Stop-band limitations of the Sallen-Key low-pass filter
We might expect the gain amplitude of an analog, lowpass anti-aliasing filter to continually decrease past the filter’s cutoff frequency. This is a safe assumption for most filter topologies, but not necessarily for a Sallen-Key low-pass filter. The Sallen-Key filter attenuates any input signal in the frequency range above the cutoff frequency to a point, but then the response turns around and starts to increase in gain with frequency.
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