Capacitor in positive feedback of comparator?

[Feifer]

So I can't for the life of me figure out what this cap (C4, upper right corner) is doing. I've never seen a set up like this before and when running a simulation of it, it sets up an oscillation on the comparator. Does anyone know the reason for it? Ray didn't say why he included it.

[PHOBoS]

Those small capacitors in the feedback loop are usually added to suppress high frequency oscillations.
I don't know why it would cause an oscillation when you simulate it.

[blue hell]

In the negative feedback path it would add some integration, which would (help to) suppress oscillations.

In the positive feedback path (as used here) it adds some differentiation. This is usually used to speed up output switching, but it has a tendency to make the thing unstable (into an oscillator).

In this case I'd assume the simulation to be less than perfect. So just leave it out for simulation, and do add it in when you'd actually build it (and maybe adjust it after checking on the scope what happens in practice).

[Feifer]

If it's there to just speed up output switching on the op amp, how about using an actual comparator there instead? Would there still be a need for it if a different device is able to switch 1000 times faster alone?

[PHOBoS]

[blue hell]