Do I need an analog switch or multiplexer?

If an application calls for a device with multiple inputs and a single output, it needs a multiplexer. A switch can be used if the need is for one input per output.

Analog switches and multiplexers are used in a wide array of systems, including industrial, medical, military/aerospace, consumer, 5G infrastructure, and more.

This article begins with a brief review of the structure of analog switches and multiplexers and then digs into important performance characteristics and specifications to consider when selecting a device.

Switch structure

Figure 1. A CMOS switch using a complementary pair (top right) to minimize on-resistance and improve linearity (bottom graph). (Image: Analog Devices)

A basic analog switch can be made using P-channel and N-channel MOSFETs. Connecting the devices in parallel forms a bilateral switch. The benefits of this design include a reduction in the combined on-resistance and improved linearity that produces an on-resistance that varies less with the signal voltage (Figure 1).

High-frequency RF analog switches are manufactured using a variety of technologies, including:

Gallium arsenide (GaAs)
Silicon on insulator (SOI)
RF CMOS
Microelectromechanical systems (MEMS)

Multiplexer structure

An analog multiplexer consists of an array of analog switches and a digital control circuit. The switches are usually integrated into a single IC but can also be discrete. Each switch corresponds to an input channel. The digital control individually addresses each switch and determines which signal appears on the output.

When using a multiplexer, several sensors can be sampled sequentially in sensor fusion applications and share a single instrumentation amplifier (INA) and analog-to-digital converter (ADC) (Figure 2).

Figure 2. A multiplexer enables several sensor inputs to share a common INA and ADC. (Image: Renesas)

Discrete multiplexer

A wide array of fully integrated multiplexers is available to meet most systems’ needs. But there are exceptions. Designing multiplexers with discrete analog switches and control circuitry can improve performance in some applications.

That can be especially useful when dealing with various sensor technologies with varying signal levels and voltages, bandwidth and impedance requirements, etc.

Key specs

Analog switches and multiplexers have several key specifications in common. In large part, that’s because multiplexers incorporate switches. For example, every analog switch, and by extension, every multiplexer, has a finite on-resistance (R_on) related to the power devices used in the switch.

Bandwidth is another important specification. It’s the frequency range between the upper and lower -3 dB frequencies. Factors like parasitic or stray resistances and capacitances in the signal path determine it. It defines the frequency range where a signal can be accurately acquired.

Switching time is determined by several factors, including the gate capacitance and on-resistance of the switching transistors, the supply voltage, and the impedance of the connected load. Switching time can be improved by using higher-quality switching transistors and optimizing the transistor drive circuit.

The slew rate and settling time are related to the switching time and, therefore, also related to the internal parasitic capacitance in the switch circuitry, along with the current driving capability of the switch transistors, which affects how quickly the capacitor can charge or discharge when transitioning between states.

The settling time is when the output voltage reaches a steady state and stays within a specified tolerance band. It can be an order of magnitude longer than the switching time.

Multiplexer challenges

Integrating multiple switches to make a multiplexer comes with new performance challenges when managing the performance characteristics of individual switches. Those challenges can be especially evident in high-performance or low-power designs. A few examples include:

R_on mismatches can lead to differences in signal attenuation across channels, impacting accuracy. This can be especially important for high-performance designs.
Leakage current variations can affect the signal integrity of adjacent channels. This can be particularly problematic in low-power designs.
Capacitive coupling of signals between adjacent channels can result in crosstalk and reduced signal integrity in high-speed multiplexers.
Switching speed variations can result in signal glitches or distortion in high-speed multiplexers.
Analog switches can fail in an open or short condition and fault protection is an important consideration.

Summary

An analog switch is used to turn individual signals on and off. A multiplexer consists of multiple switches and is used to connect several inputs, like sensors, to a single INA and ADC. The inputs can be activated sequentially or in any order based on system inputs and the digital controller connected to the individual switches. Multiplexers and switches have many common specifications based on the performance of the switches while multiplexers have additional specifications based on interactions between switches.