What is an Analog Switch?

Contel Bradford

Sometimes also referred to as a bilateral switch or transmission gate, an analog switch is an electronic device that selectively passes, or blocks, a signal level from the input to the output. A solid-state semiconductor component, it is compromised of an nMOS transistor and pMOS transistor. Its control gates are configured in a manner so that both transistors are either turned on or off. Common analog switch applications include instrumentation, process control, data acquisition, communications systems, and video systems.

Prior to 2009, televisions in the U.S. could receive analog TV signals, but now only digital signals are used and special adapters are needed for old-style televisions.
Prior to 2009, televisions in the U.S. could receive analog TV signals, but now only digital signals are used and special adapters are needed for old-style televisions.

An analog switch is also commonly used as a building block for logic circuitry such as D Flip-Flop or D Latch. Acting as a stand-alone circuit, it has the ability to isolate one or multiple components from live signals during removal, or hot insertion. When used in security applications, it can selectively prevent sensitive signals, or data, from being transmitted without valid authorization from the hardware.

Digital TV signals are either UHF, or Ultra High Frequency, or VHF, or Very High Frequency, and they generally reach up to 70 miles away from their broadcast towers.
Digital TV signals are either UHF, or Ultra High Frequency, or VHF, or Very High Frequency, and they generally reach up to 70 miles away from their broadcast towers.

The ideal analog switch has infinite resistance when off, and zero resistance when on. It also has flawless linear, volt-ampere characteristics when transmitting analog signals. The truth, however, is that most analog switches are not ideal. This can be true because they are essentially solid-state semiconductor components and tend to exhibit non-zero resistance when on, finite resistance when off, and non-linear, volt-ampere characteristics.

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Similar to a mechanical switch, the analog switch comes in many forms, that largely depending on the number of throws and poles it has. While often used to described a mechanical switch, single-pole single throw (SPST) and single-pole double-throw (SPDT) are among the most common of these switches. A single integrated circuit (IC) may also contain multiple analog switches, each of which corresponds to a particular analog channel.

The gate for an analog switch can be formed by a number of circuit configurations. Some of these configurations are very simple, and often consist of numerous resistors and a single diode. Most commercial switches, however, utilize field-effect transistors (FETs), highly advanced bipolar transistors, or a combination of both, to facilitate the passing or blocking of analog signals. FETs are employed in analog switches more frequently due to their low closed resistance and high open resistance.

An analog switch behaves a lot like a relay. The major difference is that it has no moving parts, and does not provide isolation between the analog signal and control signal. For this reason, it is not recommended for use in high-voltage circuits where electrical isolation is required. It is also important to note that because there is low current path between the input and output, the maximum current able to run through an analog switch is usually smaller than the average relay.

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