Noise margin calculation — why use worst-case VOH and VOL Why are VOH(min) and VOL(max) (rather than typical values) used when determining logic noise margins?

Introduction / Context:
Noise margin quantifies how much unwanted voltage disturbance a logic signal can withstand while still being interpreted correctly. For robust designs that tolerate temperature, supply, and process variations, margins must be computed with guaranteed limits—not rosy typical values found in marketing summaries or “typical characteristics.”

Given Data / Assumptions:

• VOH(min) is the lowest guaranteed HIGH output level.
• VOL(max) is the highest guaranteed LOW output level.
• VIH(min) and VIL(max) are input thresholds for recognizing HIGH and LOW reliably.

Concept / Approach:

Noise margins are usually defined as NMH = VOH(min) – VIH(min) and NML = VIL(max) – VOL(max) (sign conventions vary). Using worst-case output and input specs ensures that even under the least favorable conditions the logic chain will still function. Typical values would overestimate margin and risk field failures.

Step-by-Step Solution:

Verification / Alternative check:

Perform corner simulations across PVT (process-voltage-temperature). Worst-case simulations align with using min/max specs, validating the conservative approach.

Why Other Options Are Wrong:

“Normal/best-case” does not bound hardware variation; “does not matter” is unsafe; simplifying math is not a valid engineering reason when reliability is at stake.

Common Pitfalls:

Mixing typical and worst-case values; forgetting that fan-out and pull-ups affect VOH/VOL; ignoring ground bounce and simultaneous switching noise in margin estimates.

Final Answer:

These are worst-case conditions.