1 Introduction

Passing a signal through two or more multiplexers in series, often called cascading, can be used as a creative way to realize unique system designs that otherwise are not possible. While there are a wide range of circumstances that can see a designer implement cascading multiplexer in a system, there are two main instances where this implementation is used the most.

The most common situation where cascading multiplexers are used is in configuration expansion, where smaller multiplexers create a larger multiplexer with a greater number of inputs feeding into the eventual single output. Alternatively the opposite flow can also be done where many outputs feed into a single input as well. An example is highlighted below in Figure 1-1 where 3 SPDT (2:1) multiplexers are used to create a 4:1 multiplexer.

The other most common instance where a multiplexer can be cascaded is commonly called a ‘cross-point switch’. A cross-point switch is a switch that allows any input to find a path to any other input or output without interferences from any other signal path. For example, on the right in Figure 1-2 we see a type of cross-point switch called a switch array that is created by cascading many multiplexers. Here, A_in can be connected to any of A_out, B_out or C_out, depending on the logic of each multiplexer. At the same time, B_in can also be connected to any of the three outputs, given the correct logic implementation on your multiplexers, without having to be shorted to A_in at the same time. In this example, while a small array can be created by simply shorting the outputs of a couple multiplexers together and utilizing control logic efficiently without cascading (Left image in Figure 1-2), as you increase the size of the array, you can use smaller multiplexers for flexibility in PCB-routing and logic control while also limiting certain parasitics, which we explore in more depth through this report.

With cascading multiplexers, there are a few behavioral specifications that accumulate as more multiplexer stages are added. How these affect the system varies depending on how these multiplexers are being used in a given system. We’ll examine these specifications in the following section.