Introduction / Context:
Noise margin quantifies the amount of noise a logic signal can tolerate without causing a logic error. For TTL, you determine noise margins by comparing guaranteed output levels (VOH/VOL) to required input thresholds (VIH/VIL).
Given Data / Assumptions:
- Typical TTL guaranteed levels: VIH(min) ≈ 2.0 V, VIL(max) ≈ 0.8 V.
- Typical TTL outputs: VOH(min) ≈ 2.4 V, VOL(max) ≈ 0.4 V.
- Noise margin high (NMH) = VOH(min) - VIH(min) ≈ 2.4 - 2.0 = 0.4 V.
- Noise margin low (NML) = VIL(max) - VOL(max) ≈ 0.8 - 0.4 = 0.4 V.
Concept / Approach:
The statement says “between 0.8 V and 0.4 V,” which is ambiguous and misleading. TTL noise margins are not a range bounded by 0.8 V and 0.4 V; instead, both the high and low noise margins are about 0.4 V each under standard specs.
Step-by-Step Solution:
Compute NMH → 2.4 - 2.0 = 0.4 V.Compute NML → 0.8 - 0.4 = 0.4 V.Conclusion → Both margins are ~0.4 V, not “between 0.8 and 0.4.”
Verification / Alternative check:
Check any standard 74xx TTL datasheet; values are consistent across subfamilies, with variations in advanced families.
Why Other Options Are Wrong:
Correct: Would endorse an imprecise/incorrect characterization.Only correct for open-collector TTL / VCC = 3.3 V: Noise margins are defined by thresholds, not collector configuration alone, and classic TTL is 5 V.
Common Pitfalls:
Confusing threshold levels (0.8 V and 0.4 V) with noise margins themselves.Failing to compute NMH/NML explicitly.
Final Answer:
Incorrect
Discussion & Comments