Multiplexers and Demultiplexers

A variety of practical electronic applications require multiplexing, where several input signals are individually selected, one at a time but very rapidly, to be communicated through a single channel. Telephony systems use this technique to “concentrate” multiple voice conversations over a single wire pair, and most analog dual-trace oscilloscopes use this technique to allow a single-gun CRT to display more than one signal trace on the screen at a time.

In order to rapidly select (or switch) analog signals in these multiplexing applications, we need some form of semiconductor on/off switch capable of fast switching time, low pass-through (“on” state) impedance, and high blocking (“off” state) impedance. Thankfully, there is such a device commonly and inexpensively produced, called a CMOS bilateral switch:

This hybrid analog/digital device uses digital logic signals (high/low) to activate the gates of CMOS transistor assemblies to switch analog signals on and off. It is like having four low-current solid-state relays in a single integrated circuit. When the control line is made “high” (standard CMOS logic level), the respective switch goes into its conductive (“on”) state. When the control line is made “low,” the switch turns off. Because it is MOSFETs we are turning on and off, the control lines draw negligible current (just like CMOS logic gate inputs).

If we are to use such bilateral switches to multiplex analog signals along a common signal line, though, we must add some accessory components to control which switch (out of the four) is active at any given time. Take for instance this circuit where we use four bilateral switches to multiplex the voltage signals from four accelerometers (measuring acceleration on a vibration-testing jig):

Identify the necessary “mystery device” shown in the schematic, which allows a binary input (S₀ and S₁ with four combinations of high/low states: 00, 01, 10, and 11) to activate just one bilateral switch at a time.

Multiplexers and demultiplexers are often confused with one another by students first learning about them. Although they appear similar, they certainly perform different functions. Shown here is a multiplexer and a demultiplexer, each using a multiple-position switch symbol to indicate the selection functions inside the respective circuits:

After identifying which is which, provide definitions for “multiplexer” and “demultiplexer” in your own words.

The 74HC150 is a high-speed CMOS (TTL-compatible) integrated circuit multiplexer, also known as a data selector. It is commonly available as a 24 pin DIP “chip.” Identify the terminals of a 74HC150, and label them here:

In particular, note the locations of the four “select” terminals, as well as the single output terminal.

What types of electrical “data” may be “selected” by this particular integrated circuit? For example, can it select an analog waveform, such as human speech from a microphone? Is it limited to discrete TTL signals (low and high, 0 volts and 5 volts DC)? How can you tell?