Noise margin : is a parameter intimately related to the transfer characteristics. It allows one to estimate the allowable noise voltage on the input of a gate so that the output will not be affected. Noise margin (also called noise immunity) is specified in terms of two parameters - the low noise margin NM_L , and the high noise margin NM_H . Referring to Figure 2.14, NM_l is defined as the difference in magnitude between the maximum LOW input voltage recognized by the driven gate and the maximum LOW output voltage of the driving gate. That is,

NM_L =| V_ILmax - V_OLmax |

Similarly, the value of NM_H is the difference in magnitude between the minimum HIGH output voltage of the driving gate and the minimum HIGH input voltage recognizable by the driven gate. That is,

NM_H =| V_OHmin - V_IHmin |

Where V_IHmin : minimum HIGH input voltage

V_ILmax : maximum LOW input voltage

V_OHmin : minimum HIGH output voltage

V_OLmax : maximum LOW output voltage