Tech 101 From Symmetry Electronics: What is an isolator vs optocoupler?

Tyler Wojciechowicz in Blogs on October 26, 2017

About Tyler Wojciechowicz

Tyler Wojciechowicz is an Applications Engineer at Symmetry Electronics. He has his Bachelor’s in Electrical Engineering from Milwaukee School of Engineering and eight years of hands-on experience as an Electrical Engineer. He specializes in IoT applications, microcontrollers, embedded programming, timing, sensors, and power management. In his current role, he works closely with field sales to advise on optimal part substitutions, product suggestions, and sales tools. He is adept in developing instruction manuals, reference designs, tutorials, product comparison matrices, and marketing campaigns for reputable suppliers across multiple markets.

An optocoupler also called opto-isolator, photocoupler, or optical isolator is a component that transfers electrical signals between two isolated circuits by using light. A digital CMOS isolator is a component that transfers electrical signals between two isolated circuits by using a high-frequency carrier. Both optocouplers and digital isolators are used to prevent high voltages from affecting the system receiving a signal.

Figure 1a

Figure 1b

Images from Silcon Labs: https://www.silabs.com/whitepapers/isolator-vs-optocoupler-technology

Figure 1a displays the basic form of an optocoupler. The signal input can be considered as the transmit unit (LED) and the signal output can be considered as the receive unit (photo detector or some other type of photosensitive semiconductor element). Current flowing through the LED causes emitted light to pass through the dielectric where it strikes the photo detector causing current to flow that biases the output transistor on. Both the transmit and receive units are contained within a single sealed package with an insulating film or dielectric between the input and output.

Similarly, Figure 1b displays the basic form of a digital isolator. Two identical semiconductor dies are connected together within a standard IC package forming an RF transmitter and receiver separated by a differential capacitive isolation barrier. Data is transferred from input to output using simply on/off keying (OOK). When Vin is high, the transmitter generates an RF carrier that propagates across the isolation barrier to the receiver.

Optocouplers have been the unchallenged signal isolation solution for more than four decades, but digital isolators are becoming ever more popular due to their superior performance and reliability. Recent breakthroughs in silicon isolation technology allow these improvements to be possible. CMOS digital isolators give designers the ability to create lower cost, smaller size, higher performance, lower power, and more reliable isolated circuits than competing optocoupler solutions.

Advantages of SiLabs CMOS digital isolators against competing optocouplers include:

Well defined, key operating parameters that exhibit little change over voltage, temperature and device age
- Isolators vulnerable to significant changes due to temperature, current and/or device age force the designer to give up design margin, reducing system performance. This is especially true in isolated high-speed data systems that rely on tight timing relationships.
High reliability
- Isolator lifetimes of 20+ years are now required by applications, such as HEV/EV, solar and wind energy systems, and typically operate in elevated temperature environments over 120°C.
Minimal electromagnetic interference (EMI) and high external electrical and magnetic field immunity
- EMI is a source of potential data corruption, especially in medical applications that acquire low-amplitude signals, such as electrocardiographs (ECG). External magnetic and electric fields can degrade system performance, so the ability of an isolation device to reject external field interference is critical.
Ease of use
- The best isolation devices are well documented, predictable, reliable and intuitively easy to apply; these points are covered in detail in the paragraphs below.

Source: https://www.silabs.com/whitepapers/isolator-vs-optocoupler-technology

Applications include: