A Multiplexer, or in short MUX, is an integrated circuit that selects one of several analog or digital input signals and forwards the selected input into a single line. A multiplexer makes it possible for several signals to share one device or resource, for example, one micro controller input channel or one communication line, instead of having one individual channel per input signal.
Most common usage for multiplexers is to extend the number of available input channels of a micro controller (at the cost of some required digital outputs for addressing the selected input).
The counterpart of a multiplexer, a device taking a single input signal and selecting one of many output-lines, is a demultiplexer, or in short DEMUX.
Analog versus digital multiplexers
Be aware of the differences between an analog multiplexer (for example the 74HC4067) and a digital multiplexer (for example the 74HC151).
An analog multiplexer can handle both analog as well as digital (on/off) signals, whereas a digital multiplexer can handle digital signals only.
Analog multiplexers transfer the analog signal, and as such they may change the signal somewhat because of frequency dependence and because of internal resistance. Digital multiplexers don’t show this behavior as they ‘renew’ the signal. They do however have a propagation delay.
You need to indicate the multiplexer which input is selected. This is done by connecting the selection inputs of the multiplexer circuit to digital outputs of a micro controller. The channel is then selected by switching the outputs high or low based on a binary numbering system. (e.g. 000 -> input 0; 010 -> input 2; 111 -> input 7).
Most important specifications
- Analog or digital multiplexer?
- Number of inputs required. Remark: more inputs require also more available digital outputs for signal selection: for 8 inputs you need 3 digital outputs, for 16 inputs you need 4.
- Supply voltage (V)
- Maximum input voltage (V)
- Output voltage (V)
- For analog multiplexers only: the on resistance (the resistance added when an input is selected) (Ω)