Schmitt-Trigger Hysteresis: Rising and Falling Thresholds Define Two Transitions

Retain both thresholds and output state

Analog Devices explains hysteresis as separating the up-going and down-going switching points so a reversal requires crossing the other boundary. Primary reference: comparator hysteresis.

The output state between those thresholds depends on the previous transition. A single nominal threshold cannot describe this behavior. Keep the circuit’s polarity as well: an inverting and non-inverting arrangement can associate a voltage transition with different output states.

An invented state trace

Assume a fictional non-inverting model switches high when rising voltage passes 3.0 V and low when falling voltage passes 2.0 V. Start with the output low at 1.5 V. The input sequence 2.5,3.2,2.7,2.1,1.8 V gives output states low,high,high,high,low.

The middle 2.7 V does not undo the preceding transition because it has not crossed the lower threshold. The example specifies assumed thresholds for teaching and no actual comparator precision or allowable noise level. Equality at a threshold would also need the model’s explicit convention.

Make the sequence reviewable

Write starting state, polarity, both thresholds and each input value in order. Record the expected state after every step rather than treating a point between thresholds as ambiguous without history.

For an actual interface, preserve threshold variation and timing evidence. The worksheet clarifies the state rule that a proposed hysteresis feature represents while avoiding the claim that one gap removes every possible disturbance.

Customer Questions

Primary References

These references support the technical principles discussed in this guide. The worked examples and review questions are educational.

• Primary reference: comparator hysteresis