Multiplexer vs. Switch: Which Should You Use?

When telecom networks need to share scarce physical channel resources among multiple sources/receivers, they use a simple technique called multiplexing/demultiplexing. This is a fundamental technique for routing large amounts of serial or parallel data over a single physical channel. Multiplexers are often discussed in the same context as switches, which can provide the same functionality. So what makes these components different, and which do you need for your digital or analog system?

The differences between a multiplexer vs. switch can be found in their construction at the IC level and their specifications. In addition, there are different multiplexing methods for different applications (spatial, time-division, or frequency/wavelength division), making the choice between a switch and multiplexer more confusing. Here are some of the functional and electrical characteristics that differentiate a multiplexer vs. switch and when you might use each in your application.

Multiplexer vs. Switch

The difference between these components can be confusing, and it doesn’t help that the functional diagram for a multiplexer is often portrayed as a switch. This implies that they are basically mechanical switches, but this is certainly not the case with modern multiplexers and switches. The one exception is switches built as microelectromechanical systems (MEMS), which do indeed have a tiny mechanical switching element.

Aside from this point about MEMS, most switches and multiplexers are built using FETs and some supporting circuitry and are activated with a control signal, rather than the user’s hand. Both types of components are available as through-hole or SMD ICs in standard packages, so designers have access to a broad range of switches and multiplexers for different applications. The one exception is for high power applications, where electrical switching can be used with high-power transistors instead of mechanical switching for power distribution.

To understand more about the difference between these components, let’s look a bit deeper at how they operate and their specifications:

Analog vs. Digital

Both types of components come in the analog or digital varieties. An analog switch can pass both analog and digital signals, but digital switches only pass digital logic levels. The same idea applies to multiplexers, but the definition of a multiplexer is broad; multiplexers can also be analog with some defined bandwidth, while digital multiplexers are built entirely from digital logic circuits and also only pass logic states.

Signal Selection

The best way to describe the role of a multiplexer is in terms of signal selection. A multiplexer doesn’t necessarily isolate the upstream and downstream sections of the system, but its higher on-state resistance means the input impedance seen by the driver component is less dependent on the load component’s input impedance. In contrast, a switch tends to have much lower on-state resistance (as low as 1 Ohm) and reactance at relatively low frequencies.

Specifications

Here are some of the important specifications that are common to switches and multiplexers:

• Switching time. This tells you the time required for the component to switch between different channels. Ideally, the switching time should be much smaller than the signal’s rise time (for digital signals). 

• Slew rate and settling time. When a channel in one of these components is activated, there is some specific switching time during the transition to the full-scale signal level (measured as 10%-90% for digital signals). After switching, the output requires some time to settle to the signal level seen at the input. This can be a factor 10 longer than the switching time or rise time. 

• Bandwidth. The bandwidth of a switch or multiplexer refers to the -3 dB point in a channel’s transfer function. This is set by the internal input capacitance and on-state resistance. 

• Directionality. Switches are bidirectional, while multiplexers are monodirectional. Note that some newer multiplexers have the same topology as analog switches, and they can be used as bi-directional multiplexers. 

• Channel leakage. Components with lower on-state channel resistance tend to have higher channel leakage current. 

• Channel count. A switch/multiplexer will have some channel count (N:1 ratio), where N input channels are routed into a single channel. Note that a switch can be implemented with 1:N ratio thanks to its bidirectionality, but this is not the case with a multiplexer. 

• Process. CMOS switches and multiplexers are much slower than components manufactured with bipolar processes. For this reason, the fastest switches use a FET process in order to accommodate high data rate applications. 

Some switches or multiplexers can be wired in a nested topology, e.g., where multiple N:1 multiplexers are wired into another N:1 multiplexer. An example of a quad 2:1 multiplexer is shown below.

Logic diagram for the MC74ACT157DG quad 2:1 multiplexer from ON Semiconductor. Source: MC74ACT157DG datasheet.

Note that multiplexing and SerDes are not the same. A multiplexer can be implemented as a serializer by cycling through the control bits on the multiplexer in order as the component receives parallel data. In addition, multiplexing is used for bit-interleaved SerDes, where multiple slow serial data bitstreams are compressed into one high-speed bitstream. Aside from that, the two techniques are not the same. There are other applications where multiplexing is used as it eliminates the need for one or more mechanical/electromechanical switches.

Applications of Multiplexers and Switches

In summary, some applications will work just fine with either type of component. High-frequency analog applications should focus more on the bandwidth and on-state resistance than on other metrics. In addition, any application that requires selecting between multiple data streams or compressing data into a single bit stream can make use of multiplexers or switches. Some applications of multiplexers and switches include:

• High-speed video and high fidelity audio

• Cable/satellite television

• Antenna selection in phased arrays for beamforming and spatial multiplexing

When you need to compare a potential multiplexer vs. switch IC, try using the advanced search and filtration features in Octopart. You’ll have access to an extensive search engine with distributor data and parts specifications, all of which are accessible in a user-friendly interface. Take a look at our integrated circuits page to find the components you’ll need for data transmission and management.

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