Standard TTL noise margin For standard TTL logic operating at 5 V, what is the typical dc noise margin (per level)?

Difficulty: Easy

Correct Answer: 0.4 V

Explanation:


Introduction / Context:
Noise margin quantifies how much unwanted noise a logic signal can tolerate without causing a logic error. For TTL systems, understanding the nominal margin aids robust interfacing and cabling choices.


Given Data / Assumptions:

  • Standard TTL thresholds (representative): VIH(min) ≈ 2.0 V, VOH(min) ≈ 2.4 V, VIL(max) ≈ 0.8 V, VOL(max) ≈ 0.4 V.
  • Positive logic at 5 V nominal supply.


Concept / Approach:
Noise margin (high) = VOH(min) − VIH(min). Noise margin (low) = VIL(max) − VOL(max). With the representative TTL values, both margins compute to about 0.4 V.


Step-by-Step Solution:
Compute high-level margin: 2.4 − 2.0 = 0.4 V.Compute low-level margin: 0.8 − 0.4 = 0.4 V.Therefore, the standard per-level TTL noise margin is approximately 0.4 V.


Verification / Alternative check:
Datasheets for classic 74xx TTL families corroborate ≈0.4 V margins; some subfamilies may vary slightly but remain close to this value.


Why Other Options Are Wrong:
5.0 V: this is the supply, not the margin.0.2 V and 0.8 V: do not match both high and low margin calculations.


Common Pitfalls:
Confusing input thresholds with noise margins; margins are differences between guaranteed output and required input levels.


Final Answer:
0.4 V

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