Analog multiplexers are usually bidirectional. Each channel has very similar characteristics in both directions so that the multiplexer can be used as multiplexer or demultiplexer. Multiplexing are commonly used in various applications such as asynchronous data acquisition of multi-channel signals, status read or monitoring of polling system. For multiplexing application, the unselected (OFF) channels are not connected to the common terminal, which is naturally isolated from the signal conditioning circuit after the mux. However, for demultiplexing application, output channels are always connected to the signal conditioning circuit after the mux. For the OFF channels, unwanted voltage can be detected if the output channels are not properly handled.

This application note firstly provides a demultiplexing application example in solar system based on CD405xB CMOS Single 8-Channel Analog Multiplexer or Demultiplexer With Logic-Level Conversion, data sheet to show the issue of unwanted voltage at OFF channels. Then, equivalent models for the OFF channel are built to help analysis the root cause. Finally, design and measurement tips are discussed to help engineers use demultiplexer properly.