It is common to see higher-order, multistage, Sallen-Key filters designed with each stage at a gain of 1. This has component-sensitivity advantages, but always produces significant noise gain peaking within the filter stages themselves for the higher-Q stages. While it often might be the case that the SNR is already defined by the input signal before it hits an active filter stage, if the same filter response can be achieved, and produce considerably lower noise contribution to the signal path, why not?
A detailed analysis of the noise gain response within the Sallen-Key stage exposes this noise gain-peaking issue and shows it only takes a slight increase from a gain of 1 to get a significant drop in noise peaking, thus reducing the output integrated noise due to the filter stage itself.
This lengthy and detailed article, complete with numerous figures, equations, and analyses, will benefit anyone who is looking to optimize noise performance. It is presented as a single document in Word format.
If you are interested in optimizing noise performance of multistage Sallen-Key filters, and understanding the subtle tradeoffs in the design, click and read "Design Sallen-Key low-pass filters with G >1, achieve lower output noise" which is presented as a Word document. (Editor's note: you may want to consider printing this one out—it's one of those articles where the "dead-tree" version is easier to study, analyze, make notes about, and go back-and-forth than the on-screen version. But that's OK, since this is analog material, right?)
About the author
Michael Steffes is Senior Applications Manager, Intersil Corp. with more than 25 years of experience in high-speed amplifier design, applications, and marketing. Previously, he was the Market Development Manager for High-Speed Signal Conditioning, and a Distinguished Member of the Technical Staff, at Texas Instruments Inc. He currently provides product definition and customer design-in support.
Michael earned a BSEE from the University of Kansas and an MBA from Colorado State University. He shares several basic patents in high-speed op amp designs and has written more than 85 product data sheets, scores of contributed articles, applications notes and conference papers.
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