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Today I have learned that a device called "isolator" exists (eg: SI8712CC-B-IS). Its function is very similar to the optocoupler's. On the good side, it is much much faster. While it is very hard to find an optocoupler with propagation delay less than 1\$\mu\$s, almost all isolators have less than 100ns delay. On the other hand, they are slightly expensive though.

If such a device exists, why do we keep using optocouplers? Is the price the main factor of choice? Can we always use an isolator instead of an optocoupler? How do they differ in general?

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    \$\begingroup\$ If you don't need the performance of the isolator in your application, why would you pay the extra cost? \$\endgroup\$ – Dave Tweed Nov 10 '14 at 13:32
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What you show and call a "isolator" is a optocoupler with digital output. The term isolator refers to various means to isolate a signal such that there is no common connection. A opto-coupler or opto-isolator (different words for the same thing) is only one method.

Opto-isolators can be broken into a few broad catagories. The most common is a LED shining on a phototransistor. As you found, these usually have a few µs to to 10s of µs propagation delay, particularly when turning off.

Another type uses a photodiode as the detector, with active circuitry to amplify the relatively small signal from the photodiode into a digital signal. These can have less than 100 ns propagation delay. Note however that they require separate power on the receiving side, and generally cost more.

As should be no surprise when you find two different things that address the same problem, each has its advantages and disadvantages. Everything is a tradeoff.

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An optocoupler is an isolator, but not all isolators are optocouplers. As you've already indicated, when the specs of an optocoupler indicate that it won't do what you need, move on. Another special case is when you're trying to isolate a bidirectional bus. In that case, some manufacturers have specialized IC's just perfect for those jobs.

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Digital Isolators are a more modern alternative to optocouplers, offering improved performance with lower power dissipation. They’re also much easier to implement in a design since the inputs and outputs use standard digital logic. Standard digital isolators have different numbers of input and output channel combinations and can operate up to 150Mbps. Specialized products also exist for specific serial interfaces, such as USB and I2C.

You can see the entire digital isolator product line available from Analog Devices at our web site: http://www.analog.com/en/interface/digital-isolators/products/index.html

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What you provided is actually different. Yes, it is functionally equivalent to an optocoupler (and it is designed to actually be a pin to pin replacement), but the part you provided does not use light as the means of making the output go low or high. Instead, an "isolator" uses electrical means. In this case, it is probably electrically coupled via a coil (think relay or transformer). The other way is to do it with capacitive coupling. You see this a lot in chips that are used for isolating rs232 or usb (Analog Devices and Linear Technology, which is now Analog Devices, make these chips).

Why do we keep using optocouplers? Because the technology is trusted and has been used for a while. It is the same reason why we still use those old rs232 to USB converter chips. Yes, they have newer and improved versions, but you still use the older ones because the designs are documented and well understood.

In reality, if you do not need the features of an isolator, then why spend the extra dollar? It may not seem like a big deal if you make small qtys, but in huge qty, a dollar more is a lot.

Also, even if they do advertise that they can be a direct replacement to optocouplers, that is not 100% correct. With isolators (remember, they are connected via an electrical mean, not optical), you sometimes need to make sure the input is not left floating. This means you need to use a pull down (or up) or even a schmitt trigger to ensure the input is always at a low or high.

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