
Use two RC lowpass filters or an LC lowpass filter.

 

[Robby23]

Heyb Audioguru could you draw it out and help me understand how you did it. I don't quite understand your post. Thanks

also you said that at 200Hz to 650Khz that is 3.5 decades. Well in my design we can set the corner frequency to 200Hz and then that would be 4 decades. This would cause a single order low pass to have 80db reduction. But I still don't know how to build it and choose resistor and capacitor values.

I also downloaded that program but I am not sure what to set my Stopband attenuation to and what to set my stop band frequency to


should I just choose 200hz as my corner freq?


if you can help me with those values then this program should just spit out my answer. Thanks

 

[MikeMl]

Here is how I would do it. A 2 pole filter has more than enough attenuation at 550Khz to meet your spec, even though I moved the corner frequency to 220Hz, which means that 150Hz is attenuated only 1db, instead of the 3db it would be attenuated if you choose 150Hz as the cutoff freq.

 

[Robby23]

Hey Mike. I need it to be exactly 0.001 of its original value at 650Khz. Does yours accomplish this?

I am thinking I could just make the cutoff freq 55000hz. Then set the attenuation to -80db and the stopband frequency to 550Khz. Then I am pretty sure the 650Khz signal is reduced by -80db. Making it 0.001 of its original value. And this still passes the 100-200 hz signal. So that should work I think. What does everyone else think?

 

[MikeMl]

So you need to use trial&error to move the corner frequency to just the right place to make the attenuation at 550KHz be 80db.

I make it a 2pole Butterworth with a cutoff freq of 5500Hz

 

[Robby23]

cool I think those parameters above seem to work. It makes it 80.5 or something. Close enough. Thanks a lot for the help everyone!