Analog Switches and Multiplexer

Analog switches are ICs designed to switch analog signals via digital control. The internal structure of these devices typically consists of a number of logic control gates interfaced with transistor stages used to control the flow of analog signals. The CMOS 4066B quad bilateral switch uses a single supply voltage from 3 to 15 V. It can switch analog or digital signals within _7.5 V and has a maximum power dissipation of around 700 mW. Individual switches are controlled by digital inputs A through D. The TTL compatible AH0014D DPDT analog switch can switch analog signals of _10 V via the A and B logic control inputs.

Note that this device has separate analog and digital supplies: V+ and V− are analog; VCC and GND are digital. The DG302A dual-channel CMOS DPST analog switch can switch analog signals within the _10-V range at switching speeds up to 15 ns. There are a number of circuits that use analog switches. They are found in modulator/demodulate circuits, digitally controlled frequency circuits, analog signal-gain circuits, and analog-to-digital conversion circuits where they often act as sample-hold switches. They can of course be used simply to turn a given analog device on or off while a digital demultiplexer acts like a data distributor.

Analog multiplexers and demultiplexers act the same way but are capable of selecting or distributing analog signals. (They still use digital select inputs to select which pathways are open and which are closed to signal transmission.) This device functions as either a multiplexer or demultiplexer, since its inputs and outputs are bidirectional (signals can flow in either direction). When used as a multiplexer, analog signals enter through I/O lines 0 through 7, while the digital code that selects which input gets passed to the analog O/I line (pin 3) is applied to digital inputs A, B, and C. See the truth table in the figure. When used as a demultiplexer, the connections are reversed.

The analog input comes in through the analog O/I line (pin 3) and passes out through one of the seven analog I/O lines-the specific output is again selected by the digital inputs A, B, and C. Note that when the inhibit line (INH) is high, none of the addresses are selected. The input/output analog voltage levels for the 4051B are limited to a region between the positive supply voltage VDD and the analog negative supply voltage VEE. Note that the VSS supply is grounded. If the analog signals you are planning to use are all positive, VEE and VSS can both be connected to a common ground. However, if you plan to use analog voltages that range from, say, −5 to +5 V, VEE should be set to −5 V, while VDD should be set to +5 V. The 4051B accepts digital signals from 3 to 15 V, while allowing for analog signals from −15 to −15 V.