multiplexer

A multiplexer is a switching device that selects one of several input signals and routes it to a single output line. The selection is controlled by address inputs, typically binary signals that tell the multiplexer which input channel to connect at any given moment. In essence, it acts as a fast, electronic rotary switch, cycling through inputs under digital control rather than mechanical intervention.

Multiplexers come in various sizes defined by their number of inputs. A 2-to-1 multiplexer handles two inputs and chooses between them; a 4-to-1 handles four; an 8-to-1 handles eight; and so on. The address lines required scale logarithmically, so a 4-to-1 multiplexer needs 2 address bits to specify one of four options, while an 8-to-1 needs 3 bits. Common integrated circuits include the 74LS157 (quad 2-to-1) and the 74LS151 (8-to-1), though modern designs often use programmable logic or microcontrollers for similar functions.

In analog signal conditioning, multiplexers route voltage or current signals from multiple sensors into a single data acquisition channel, allowing one analog-to-digital converter to sample many inputs sequentially. In digital systems, they select between data buses, control lines, or memory addresses. The speed of switching depends on the circuit technology and load conditions; propagation delays through commercial multiplexers typically range from nanoseconds (modern logic families) to microseconds (older components).

Common problems and design trade-offs

Signal crosstalk occurs when inadequate isolation between channels allows one input to influence another, particularly problematic in high-impedance analog circuits. On-resistance in analog multiplexers varies with supply voltage and temperature, introducing gain errors in precision measurements. Switching transients and glitches appear during transitions if address lines do not change synchronously, momentarily connecting incorrect input combinations. In audio and video work, multiplexing can introduce audible clicks or visible artifacts if switching is not carefully timed relative to data flow.

The term comes from telecommunications, where the concept of combining multiple signals over shared infrastructure originated. Today multiplexers remain essential in test equipment, data loggers, oscilloscopes, and control systems wherever multiple analog or digital inputs must feed into limited downstream processing. Understanding input impedance, propagation delay, and on-resistance is vital when selecting or designing multiplexer circuits for low-level signal work.