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Fan Out

In logic circuits, a gate output often has to drive a number of other gate inputs. However, it can only supply a limited amount of current. The fan out  is the maximum number of gates that can be driven reliably, i.e. without incorrect or ambiguous voltage levels.

Fan out depends on both the driving and driven devices, since different devices require different input currents.

Fan out is given by

\begin{displaymath}{\rm FANOUT} \ =
\min \left( \frac{ \vert I_{OH} \vert }{ \v...
\frac{ \vert I_{OL} \vert }{ \vert I_{IL} \vert } \right) .

The current values are available on data sheets.

ANU Engineering - ENGN3213