Low Pass Active Filter Design Help

Discussion in 'Electronic Basics' started by [email protected], Feb 22, 2006.

1. Guest

Hi everyone,
Need some help with this problem. I need to design a low-pass active
filter circuit such that its roll off attenuates frequency components
at 14kHz or more by more than 17dB, Also it is desired to have the
passband gain positive that doesnot deviate from 40dB by more than 3dB,
The passband frequency needed is 280Hz and below.

Hope that someone can help me ASAP . Thanks.....

2. John PopelishGuest

Download and install a copy of FilterPro, a free design aid from Texas
Instruments.
http://focus.ti.com/docs/toolsw/folders/print/filterpro.html

4. Walter HarleyGuest

Do I correctly understand that you want 40dB of gain (+/-3dB) from DC to
280Hz, and 23dB or less of gain above 14kHz? A simple RC lowpass filter
rolls off at 6dB/octave, and there are more than 5 octaves between 280Hz and
14kHz, so you don't need anything fancy. For instance:

___ |\
--|___|--o-------|+\
10k | | >---o--
--- .---|-/ |
33nF --- | |/ |
| | ___ |
GND o---|___|--'
| 100k
.-.
| |
| |1k
'-'
|
GND

This is within 1.5dB of 40dB gain from DC to above 280Hz; and at 14kHz, it's
down by about 29dB. It's not really an "active filter" at all - it's just a
passive RC lowpass, followed by an amp stage with a gain of slightly more
than 40dB.

One thing you have to watch out for is that you must use an opamp with
enough bandwidth that it still has 40dB of gain at 14kHz - that is,
gain-bandwidth product of at least 1.4MHz, preferably a lot more.

But if I misunderstood your question and you want to go from 40dB of gain at
280Hz down to -17dB of gain at 14kHz - that is, a total difference of 57dB -
then you do need an active filter. With 5.5 octaves to work with, a
two-pole filter (12dB/octave) will just manage, but you might be happier
with three poles.

Don Lancaster's "Active Filter Cookbook" is handy for things like this.