Precision labs series: Understanding switches and multiplexer's features and parameters

Hello, and welcome to the Multiplexer Series on Basics of Multiplexers and Signal Switches. This video series explains basics of analog signal switches offered by Texas Instruments. In this video, we will discuss bi-directional switches and multiplexers. We will begin with a basic overview of the concepts and architecture of bidirectional switches. Finally, we will learn how to spot a bi-directional switch.

The goal of this video is to understand how switches are bidirectional, and how to identify bi-directional switches and multiplexers. Bi-directional means that the channel inputs of a switch or mux are interchangeable with the outputs. The signal may flow from either terminal to the opposite terminal when the switch is in the on state.

First, let's think about this on a high level by looking at an ideal single pole, signal throw, or one to one switch. An ideal switch in the on state acts as a low impedance wire, and current may flow in either direction. The output and the input nodes are connected, so that the input and output can be used interchangeably.

Direction of current is entirely dependent on which side is driving. This basic idea can be applied to analog muxes as well. Let's look at a diagram of an ideal 3 to 1 mux. Only one path is conducting at a time, and currently may flow in either direction. Either terminal may be used as the input or output.

So for this diagram, this means that one of three inputs, A, B, or C, may be selected to conduct to the output, COM, or one input, COM, may conduct to the selected output, either A, E, or C. Bi-directional muxes may be used as both multiplexers and demultiplexers.

Let's think about switch architectures and how they enable bi-directional operation. All FET switches are bi-directional, because both NFETs and PFETs are symmetrical. For this example, we will discuss a single NFET switch. V GS is indicated as the voltage between the gate to source, and VDS is the voltage between drain and source. VTH is the minimum value for VGS for which the conducting path is created between drain and source.

In addition, the voltage between drain and source must be positive. Because NFETs are symmetrical, the drain and source are interchangeable. This distinction happens when one side of the NFET becomes more positive than the other side. The more positive side is called the drain, and the less positive side is called the source.

This example also applies to transmission gate switches, because the symmetrical PFET and NFET are in parallel. So the NFET and the PFET will share a common drain node and a common source node. For PFETs to conduct current, the voltage between gate and source must be below the threshold voltage, and the voltage from drain to source must be negative.

For more information about common switch architectures, please watch the TI Precision Labs Video, "What Are Common Switch Architectures?"

Finally, let's discuss how to determine if a switch or mux is bi-directional. All TI analog switches and muxes are passive switches, and are therefore bi-directional. So if a switch's architecture includes a single FET, including the single FET with charge pump, or two FETs in parallel, as shown in the previous slide, the switch is bi-directional.

The switch architecture or topology may sometimes be found in the datasheet and the detailed description section. The datasheet of the switch may mention the word bi-directional in the title, features, or device description. Other indicators that the switch is bi-directional include the channel current rating is both positive and negative, or the pin function table will indicate channel pins are capable of being both inputs and outputs.

For muxes, the data sheet may include the word demultiplexer in addition to multiplexer in the title or description. The bottom line is that all analog switches and muxes are bi-directional. Thank you for watching the TI Precision Labs video, Are Multiplexrs and Switches Bi-directional? To find more switches and multiplexing, technical resources, and search products, visit ti.com.