Noise Margin

The noise margin is a measure of the extent to which a logic circuit can tolerate noise or unwanted spurious signals. The high state noise margin is defined as

V_NH = V_OH(min) - V_IH(min)

and the low state noise margin is

V_NL = V_IL(max)- V_OL(max)

Thus V_NH and V_NL give the maximum noise voltage that will not cause incorrect or ambiguous communication of logic level in the H and L states, respectively.

**Figure 139:** Digital voltage levels showing noise margin and error caused by spurious excursion.
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