Optocouplers should be well-known to most engineers. Texas Instruments ISO7420, ISO7420M and ISO7421 are a nice alternative for digital signals.

They are optimized for digital signals with a speed of up to 1Mbps. Understanding the parts function is best accomplished by looking at a figure showing the pinout – figure one, taken straight from the datasheet, should work fine.

The two versions of the part differ in the arrangement of the isolated channels

The most important difference to a traditional optocoupler is the strictly digital behavior of the parts. Instead of transmitting analog signals, the input is “disciplined” with a comparator, and the output is either high or low.

Texas Instruments shows itself “friendly” by providing multiple arrangements across the isolated channel. While this might look like a technicality at first, understand that it is beneficial – the area marked with “isolation” can be used to house a cutout separating the two sides. The SOIC housing used is large enough to make this technically possible even for less than sophisticated PCB design houses.

Of course, two power supplies are needed to maintain isolation. Input voltages can be either 3V3 or 5V0, making the ISO7420 family ideally suited to switching between various voltage levels found in a circuit.

Published: March 12, 2021