Schmitt trigger threshold behavior: How many distinct trigger points (thresholds) does a Schmitt trigger comparator use to implement hysteresis and noise immunity?
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Athree trigger points
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Bno trigger points
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Ctwo trigger points
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Done trigger point
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Evariable number that changes every cycle
Answer
Correct Answer: two trigger points
Explanation
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:
Establish that a plain comparator has one nominal trip point.Add positive feedback to create state-dependent references.Identify resulting thresholds: one for rising transitions (upper), one for falling transitions (lower).Conclude: a Schmitt trigger has two trigger points, delivering hysteresis.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:
No trigger points: contradicts switching behavior.One trigger point: describes a basic comparator without hysteresis.Three trigger points or variable number: not how standard Schmitt triggers operate.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