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I need to heavily low pass filter an input to a hysteresis comparator, but due to the fact that R1 is one of the components setting the hysteresis, I can't add additional RC-circuits in series with it.

My question is if it is suitable to double-up the function of the R1 resistor as in circuit below? The resistor is now also intended to function as a part of the low-pass filter.

The circuit displayed below is a basic comparator with noninverting hysteresis where I have added a sizable capacitor marked by the red circle.

enter image description here

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  • \$\begingroup\$ Ask yourself why you want hysteresis and what adding the capacitor does to affect the efficacy of the hysteresis. \$\endgroup\$
    – Andy aka
    Sep 11, 2020 at 11:00

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This is generally a bad circuit. C1 will slow the operation of the hysteresis through R2, so that as the circuit gets to the threshold, it's likely to amplify noise, and give a very ragged output, while C1 prevents the inputs moving to different voltages quickly.

Split R1 into two series resistors, and apply C1 to the mid point. Now the non-inverting input can move quickly when the output changes.

A good plan will be to put a small capacitor, a few pFs to a few 10s of pFs, across R2, to speed up the application of the hysteresis.

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    \$\begingroup\$ Thank you, I think your advice sounds very reasonable. After posting the question and some further search I was able another source supporting this idea: tablix.org/~avian/blog/archives/2006/05/comparator_tricks. \$\endgroup\$ Sep 11, 2020 at 12:38
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    \$\begingroup\$ @NoobPointerException a good find for that link. They have R1 and R4 very different sizes, which eases approximate computation of what the circuit will do, but it's not really necessary. Having them a similar size, as I implied, will cause the amount of hysteresis available to shift with frequency, but only over a 2:1 range if they are equal. \$\endgroup\$
    – Neil_UK
    Sep 11, 2020 at 12:50

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