Optocouplers are undoubtedly effective, but alternative isolation technologies may be a better choice for your application.

Related Information

 

Achieving galvanic isolation is fairly easy when we’re dealing with AC signals. Smoothly varying voltages and currents are so eager to move beyond the confines of conductive interconnections that they naturally allow for three types of isolated transmission: they generate magnetic fields that can be coupled via transformer coils, electric fields that can be coupled via capacitor plates, and electromagnetic radiation that can be transferred via antennas.

The trouble is, we often need to electrically isolate signals that do not have constantly varying waveforms. Typically these are digital signals that might remain at logic low or logic high for an extended period of time. A standard solution to this problem is to use light, which has the convenient ability to provide low-frequency or even steady-state communication between transmitter and receiver without establishing a direct electrical connection.

 

What Is an Optocoupler?

An optocoupler is a straightforward device consisting of an LED, an insulating barrier, and a photosensitive semiconductor device (i.e., a photodiode or a phototransistor).

 

An X-ray image of an optocoupler, taken from a Silicon Labs document entitled “CMOS Digital Isolators Supersede Optocouplers in Industrial Applications.”

 

Drawbacks of the Optocoupler in Practice

Optocouplers are adequate in many systems, but they do have significant disadvantages:

  • In the context of modern low-power electronics, an LED’s current requirements are quite high, and an optocoupler’s LED must be on whenever the input signal is logic high. In some systems, this inefficient use of power is simply unacceptable.
  • Optocouplers have reliability issues. Perhaps the primary concern is failure of the LED, but the abstract for this research paper mentions, among other things, interface contamination and thermo-mechanical stress associated with moisture absorption.
  • The propagation delays involved in optocoupler operation impose irksome data-rate restrictions. I don’t know if it’s fair to say that optocouplers are inherently “slow,” but they are indeed slow compared to alternative devices.
  • An optocoupler’s input and output are not typical logic gates and, consequently, the interface between the optocoupler and the rest of the system may require components or design effort that can be eliminated when digital isolators are used.
  • Optocoupler manufacturing techniques make it difficult to integrate multiple channels into the same package.

 

The RF Approach

We usually associate radio-frequency communication with long-distance systems, but there is no reason why you can’t use it for (very) short-range applications such as digital-signal isolation. The idea here is to modulate a carrier according to the digital input signal, transmit the modulated signal across an isolation barrier, and then demodulate the signal.

 

Diagram taken from the datasheet for the Si864x digital isolator family, from Silicon Labs.

 

The use of on-off keying reduces power consumption because the device doesn’t transmit an RF signal when the input is logic low.

 

Diagram taken from the datasheet for the Si864x digital isolator family, from Silicon Labs.
 

The part description on page one of the Si864x datasheet indicates that these devices are better than optocouplers in just about every way. The only possible disadvantages that I can think of are related to increased generation of or susceptibility to electromagnetic interference. However, this document (page 10) claims that these isolators are designed in such a way as to ensure low-EMI operation and high resistance to RF interference.

Before we move on, it’s interesting to note that these RF isolators provide major improvements over optocouplers despite the fact that the fundamental difference between the two technologies is simply wavelength: an optocoupler allows a digital signal to enable and disable a source of shorter-wavelength electromagnetic radiation (i.e., light), and the SiLabs device allows a digital signal to enable and disable a source of longer-wavelength electromagnetic radiation (i.e., an RF signal).

 

Magnetic Isolation

Analog Devices uses magnetic coupling to overcome the limitations of optocouplers. Their iCoupler technology combines tiny transformers with control circuitry in such a way that low-frequency digital signals can be transferred, despite the fact that you need a changing magnetic field to induce current. The following diagram provides a good summary of their technique:

 

Diagram taken from this article published by Analog Devices.

 

The output signal follows the input signal by keeping track of transitions. The logic on the input side encodes rising and falling edges and transfers them across the barrier via magnetic coupling, and the output side decodes these signals into normal logic transitions. As with RF isolators, iCoupler technology surpasses optocouplers in terms of size, operational frequency, power consumption, etc. Honestly, after reading the manufacturer literature on alternative isolation techniques you almost pity the poor engineers who had to subsist for so long on light-based isolation.

You could probably spend the better part of a day trying to thoroughly analyze the pros and cons of magnetic and RF isolators, but I don’t think that’s a good use of time because in most applications both will be highly effective. I will mention the following two points of comparison, though:

  • RF isolators (or at least those using on-off keying) are somewhat worse in terms of power consumption because an RF signal is being transmitted whenever the input signal is logic high.
  • iCoupler devices are more sensitive to magnetic interference (this is not too surprising since they use transient magnetic fields to indicate logic transitions). This is an important consideration if you need a device that can be used near large motors.

 

Conclusion

If optocouplers are completely satisfactory for your application, by all means, keep using them. I’m the last person who will recommend that you transition to a new product simply because it is new. However, it’s important to be aware of the alternatives because RF- and magnetic-field-based isolation technologies really do offer significant benefits.

 

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