When multi-sensor digitization is performed or when multiple transceivers are connected to the public communication bus, the designer is often difficult to find the most effective way to save costs, power and space. The solution is shared public resources to avoid repeating the build of the entire signal chain and its related components.
Implementation is to use an analog multiplier multiplexer to multiplex into input. This can connect a plurality of sensors to an input of an analog-to-digital converter (ADC), which is sequentially processed for each sensor. The same method can also be applied to a communication bus, each transceiver, can be used in a fixed time interval.
The key features of the analog switches and multiplexers are all bidirectional paths between inputs and outputs, and also have high signal integrity, extremely small crosstalk, and leakage current.
This article first describes the analog multi-channel multiplexer and switch configuration, then introduces the relevant solutions of Texas Instruments to demonstrate the functionality and flexibility of these devices. This article will then make some guidelines on the application of the analog switch and multiplexer implement resource sharing.
Analog multiplex multiplexer
A multiplexer is an electronic switch that selectively connects multiple input sources to the public output line (Figure 1).
Figure 1 shows that four sensors are connected to the public ADC through the 4: 1 analog multiplier multiplexer. A pair of logic signals A0 and A1 control which sensor connects to the ADC. Since the physical properties of the sensor report will not change quickly over time, the sequential samples do not cause the risk of data loss. Its main advantage is that only one ADC and related circuitry can handle all four sensors, the total number of components is reduced, and the total cost of design is also reduced.
Multiplexer and switch configuration
The analog multiplexer belongs to a wider range of electronic switch categories, providing a large number of configurations as shown in Figure 2.
The multiplexer is configured to select any of 2N inputs, and the common model is from 2: 1 to 16: 1. For each multiplexer 2N configuration, the number of digital control lines is equal to N. Therefore, 8: 1 multiplexer requires three control lines. The switch configuration is described by the number of inputs (or "knife") and output (or "throw"). Single-knife single throw (SPST) switch has all the input and all the way. Single knife double throw (SPDT) switch has all the way and two outputs. Integrated circuit (IC) manufacturers often load multiple switches into a single IC package and describe these switches as having multiple channels, as shown in FIG. 2, four channel SPST switches.
SPST and SPDT switches are two most common switch configurations. There is also a single-knife throw (SP3T) and a single-knife four-hit (SP4T) switch for radio frequency (RF) applications.
The switch can be designed to have a specific dynamic characteristic, which can affect the operation that occurs when the switch contact changes. If the switch is designed as "first-opening", it means that the initial connection will remain until the establishment of a new connection. The moving contact will never be in the open state. On the contrary, the "first-opening" switch will first turn off the original connection, then establish a new connection, so that adjacent contacts will not be shorted.
CMOS switch
Most of the analog switches and multiplexers are designed with a complementary metal oxide semiconductor (CMOS) field effect transistor (FET). Representative bidirectional switching elements use two complementary CMOS FETs: an N-channel device and a p-channel device, both in parallel (Figure 3).
The conductive path generated by the parallel assembly can handle the signal of any polarity. Such a combination also minimizes the tandem electrical resistance (RON) and reduces its voltage sensitivity. An important component of the equivalent circuit has Ron and channel capacitance CD.
The on-resistance is associated with the source resistor RSource and the load resistor RLoad affects the gain of the shutdown time. The on-resistance also varies with the applied signal voltage. The parallel combination of on-resistance and CD and load capacitance Cload affect bandwidth and switch dynamic characteristics, mainly switching time. In general, designers should make RON and CD minimize. At the same time, there will be a leakage current into the signal path, affecting the direct current (DC) offset.
When the switch is turned off, the feedthrough capacitor CF provides a path around the switch to limit its isolation. During the shutdown period, the source capacitor CS is shared with the channel and load capacitance to generate a switch transient.
As shown in FIG. 1, the buffer amplifier buffer switch output with ultra-high input resistance can minimize the impact of the switch turning resistance. This circuit configuration reduces gain loss and minimizes the impact of on-to-conductivity changes. However, the compensation voltage caused by leakage current may increase. Here you need to trade out in engineering, usually solved by selecting the elements that the leakage current is as small as possible.
Analog multiplexer and switching solutions
Texas Instruments TMUX1108PWR 8: 1 multiplexer is an example of a precision multiplexer intended to be used with the ADC. Its supply voltage (VDD) ranges from 1.08 V to 5 V. The signal voltage range is from 0 V to VDD, supports two-way analog or digital signals. The typical value of the channel series resistance RON is 2.5 Ω, and the leakage current is less than 3 Pa. The conduction capacitance is 65 pf, so the passage between the channels is typically 14 NS, with a bandwidth of 90 MHz.
TMUX11xx Series Multiplexers have multiple configurations available. For example, TMUX1109RSVR is a dual channel 4: 1 multiplexer; having the same power supply range and leakage current specifications as TMUX1108PWR, but the on-resistance is 1.35 Ω (typ), the maximum bandwidth is 135 MHz. The device has two 4: 1 multiplexer, which can be used as a 4: 1 differential multiplexer or two 4: 1 single-end multiplexer (Figure 4).
This is an application example of a differential four-channel data acquisition system, wherein the system is based on a dual channel synchronous sample number approximation type ADC. Each ADC has four differential channels. The signal sample rate of each 16-bit ADC is 3 ms / s, the amplitude is as high as ± 3.8 V. Applications of such acquisition systems include optical, industrial and motor control.
The simplest multiplexer topology is a single channel 2: 1 multiplexer. This is basically a SPDT switch. TMUX1119DCKR of Texas Instruments is a sophisticated version 2: 1 multiplexer. Its power supply range and leakage current specifications are the same as other members of the TMUX11xx series. The conductive resistance value is 1.8 Ω, the maximum bandwidth is 250 MHz.
2: 1 Multiplexer has an application that uses two such devices as reverse switches (Figure 5). This circuit is a gas metering system that determines the flow rate using a differential flight time measurement. Two ultrasonic transmitters are placed in a pipe, and the separation distance is known. First measuring the propagation time from one transmitter to another transmitter, and then reverse transitioners to measure the propagation time in the other direction. Calculate the gas flow rate in the pipeline according to the time difference. Two TMUX1119 multiplexers are used to reverse transition. This is an example of a multiplexer to route the signal to the airflow analyzer. The multiplexer has ultra-low leakage current and flat on-resistance, thus becoming excellent selection of such applications.
In addition to a wide variety of multiplexer configurations, multiple independent switches can be packaged into an IC. Tmux 6111RTER four-circuit SPST switch in Texas instruments (Figure 6). The device has a lower leakage current of 0.5 Pa and a bandwidth of 800 MHz. Turn on resistance, is 120 Ω.
This is one of the three devices in the product series, providing four independent switches. This version has four normally open switches. Another version has four normally closed switches, and the third version is two of the switches.
Summarize
The analog switch and multiplexers support multiple sensors to share a public analog-to-digital converter to bring great economy in terms of components, cost, and power consumption. These devices can also provide great flexibility that can be used under computer control, whether the shared communication bus or change the transmitter connection.
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