What is a Demultiplexer

    A demultiplexer is also a combinational logic circuit designed to switch a common input line to one of several independent output lines.

    A Data Distributor (often shortened to a Demultiplexer or just a "Demux") is the exact opposite of the Multiplexer we looked at in the previous tutorial.

    A demultiplexer takes one input data line and switches it to any one of multiple independent output lines at a time. The demultiplexer converts the serial data signal at the input into parallel data at its output lines as shown below.

    1 to 4 channel demultiplexer

    The Boolean expression for this 1-to-4 demultiplexer above with outputs A to D and data select lines a, b is:

    F=abA+abB+abC+abD

    In the example above, the function of the demultiplexer is to switch one common data input line to any one of the 4 output data lines A through D. As with the multiplexer, the individual solid-state switches are selected by the binary input address code on the output select pins "a" and "b" as shown.

    Demultiplexer Output Line Selection

    As with the previous multiplexer circuit, adding more address line inputs allows more outputs to be switched, providing 1 to 2n data line outputs.

    Some standard demultiplexer ICs also have an additional "enable output" pin that disables or prevents the input from passing through to the selected output. Still others have latches built into their outputs to hold the output logic level after changing the address input.

    However, in a standard decoder type circuit, the address input will determine which single data output will have the same value as the data input, while all other data outputs have a value of logic "0".

    As shown above, when implementing the above Boolean expression using separate logic gates, it would require the use of six separate gates consisting of AND and NOT gates.

    4-channel demultiplexer using logic gates

    The symbols used to identify demultiplexers in logic diagrams are as follows.

    Demultiplexer symbol

    Again, as with the previous multiplexer example, we can also use a demultiplexer to digitally control the gain of an op amp as shown.

    Digitally adjustable amplifier gain

    The circuit above illustrates how a demultiplexer can be used to provide a digitally controlled adjustable/variable op amp gain. The voltage gain of an inverting op amp is determined by the ratio of its input resistance, RIN, and its feedback resistance, Rƒ, as determined in the Op Amp Tutorial.

    The digitally controlled analog switches of the demultiplexer select an input resistor to vary the value of Rin. The combination of these resistors will determine the overall voltage gain (Av) of the amplifier. The voltage gain of the inverting op amp can then be adjusted digitally simply by selecting the appropriate combination of input resistors.

    Standard demultiplexer IC packages available are the TTL74LS138 1 to 8 output demultiplexer, the TTL74LS139 dual 1 to 4 output demultiplexer or the CMOSCD4514 1 to 16 output demultiplexer.

    Another type of demultiplexer is the 24-pin 74LS154, which is a 4-bit to 16-line demultiplexer/decoder. Here, a 4-bit binary coded input is used to select the individual output positions. Like multiplexers, demultiplexers can also be cascaded together to form higher order demultiplexers.

    Unlike multiplexers which convert data from one data line to multiple lines and demultiplexers which convert multiple lines to one data line, there are devices available which convert data to and from multiple lines, which we will cover in the next tutorial on Combinational Logic Devices. In encoders, they convert multiple input lines to multiple output lines and convert data from one form to another.