The Demultiplexer

The data distributor, known more commonly as a Demultiplexer or “Demux” for short, is the exact opposite of the Multiplexer we saw in the previous tutorial. The demultiplexer takes one single input data line and then switches it to any one of a number of individual output lines one at a time. The demultiplexer converts a serial data signal at the input to a parallel data at its output lines as shown below.

1-to-4 Channel De-multiplexer

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

F = abA + abB + abC + abD

The function of the Demultiplexer is to switch one common data input line to any one of the 4 output data lines A to D in our example above. 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 it is possible to switch more outputs giving a 1-to-2n data line outputs.

Some standard demultiplexer IC´s also have an additional “enable output” pin which disables or prevents the input from being passed to the selected output. Also some have latches built into their outputs to maintain the output logic level after the address inputs have been changed. However, in standard decoder type circuits the address input will determine which single data output will have the same value as the data input with all other data outputs having the value of logic “0”.

The implementation of the Boolean expression above using individual logic gates would require the use of six individual gates consisting of AND and NOT gates as shown.

4 Channel Demultiplexer using Logic Gates

The symbol used in logic diagrams to identify a demultiplexer is as follows.

The Demultiplexer Symbol

Again, as with the previous multiplexer example, we can also use the Demultiplexer to digitally control the gain of an operational amplifier as shown.

Digitally Adjustable Amplifier Gain

The circuit above illustrates how to provide digitally controlled adjustable/variable op-amp gain using a demultiplexer. The voltage gain of the inverting operational amplifier is dependent upon the ratio between the input resistor, Rin and its feedback resistor, Rƒ as determined in the Op-amp tutorials.

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

Standard Demultiplexer IC packages available are the TTL 74LS138 1 to 8-output demultiplexer, the TTL 74LS139 Dual 1-to-4 output demultiplexer or the CMOS CD4514 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 the individual output positions are selected using a 4-bit binary coded input. Like multiplexers, demultiplexers can also be cascaded together to form higher order demultiplexers.

Unlike multiplexers which convert data from a single data line to multiple lines and demultiplexers which convert multiple lines to a single data line, there are devices available which convert data to and from multiple lines and in the next tutorial about combinational logic devices, we will look at Encoders which convert multiple input lines into multiple output lines, converting the data from one form to another.