How to Calculate Fanout

Typical circuit board

Fan-out is a concept in computer science concerning the operation of circuits and, more specifically, components of circuits called logic gates. At its most basic level, a circuit takes input in the form of an electrical pulse and passes it through various logic gates to construct a series of binary settings. Fan-out is a measure of the ability of a single logic gate to pass an input out to other logic gates in the same family. The measurement is useful in determining how many inputs can be linked together without losing necessary voltage levels.

Things You'll Need

• Circuit
• Ammeter

Instructions

1. Turn off the power to the circuit you intend to measure.

2. Determine the point on the circuit you want to measure and disconnect that circuit from surrounding connections, making sure to maintain appropriate distance between the two ends of the circuit and any other part of the of the circuit board.

3. Connect one of the ammeter probes to each end of the circuit. Measure the amount of current going into the logic gate (Ai) and the amount of current coming out of the logic gate (Ao).

4. Divide Ao by Ai. That number is the fan-out of the logic gate.

References

• Knol: Digital Electronic Design for Beginners---Chapter 4
• ASIC World: Digital Logic Gates---Part IV
• All About Circuits: CMOS Logic Gates