how to make a filter with unity gain below 5Hz and attenuate 60Hz by factor of 100?

Question

Hopefully any of u know about filter design cuz I'm really stuck ...I'm trying to design a filter in circuits following the above specs in my question.The purpose is to minimize noise while providing low ouput impedence to recording device. I know I need to have attenuation of -40dB/decade by 60 Hz, but I dont know how to do it ...what R and C to use, how to align them, etc. Right now all i have is a simple low pass RC filter with transfer function(1/sqrt(1 +(wRC)^2)), but i'm getting a -20dB/decade and the rolloff isn't fast enough to reduce the input by 100 at 60Hz. Any help appreciated

Answer 1

If you want to restrict your components to only R and C, you should use an op-amp (active) filter. You will need a multi-pole filter; each pole provides 10db/decade, so you will need a 4-pole filter. A cascaded R-C circuit will have a very long transition region from flat to max slope. You need an active filter design handbook that will give you component values for various fiters (bessel, butterworth, chebychef, etc) depending on the characteristics you need.

Start here http://en.wikipedia.org/wiki/Active_filter

You might also look for a "notch" filter design tuned to 60Hz.
and look up some of the reference. Also google "active filter design".

Answer 2

Exactly. You need to cascade to get -40dB/decade. Just put 2 op-amp filters in series. Your output should give you -40dB/decade based on your R and C values. You can do the same thing with 2 R/C passive filters in series, but good luck getting unity gain below 5Hz. You definitely want to use an op-amp for your low input impedance to recording device. Op-amps have low output impedance.

Answer 3

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