Visualizing comparator and Op Amp hysteresis


Kerry Wong writes:

Hysteresis can be added to a comparator circuit to improve its stability, especially when the input signal is noisy. In this post, we will examine the hysteresis characteristics of some common comparator and Op Amps using an oscilloscope.
Perhaps the most intuitive way to visualize the hysteresis in a circuit is to plot the input signal (x axis) against the output signal (y axis). So, if we sweep the input voltage we should be able to see the characteristics of the transitioning of the output voltage due to hysteresis.

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  1. That is a lot more hysteresis than I would have expected. I wonder if slew rate and internal damping (to prevent oscillations) is an issue here. What is the hysteresis as a function of frequency and input signal magnitude? I am having a hard time squaring the results here with actual behavior of op amps in their usual negative feedback topologies. Can’t possibly be that bad. Can it?

    1. Well, that’s what I measured. And yes it was quite interesting to see the different results using different Op Amps. Hysteresis shouldn’t be affected by the input signal magnitude though as the trip point is independent of the input signal. Frequency could play a role though. Although I think the hysteresis is more affected by the different output range limitations of these Op Amps and some other internal compensation differences.

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