Designing with Multiple Multiplexers in Series: A Guide to Cascading Multiplexers

Abstract

Passing a signal through multiple multiplexers, often called cascading, is a useful way to create new and creative designs for complex system needs. While using this technique does allow for more flexibility in design it is important to note the effects of multiplexer parasitics as more multiplexers are added to the bus. This document will expand on the effects of accumulate On-Resistance, On-Capacitance and On-Leakage when cascading multiplexers while also showing both simulated and bench results of the impact on insertion loss. A table of recommended devices for cascading applications is highlighted at the end of the document.

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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.

Figure 1-1 Expanding Configuration With Cascading Multiplexers

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.

Figure 1-2 Creating Switch Arrays With Multiplexers

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.