Fanout vs Noise Margin-Difference between Fanout,Noise Margin

This page on Fanout vs Noise Margin describes basic difference between Fanout and Noise Margin used for logic gates. It mentions formulas for the fanout and noise margin.


Fan Out

It is the maximum number of logic gates which can be driven by logic gate output without affecting its performance. This number is known as fanout. The driver logic gate and loaded logic gates should be similar type.

High Fanout is advantageous as it reduces the need for additional drivers to drive more number of gates.Figure-1 depicts the concept of fanout.

Fanout (High) = IOH(max) / IIH(max)

Fanout (Low) = IOL(max) / IIL(max)

Overall Fanout is lowest among Fanout(High) and Fanout(Low).

Noise Margin

Noise Margin

Noise immunity of logic circuit refers to ability of the circuit to tolerate noise without introducing any spurious changes in output voltage. A quantitative measure of noise immunity is known as Noise Margin.

The high state noise margin ( VNH ) is defined as follows:
VNH = VOH(min) - VIH(min)

The low state noise margin ( VNL ) is defined as follows:
VNL = VIL(max) - VOL(max)

In a logic family following relation is maintained,
VOH(min) > VIH(min) > VIL(max) > VOL(max)

Overall noise margin is lowest among high state noise margin and low state noise margin.
Refer Difference between TTL ECL CMOS➤

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