OP484 as Comparator With Hysteresis: Input Protection, Resistor Values, and Limitations

Hello,

I wanted to use one spare op of a OP484 as a comparator. I was wondering about what should care about when using it with positive feedback to have some hysteresis.

I know that the input of the 484 can't be at a different voltage due to their diodes. As long as I use resistors in the inputs of enough value to limit the current to not stress it (for example, 10k, with inputs and supply at 13V), the OP484 will work OK as a comparator? Is there any other problems I should be aware of?

Look at analog devices application note AN-849.

Ideally, op-amps should not be used as comparators. Op-amps are designed to work with feedback - i.e., never open-loop, whereas a true comparator is designed to work open-loop. As such op-amps have an internal miller capacitor (unity gain compensation) and that limits their speed when used open-loop.

When you compare an op-amp and a comparator you will notice that the inverting & non-inverting inputs have a different meaning - where in an op-amp, the o/p will swing to positive Vsat if the non-inv input is greater than the inverting input and vice-versa, while it is the exact opposite for a comparator. Also a comparator has facility to add external pull-up/down resistors for CMOS/TTL compatibility whereas an op-amp does not.

In short an op-amp _can_ be used as a comparator but with certain caveats.

Thanks, and thanks J. Schmitz for the note. I will take a good look at the problems...