Optocouplers are ICs used in analog and digital signal isolation applications. They operate on the principle of emission from an LED light source transmitted to a phototransistor through a dielectric insulating material of air, epoxy or mold compound. You can see in Table 3 that these materials have the lowest dielectric strength, and therefore require more physical separation to achieve higher levels of isolation. TI’s portfolio does not currently include optically isolated products, but does include Opto-emulators that are pin-to-pin solutions emulating the behavior of optocouplers, while using TI’s SiO₂ isolation technology.

Although light-emitting photons are the fastest known vehicles for electromagnetic energy transfer, LED switching speeds, forward bias requirements and drive circuitry limit their signal rate to less than a few megabits per second. Furthermore, optical transmission efficiency does not transfer enough power for effective use as a power supply, so typically optocouplers are used only for transmitting data.

Combining functions such as LED drive circuitry and amplifiers inside an optocoupler package helps achieve higher data rates, but at a higher cost. The input-to-output current transfer ratio is a measure of an optocoupler’s gain and will vary and degrade over time. Designers sometimes compensate for this aging effect by overspecifying the required bias current. Thus, optocouplers tend to have higher power consumption compared to capacitive or magnetic isolators.