Schmitt trigger threshold behavior: How many distinct trigger points (thresholds) does a Schmitt trigger comparator use to implement hysteresis and noise immunity?

Introduction / Context:
A Schmitt trigger is a comparator with hysteresis. It cleans up noisy or slowly varying input signals by switching its output only when the input crosses well-defined thresholds on the rising and falling edges. This behavior provides robust noise immunity and debouncing in digital interfacing.

Given Data / Assumptions:

• Input can be noisy or slowly ramping.
• Hysteresis is implemented by positive feedback.
• Upper threshold (VUT) applies when the input is rising; lower threshold (VLT) applies when falling.
• Output swings between defined logic levels when thresholds are crossed.

Concept / Approach:
Positive feedback shifts the effective reference of the comparator based on output state, creating two distinct switching thresholds. The difference VUT - VLT is the hysteresis width. This prevents multiple rapid toggles when the input hovers near a single trip point, dramatically improving noise tolerance and timing determinism.

Step-by-Step Solution:

Verification / Alternative check:
Examine the transfer characteristic: the I/O plot forms a loop due to hysteresis, with distinct transition points on the up and down trajectories, confirming two thresholds.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing Schmitt triggers with window comparators; assuming hysteresis width is arbitrary without considering resistor ratios and supply levels; forgetting that input common-mode limits still apply.

Final Answer:
two trigger points