Programmable logic controllers (PLCs) incorporate analog input or output modules that are implemented in fairly harsh factory automation environments and will routinely be exposed to high common mode voltages. A typical analog I/O module can incorporate a multiplexer being used in conjunction with an ADC. Usually, this type of use case has a single ADC that can use a multiplexer to switch between several different inputs as shown in
Figure 3-1.

As seen above, the multiplexer can add much needed functionality to this application by expanding the amount of inputs the ADC can sample, however, the multiplexer also needs to be able to handle a potential common mode fault that can be introduced from the surrounding environment such as shown in Figure 3-2.

The high common mode fault can shift the operating point of the application and the multiplexer must either have the supply capability to handle this shift, fault protection, or external components added to survive this event as illustrated in Figure 3-3.