The data distributor, known more commonly as the 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
Output Select | Data Output Selected |
b | a |
0 | 0 | A |
0 | 1 | B |
1 | 0 | C |
1 | 1 | D |
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 it 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 a demultiplexer selects an input resistor to vary the value of Rin. The combination of these resistors will determine the overall voltage 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, they can also be cascaded together to form higher order devices.
Unlike demultiplexers which convert a single data line into multiple output lines, or the multiplexer which can transfer multiple input lines into one single data line, there are also logic devices available which can switch data to and from multiple input and output lines depending on their data select control inputs.
In the next tutorial about Combinational Logic Devices, we will look at Encoders which can convert multiple input lines into multiple output lines, converting the data from one form to another such as BCD or binary.
I would to know , why race condition isn’t good for the circuit . And also to explain level and edge triggering
Race condition occurs when an output state changes in an unpredictable manner as a result of two or more input conditions (level 1 or 0) is being sampled when in the metastable state. That is, a race condition is were an expected output state does not occur but instead switches to an unknown value determined by noise in the system.
Please how to understand studio broadcast swicher or vision mixer
Am interesting
Please please request
Great that demultiplexer is clear and to the point
Dumux
Shivu
8 dərəcəli sxemdə 128 dərəcəli demultipleksorun necə qurulur?
It’s a superb learn it a notes
Hi
Convert decoder into demultiplexer
I think this “Unlike multiplexers which convert data from a single data line to multiple lines and demultiplexers which convert multiple lines to a single data line ..” is incorrect?
Thanks 🙂
Wants to learn digital logic
Nice
O
this tutorials very good for all kind of students.
Draw the Logic circuit for 1:8 Demultiplexer using Logism and analysis the circuit.
Gud
What is the Design for 1:16 Demux on the basis of the following expression ?
f1(a b c)=∑m(1,2,5,6,7,11,14) & f2(a b c)=∑m(0,1,2,5,6,7,8,11,12,15)
1:5 demultiplexers are possible? If yes how?