Signals-from-Noise: What Sallen-Key Filter Articles Don't Tell You, Part I
What are Sallen-Key filters, what makes them tick, and what makes them so tricky to use?
by Dave Van Ess, Principal Application Engineer, Cypress Semiconductor

It seems that six months can't go by without some article in an electronic design magazine showing how to select components for a Sallen Key filter. Articles written before the 70s generally relied on nomograms to select component values or maybe a FORTRAN program that ran on some big mainframe computer. In the late 70s, articles started showing up with programs written for the venerable Radio Shack Trash 80. The advent of the IBM personal computer led to the programs being written in either visual basic or C. Articles now are most likely programs written in JavaScript. (Acquire some electronic magazines of the 80s vintage, find an article to paraphrase, rewrite the program into JavaScript and in no time at all you are a published engineer.)

These articles make designing a filter seem trivial. What are left out are the real world constraints that keep the designer from getting the best performance. Below are the answers to the questions other articles don't explain.

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