Electronics > Projects, Designs, and Technical Stuff
Voltage comparator
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Zero999:
Why are you against varying the gain of the pre-amplifier?

To vary the hysteresis, simply adjust the feedback resistor value. A digital potentiometer, with sufficient bandwidth and low parasitic capacitance could theoretically be used, but I admit, I haven't investigated whether it's practical or not.
http://www.ti.com/lit/ug/tidu020a/tidu020a.pdf

Another method of adjusting the hysteresis, is to use two comparators, each with their own reference, connected to an RS flip-flop.
https://www.edn.com/design/analog/4312255/Add-hysteresis-to-a-voltage-comparator
OM222O:
as I mentioned, in the schematics that have been posted non of them seem to have adjustable hysteresis or even if they do, it doesn't seem to do much  :-//
Zero999:

--- Quote from: OM222O on April 30, 2019, 12:38:53 pm ---as I mentioned, in the schematics that have been posted non of them seem to have adjustable hysteresis or even if they do, it doesn't seem to do much  :-//

--- End quote ---
I've just told you how to implement adjustable hysteresis and what do you mean by "it doesn't do much"?
OM222O:

--- Quote from: rcbuck on April 30, 2019, 12:37:20 am ---I used this circuit for the front end of a counter I built last year. It is a slightly modified version of my 1970s Simpson 710 counter that still works fine today. The pot is supposed to be an input sensitivity adjustment. However, I found it has very little effect on the sensitivity. Input sensitivity is about 50 mV from 20 Hz to 60 MHz. It is more sensitive above 1 kHz.

You may be better off to just copy the Simpson version. I built an exact copy and it operates the same as my circuit. The Simpson operating manual can be found online and the schematic, although not very clear is at the back of the manual. The sensitivity control in the Simpson also had very little effect on sensitivity so you could just replace it with a fixed 100 ohm resistor. The MC10H116P is available from Mouser.

--- End quote ---

as he pointed out, the adjustment seems to be pointless for the overall function of the end product  :scared: |O so if possible, I would also like to not include it and change my input circuit to take care of that (presumably with some sort of amplification?)
duak:
David noted something about the AM687 comparator.  I remember a story about one of AMD's fast comparators, maybe the AM686 - the one with TTL outputs.  In the mid 80's I had just been hired at a startup and a contractor was battling with one of these comparators for a different project.  AMD was not kidding when they talked about sensitivity to layout and choice of bypass caps.  My contribution was to suggest adding a shield over the pin side of the PCB which helped.  I believe the ultimate solution was to go surface mount when the part became available in that form.   The DIP package leadframe just had too much mutual inductance.
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