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 http://www.burr-brown.com/download/ABs/AB-034.pdf calls this circuit topology Sallen-Key Jim Hannon http://soli.inav.net/~jmhannon/ 42,11.90N,91,39.26W WB0TXL __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>