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Subject: Loomis Filter mods
From: jmhannon@........
Date: Thu, 14 Sep 2000 10:15:28 -0500 (CDT)

Analysis of A Loomis low pass filter.
The filter sections in the A Loomis filter match the schematic shown in
"The Art of Electronics" Second 
Edition by Horowitz and Hill on page 274. The book refers to this circuit
topology as a VCVS (Voltage 
Controlled Voltage Source) type of filter. Page 274 also includes a table
of design values for 2,4,6 and 8
pole Butterworth, Bessel and two types of Chebyshev filters. If you use
the design equations given backwards 
for the component values in the A Loomis filter you get the design values
for a 1 Hz Butterworth low pass 
filter. (Now that I see that the resistors in series with the plus input
of the op amps are 1.6 Meg Ohm 
not 1.0 Meg Ohm.) Given this the design values I provided earlier for the
Bessel filter are correct except
that you also need to change the 1.6 Meg Ohm resistors to 1.0 Meg Ohm
(Which is what I thought they were
when I did the Bessel calculations.)

A Burr Brown App note
 calls this circuit topology Sallen-Key

Jim Hannon


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Larry Cochrane <cochrane@..............>