Summary of knowledge points of multiplexer

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

    Multiplexing is a general term used to describe the operation of sending one or more analog or digital signals at different times or speeds over a common transmission line, so the device we use to do this is called a multiplexer.

    The multiplexer, abbreviated as "MUX" or "MPX", is a combinational logic circuit designed to switch one of multiple input lines to a single common output line through the application of a control signal. A multiplexer works like a fast-acting, multi-position rotary switch, connecting or controlling multiple input lines, called "channels", to the output at one time.

    A multiplexer, or MUX, can be a digital circuit made of high-speed logic gates used to exchange digital or binary data, or it can be an analog type that uses transistors, MOSFETs, or relays to switch one of the voltage or current inputs to a single output.

    The most basic type of multiplexer device is a one-way rotary switch as shown in the figure.

    Basic multiplexing switch

    A rotary switch, also known as a wafer switch because each layer of the switch is called a wafer, is a mechanical device whose input is selected by rotating a shaft. In other words, a rotary switch is a manual switch that you can use to select a single data or signal line by simply turning its input "ON" or "OFF". So how can we use digital devices to automatically select each data input.

    In digital electronics, multiplexers are also called data selectors because they can "select" each input line, constructed from a single analog switch enclosed in a single IC package, as opposed to "mechanical" type selectors such as regular conventional switches and relays.

    They are used as a way to reduce the number of logic gates needed in a circuit design or when a single data line or data bus is required to carry two or more different digital signals. For example, an 8-channel multiplexer.

    Typically, the selection of each input line in a multiplexer is controlled by another set of inputs called control lines, and depending on the binary condition ("high" or "low") of these control inputs, the appropriate data input is connected directly to the output. Typically, a multiplexer has an even number of 2n data input lines and a number of "control" inputs corresponding to the number of data inputs.

    Note that the operation of a multiplexer is different from that of an encoder. An encoder is able to switch an n-bit input pattern into multiple output lines that represent a binary coded (BCD) output equivalent to the active input.

    We can build a simple 2-line to 1-line (2-to-1) multiplexer from basic logic NAND gates as shown in the figure.

    Input Multiplexer Design

    This simple 2-1 wire multiplexer circuit constructed from standard NAND gates has input A used to control which input (I 0 or I 1 ) is passed to the Q output.

    From the truth table above, we can see that when the data select input A is LOW at logic 0, input I1 passes its data to the output through the NAND gate multiplexer circuit, while input I0 is blocked. When data select A is HIGH at logic 1, the reverse operation occurs and now input I0 passes data to output Q, while input I1 is blocked.

    Therefore, by applying a logic “0” or a logic “1” at A, we can select the appropriate input, I 0 or I 1, and the circuit acts somewhat like a single-pole double-throw (SPDT) switch.

    Since we only have one control line (A), we can only switch 2 1-bit inputs, in this simple example, the 2-input multiplexer connects one of the two 1-bit sources to a common output, producing a 2-to-1 in-line multiplexer. We can confirm this in the Boolean expression below.

    Q = AI 0 .I 1 + A .I 0 .I 1 + AI 0 .I 1 + AI 0 .I 1

    For the 2-input multiplexer circuit above, this can also be simplified:

    Q = A.I 1 + AI 0

    We can simply increase the number of data inputs to be selected further by following the same steps and can implement a larger multiplexer circuit using the smaller 2 to 1 multiplexer as its basic building block. Therefore, for a 4-input multiplexer we will need two data select lines as the 4 inputs represent 2 2 data control lines, thus giving a circuit with 4 inputs i.e. I0, I1, I2, I3 and two data select lines A and B as shown.

    4-to-1 channel multiplexer

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

    Q = ab A + ab B + a bC + abD

    In this example, at any instant in time, only one of the four analog switches is closed, connecting one of the input lines A through D to only the single output at Q. Which switch is closed depends on the addressing input codes on lines "a" and "b".

    So for this example, to select input B to output Q, the binary input addresses would need to be "a" = logic "1" and "b" = logic "0". We can therefore show how the selection of data through the multiplexer relates to the data select bits shown.

    Multiplexer Input Line Selection

    Adding more control address lines, (n) will allow the multiplexer to control more inputs since it can switch 2n inputs, but each control line configuration will only connect one input to the output.

    Then, implementing the above Boolean expression using separate logic gates would require the use of seven separate gates consisting of AND, OR, and NOT gates as shown in the diagram.

    4-Channel Multiplexer Using Logic Gates

    The symbols used to identify multiplexers in logic diagrams are as follows:

    Multiplexer symbol

    Multiplexers are not limited to switching multiple different input lines or channels to a common single output. There are also types that can switch their inputs to multiple outputs and have configurations such as 4 to 2, 8 to 3, and even 16 to 4 configurations, as well as a simple dual channel 4-input multiplexer (4-to-2) as follows:

    4 to 2 channel multiplexer

    In this example, here 4 input channels are switched to 2 separate output lines, but larger arrangements are possible. This simple 4 to 2 configuration can be used, for example, to switch audio signals for a stereo preamplifier or mixer.

    Adjustable amplifier gain

    In addition to sending parallel data in a serial format over a single transmission line or connection, another possible use for a multi-channel multiplexer is as a mixer in digital audio applications, or to digitally control the gain of an analog amplifier, for example.

    Digitally adjustable amplifier gain

    Here, the voltage gain of the inverting op amp is determined by the ratio of its input resistance, R IN , and its feedback resistance, R ƒ , determined in the Op Amp Tutorial.

    A single 4-channel (Quad) SPST switch configured as a 4-to-1 channel multiplexer is placed in series with resistors to select any feedback resistor to change the value of Rƒ. The combination of these resistors will determine the overall voltage gain (Av) of the amplifier. The voltage gain of the amplifier can then be digitally adjusted by simply selecting the appropriate resistor combination.

    Digital multiplexers are sometimes called "data selectors" because they select the data to be sent to the output lines and are often used in communications or high-speed network switching circuits such as LAN and Ethernet applications.

    Some multiplexer ICs have a single inverting buffer (NOT gate) connected to the output to provide a positive logic output (logic "1", HIGH) at one terminal and a complementary negative logic output (logic "0", LOW) at a different terminal.

    As mentioned above, simple multiplexer circuits can be made from standard AND and OR gates, but common multiplexers/data selectors are available as standard ic packages, such as the common TTL 74LS151 8 input to 1 line multiplexer or the TTL 74LS153 Dual 4 input to 1 multiplexer. Multiplexer circuits with a large number of inputs can be obtained by cascading two or more smaller devices together.

    Multiplexer Summary

    Then we can see that multiplexers are switching circuits that only switch or route signals through themselves, and as a combinational circuit, they have no memory as there is no signal feedback path. Multiplexers are very useful electronic circuits that have been used in many different applications such as signal routing, data communications, and data bus control applications.

    When used with a demultiplexer, parallel data can be transmitted in serial form over a data link (such as a fiber optic cable or telephone line) and then converted back to parallel data again. The advantage is that only one serial data line is required, rather than multiple parallel data lines. Multiplexers are sometimes called "data selectors" because they select the data in a row.

    Multiplexers can also be used to switch analog, digital, or video signals, and the switching current in the analog power circuit is limited to less than 10mA to 20mA per channel to reduce heat dissipation.