As many systems such as automated test and measurement and factory automation become smaller and more cost constrained, one potential for savings can come from shrinking the signal chain. Almost every system contains some type of switching and historically, mechanical relays were used. This implementation can be good from a performance perspective, but comes with a significant tradeoff on size, cost, and reliability. In more modern designs, Photorelays are often used instead of mechanical relays. These typically take a small hit on signal performance to drastically reduce size and cost while increasing reliability. Many systems can be further optimized by using TI's integrated Flat On-Resistance Multiplexers instead of Photorelays. In this application note, we go over the differences in performance, cost, size, and reliability between these three solutions.

Though these three devices can serve similar purposes, their internal designs are very different. Mechanical relays typically consist of an inductive coil and a physical switch. When current excites the coil, it becomes magnetic and pulls two pieces of metal together. Photorelays have a similar operation except instead of a coil, an LED is used to drive the gate of a MOSFET on/off. A multiplexer integrates multiple drivers and MOSFETs onto the same chip and drives the gate with a constant voltage source. As a result, Multiplexers save PCB space compared to Photorelay and mechanical relays.

Figure 1-1 Equivalent Circuits (from left to right): Mechanical Relay, Photorelay, Multiplexer