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I have a project on filtering out frequency above 2.5 KHz. The filter should be low pass butterworth. I am confused here as there are two cutoff frequencies I need to use. 1st cutoff freq. 1KHz and 2nd cutoff freq. 5KHz.
The passband is 5 amd I/p voltage is 0-5 V.
If any one can provide me some idea about this filter design? And what kind of configuration should I use?
Question: Is it possible that the required corner frequency (3 dB) is 1 kHz and the stopband frequency (with a certain required damping specification) is given with 5 kHz ? Please note that it is NOT possible to "filter out" a frequency of 2.5 kHz. You only can provide a certain degree of damping - unless you design a filter with a passband zero at 2.5 kHz. However, this has no Butterworth response.
The filter order necesary to meet your requirements is determined by the stop band attenuation, which is not given up to now.
As an example: A Butterworth low pass of 4th order can be realized as a series connection of two active lowpass stages with the same 3dB cut-off frequency (1 kHz) but with two different Qp values (Qp: pole Q).
A second order low-pass filter with two real, negative and distinct poles (corner frequencies) shows an overdamped response (no overshoot). Damping ratio > 1 (or Q < 0.5). Attenuation 20 dB/decade above the first pole and 40 dB/decade above second pole. Remember that, in general, "cut-off frequency"should not be confused with "corner frequency". Just for first order systems or Butterworth responses.
