Designers are often challenged with finding the most cost, power, and space efficient way to digitize multiple sensors or to route multiple transceivers onto a common communications bus. The solution is to share common resources to avoid duplicating entire signal chains and their related components.
This is accomplished by multiplexing the inputs using analog multiplexers. These can tie multiple sensors to the input of an analog-to-digital converter (ADC), digitizing each transducer in turn. The same approach can be applied to communications buses, where each transceiver can share the bus for a fixed time interval.
The key characteristics of analog switches and multiplexers is that they both offer bidirectional paths between inputs and outputs, and they also feature high signal integrity with minimum crosstalk and leakage currents.
This article describes analog multiplexers and switch configurations before introducing suitable solutions from Texas Instruments that demonstrate the capabilities and flexibility of these devices. It then provides insight on the application of analog switches and multiplexers to share resources.
A multiplexer is an electronic switch that selectively connects multiple input sources to a common output line (Figure 1).
Figure 1: A typical analog multiplexer application using a 4:1 multiplexer to sequentially digitize the analog outputs of four sensors. The binary state of the logic signals A0 and A1 determines which input is connected to the ADC. (Image source: Texas Instruments)
Figure 1 shows four sensors connected via a 4 to 1 analog multiplexer to a common ADC. A pair of logic signals, A0 and A1, controls which sensor is connected to the ADC. As the sensors report physical characteristics that do not change rapidly with time, sequential sampling does not pose any risk of data loss. The principal benefit is the lowering of the overall parts count by using only a single ADC and related circuitry for all four sensors, thereby reducing the overall cost of the design.
Multiplexer and switch configurations
Analog multiplexers are part of a broader category of electronic switches that are available in a large number of configurations as shown in Figure 2.
Figure 2: Some common switch and multiplexer configurations. Switches differ from analog multiplexers in that the outputs are not tied together; they can be routed independently. (Image source: Digi-Key Electronics)
Multiplexers are configured to select any of 2N inputs with commonly available models ranging from 2:1 to 16:1. For each multiplexer 2N configuration the number of digital control lines equals N. So, an 8:1 multiplexer needs three control lines. Switch configurations are described by the number of inputs or “poles” and the number of outputs or “throws.” A single-pole single-throw (SPST) switch has a single input and a single output. A single-pole double-throw (SPDT) switch has a single input and two outputs. Integrated circuit (IC) manufacturers often package multiple switches into a single IC package and describe the multiple switches as having multiple channels, like the four-channel SPST switch shown in Figure 2.
SPST and SPDT switches are the two most common switch configurations. There are also single-pole three-throw (SP3T) and single-pole four-throw (SP4T) switches that are used in radio frequency (RF) applications.
Switches can be designed to have specific dynamic characteristics affecting what happens when switch contacts change. If the switch is designed to “make before break” it means that the initial connection is maintained until the new connection is made. The moving contact never sees an open condition. On the other hand, a “break before make” switch severs the original connection before a new one is made so that no adjacent contacts are short circuited.
Most current analog switch and multiplexer designs employ complementary metal-oxide semiconductor (CMOS) field effect transistors (FETs). A representative bilateral switch element employs two complementary CMOS FETs, an N-channel and a P-channel device, connected in parallel (Figure 3).
Figure 3: A basic multiplexer switch element and its equivalent circuit. Complementary FETs allow for bilateral operation so it can switch signals in either direction. (Image source: Digi-Key Electronics)
The parallel arrangement produces a conduction path that can handle signals of either polarity. This combination also minimizes the series on-resistance (ROn) and reduces its voltage sensitivity. The significant elements of the equivalent circuit are ROn and the channel capacitance, CD.
The on-resistance along with the source resistance, RSource, and load resistance, RLoad, affects the gain of the switch when it is closed. The on-resistance also varies with the applied signal voltage. The on-resistance and parallel combination of CD and load capacitance CLoad affect the bandwidth and switching dynamics, primarily the switching time. In general, designers should look to minimize both ROn and CD. There is also a leakage current into the signal path that affects direct current (DC) offset.
