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.
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One of the largest benefits to an integrated multiplexer switching design is size. For example, in a 16-channel system, 16 SPST (1:1) mechanical relays would be needed, accounting for >450mm2 board area. This is before including any relay driver ICs. Compare this to a Photorelay design and the user can see that the size requirements reduce dramatically. But Photorelays can not be driven directly through GPIO and need an LED driver to control the state of each switch. This adds to the design size and increases the system complexity, as well as power requirements. TI’s Multiplexer design such as the TMUX821x and TMUX7612 do not require any additional ICs. Each individual switch can be controlled by standard digital pins with 1.8V Logic. As a result, TI’s switch design is even smaller than a comparable Photorelay design.
As design size becomes a larger concern, routing and layout become a major consideration. The table below compares the actual design size of these three options with routing. You can see while the package density of the TMUX7612 and Photorelay are similar, when routing and layout is considered the space savings is more significant. And the TMUXS7614D saves significantly more area on top of this by integrating the passive components and optimizing the pinout.
TMUXS7614D |
TMUX7612 TMUX8212 |
Photorelay | |
---|---|---|---|
Total Package Area | 2.5mm2/ch | 4.0mm2/ch | 4.2mm2/ch |
Total PCB Area | 3.18mm2/ch | 9.13mm2/ch | 11.22mm2/ch |
Area Savings | 129mm2 (70% reduction) | 33.5mm2 (20% reduction) | - |