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How to read datasheets

The driving IC has VOH(min) = 2.9 V and VOL(max) = 0.4 V, and the receiving IC has VIH(min) = 2.0 V and VIL(max) = 0.8 V. What is the low-level noise margin?

  1. A0.9 V
  2. B0.4 V
  3. C0.8 V
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Answer: B. 0.4 V

The low-level noise margin is VIL(max) − VOL(max) = 0.8 − 0.4 = 0.4 V. The output drives an L of 0.4 V or less even in the worst case, and the input recognizes up to 0.8 V as L, so noise up to the 0.4 V difference will not cause a misread. The high-level noise margin is VOH(min) − VIH(min) = 2.9 − 2.0 = 0.9 V. Noise margin is thus found by combining the guaranteed values of the driver and receiver, and the smaller one is the weak point of that connection. 0.8 V is VIL(max) itself.

Logic levels and noise margins H output L output Read as H Undefined Read as L VDD 0 V VOH(min) VOL(max) VIH(min) VIL(max) NMH NML Output guarantees Input thresholds NMH = VOH(min) − VIH(min) NML = VIL(max) − VOL(max)
Logic levels and noise margins · View in diagrams
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