Choosing Between An Optocoupler And A Digital Isolator

Choosing Between An Optocoupler And A Digital Isolator

Isolation circuits designed to withstand the rigors of industrial environments are the focus of this white paper. Industrial electronic equipment commonly uses galvanic isolators to protect systems and users from potentially hazardous voltages. It is well known that industrial equipment must operate reliably in the harshest environments, where strong electromagnetic fields, surges, fast transients, and high noise floors are the norm. This environment presents challenges for designing reliable isolation circuits that deliver error-free operation over long equipment lifetimes.

Over the last four decades, optocouplers have been the "default" signal isolation device, but recent breakthroughs in silicon isolation technology have spawned smaller, faster, and more reliable and cost-effective solutions that have already begun supplanting optocouplers in many end applications. This white paper discusses industrial isolation issues and ways RF isolation technology can be applied to increase system robustness and performance.

RF Isolator and Optocoupler Basics

The optocoupler is a hybrid assembly having a light-emitting diode (LED) that emits light when forward biased, with brightness being proportional to LED forward current. Emitted light passes through an optically transparent insulating film (or dielectric), striking a photo detector and causing a current flow that biases the output transistor on. When LED forward current no longer flows, light emission ceases, and the output transistor turns off.

Basic Operation of Optocoupler vs. ISOpro Isolator

The basic operation of the ISOpro isolator is analogous to that of the optocoupler, except that an RF carrier is used instead of light. The ISOpro isolator consists of two identical semiconductor dies 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 simple on/off keying (OOK). When VIN is high, the transmitter generates an RF carrier that propagates across the isolation barrier to the receiver. The receiver asserts logic 1 on VOUT when sufficient in-band carrier energy is detected. When VIN is low, the transmitter is disabled, and no carrier is present. The receiver, therefore, detects no in-band carrier energy and drives VOUT low.

The basic operation of the ISOpro isolator is analogous to that of the optocoupler, except that an RF carrier is used instead of light. The ISOpro isolator consists of two identical semiconductor dies 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 simple on/off keying (OOK). When VIN is high, the transmitter generates an RF carrier that propagates across the isolation barrier to the receiver. The receiver asserts logic 1 on VOUT when sufficient in-band carrier energy is detected. When VIN is low, the transmitter is disabled, and no carrier is present. The receiver, therefore, detects no in-band carrier energy and drives VOUT low.

Unlike the optocoupler, each ISOpro channel occupies little die area, making possible costeffective, high-channel-count isolators. Additionally, monolithic semiconductor process technology enables ISOpro technology to be combined with other semiconductor functions and processes to create highly integrated products with built-in isolation, such as isolated analog data converters and communication ISM Band transceiver ICs. These integration advantages enable a broader, more comprehensive ISOpro product roadmap than the optocoupler.

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