Difference Between Fan In and Fan Out in Digital Electronics

Fan In and Fan Out are characteristics of Digital ICs. Digital ICs are complete functioning logic networks. Typically, a Digital IC requires only a power supply, I/P (input) and O/P (output). Here are the definitions of Fan In and Fan Out.

Fan In: The fan-in defined as the maximum number of inputs that a logic gate can accept. If number of input exceeds, the output will be undefined or incorrect. It is specified by manufacturer and is provided in the data sheet.

Fan Out: The fan-out is defined as the maximum number of inputs (load) that can be connected to the output of a gate without degrading the normal operation. Fan Out is calculated from the amount of current available in the output of a gate and the amount of current needed in each input of the connecting gate. It is specified by manufacturer and is provided in the data sheet. Exceeding the specified maximum load may cause a malfunction because the circuit will not be able supply the demanded power.

The difference between these two characteristics of a digital IC is significant from the definitions above.

Hope you find the information presented here useful. Feel free to leave your footprints in the comments section below for any further queries, feedback or suggestions.

    • Thanks Dhanushka for stopping by. As explained in the post above, Fan Out is calculated from the amount of current available in the output of a gate and the amount of current needed in each input of the connecting gate. So if,
      Ix = amount of current needed in input of each connecting gate and Io = amount of current available in output of gate for fan out is being calculated
      Then,
      Fan-Out = Io/Ix

      Thus, a DTL logic gate with a fan-out of 5 can drive 5 DTL logic gates.

      Fan In is calculated by the number of input pins available in a gate.

      Do let us know for any further clarification. Keep visiting Vertical Horizons…!!

  1. If ratio of Io/Ix is a whole number , then its fine , but what if ratio is not whole number i.e. e.g ratio (fanout) is 1.7 or 1.3 , then should it be rounded off to 2 or 1? … then how many device (load) it can drive).

  2. Hi Shashank,

    Fan out is the maximum number of loads that can be run. Lets understand it by taking few examples:

    1. Fan out is 2
    This implies that maximum 2 load can be run.

    2. Fan out is 1.7
    This implies that maximum of first load and 0.7 of second load can be run. Now 0.7 of second load doesn’t mean anything. This is why we round off 1.7 to 1.

    Please let us know if that makes you understand the concept better and you have any further concerns. :)

  3. Admin, thanks for an easy explanation. I have a question on design optimization done by synthesis tool. I observed that when number of inputs driven by an output exceeds fan-out of that output, the synthesis tool adds buffers on the exceeding nets. Can you please explain how a buffer solve the problem of fan-out violation? Thanks.

  4. All defination of fan-in and fan-out is correct. I want sy simply, number of input to a logic gate is called fan-in nd that number of gate inputs, the out of logic gate is called fan-out.

Leave a Reply

Your email address will not be published. Required fields are marked *

*

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code class="" title="" data-url=""> <del datetime=""> <em> <i> <q cite=""> <strike> <strong> <pre class="" title="" data-url=""> <span class="" title="" data-url="">