When the switch is open, the feed-through capacitance, CF, provides a path around the switch, limiting its isolation capabilities. During the closing of the switch, charge is shared between the source capacitance, CS, and the channel and load capacitances, resulting in switching transients.
As shown in Figure 1, the effects of a switch’s on-resistance can be minimized by buffering the switch output with a buffer amplifier having a very high input resistance. This circuit configuration reduces gain loss and minimizes the effects of on-resistance variation. It can, however, increase the offset voltage due to the leakage current. There is an engineering trade-off here that is usually solved by selecting components with the minimum possible leakage current.
Analog multiplexer and switch solutions
The Texas Instruments TMUX1108PWR 8:1 multiplexer is an example of a precision multiplexer intended to mate with an ADC. It has a supply voltage (VDD) range of 1.08 to 5 volts. Signal voltages can range from 0 volts to VDD, supporting bidirectional analog or digital signals. Channel series resistance, ROn, is typically 2.5 ohms (Ω) and leakage current is less than 3 picoamps (pA). On capacitance is 65 pF, resulting in a transition time between channels of typically 14 nanoseconds (ns) and a bandwidth of 90 megahertz (MHz).
There are a number of configurations available in the TMUX11xx series of multiplexers. For example, the TMUX1109RSVR is a dual-channel 4:1 multiplexer. It has the same power supply range and leakage specifications as the TMUX1108PWR but has an on-resistance of 1.35 Ω (typically) and a maximum bandwidth of 135 MHz. This device features two 4:1 multiplexers, which can be used as a 4:1 differential multiplexer or as two 4:1 single-ended multiplexers (Figure 4).
This is an application example of a differential four-channel data acquisition system based on a dual simultaneous sampling successive approximation ADC. There are four differential channels per ADC. Each 16-bit ADC has a 3 megasample per second (MS/s) sampling rate for signals with amplitudes of up to ±3.8 volts. Applications for this type of acquisition system include optical, industrial, and motor control.
Figure 4: An application for two dual 4:1 multiplexers is a four-channel differential signal acquisition system with a bandwidth of 16.45 MHz intended for handling optical, industrial, or motor control signals. (Image source: Texas Instruments)
The simplest multiplexer topology is a single-channel 2:1 multiplexer. This is basically an SPDT switch. The Texas Instruments TMUX1119DCKR is a precision version of a 2:1 multiplexer. It shares the same power supply range and leakage current as the other members of the TMUX11xx family. Its on-resistance is typically 1.8 Ω and its maximum bandwidth is 250 MHz.
Among the applications for the 2:1 multiplexer is using two of them as a reversing switch (Figure 5). The circuit is that of a gas metering system using differential time-of-flight measurements to determine the flow velocity. There are two ultrasound transducers placed in a pipe a known distance apart. The propagation time from one transducer to the other is measured, then the transducers are reversed to measure the propagation time in the other direction. The flow velocity of the gas in the pipe is computed from the time difference. Two TMUX1119 multiplexers are used to reverse the transducer connections. This is an example of a multiplexer routing signals to the inputs of the gas flow analyzer. The ultra-low leakage current and flatness of the on-resistance of this multiplexer make it an excellent choice for this application.
Figure 5: Schematic shows the use of two 2:1 multiplexers to reverse the connections on a pair of ultrasonic transducers in a gas flow analyzer. (Image source: Texas Instruments)
In addition to these various multiplexer configurations, multiple independent switches can be packaged into an IC. Consider the Texas Instruments TMUX6111RTER four-circuit SPST switch (Figure 6). This device features a very low leakage current of 0.5 pA and a bandwidth of 800 MHz. The on-resistance is a moderate 120 Ω.
Figure 6: The TMUX611RTER four-circuit SPST switch includes four independent switches featuring very low leakage current and a bandwidth of 800 MHz. (Image source: Texas Instruments)
This is one of three devices in this product series offering four independent switches. This version has four normally open switches. Another version has four normally closed switches, while a third version comes with two of each type of switch.
Analog switches and multiplexers offer great economy in terms of component space, cost and power by allowing multiple sensors to share a common analog-to-digital converter. They also offer a great deal of flexibility in changing circuit connections under computer control, whether it be sharing communications buses or changing transducer connections.