So when you’re working on a high-volume product, where pennies matter, and the accountants and managers come knocking at your door, you may not get to use those nice easy black-box ICs. Whereas the cheapest optoisolators are the LTV-817 and EL-817 at just under 10 cents each in 1000-piece quantity. ![]() A quick glance at Digi-Key shows the Si8715 (single-channel 15Mbps) shows up at around $1.07 at 1000-piece quantity, the ISO7220C (dual-channel 25Mbps) at $1.18, and the Si8620BB (dual-channel 150Mbps) at $1.26, with prices going up from there. The capacitive-coupling digital isolators are supposed to be more reliable, and don’t show signs of temperature dependence and aging that are seen in optoisolators. In fact, if I want to send a high-speed digital signal across a voltage isolation barrier, I would probably not use optoisolators, but instead something like the Si8715 or Si8620BB or ISO7220C, which are actually based on capacitive coupling, rather than optoelectronics. Logic signal in, logic signal out, galvanic isolation in between. The easiest way to work with optoisolators is to buy the kind that has built-in input and output signal conditioning, and that way, you can pretend that they’re not optoisolators, and just treat them like a black box. Most optoisolators have an input-output rating anywhere between 500V and 5000V. If you’ve ever used the old transparent glass-packaged 1N914 / 1N4148 diodes and found that your circuit output changes when the fluorescent lights are on, you’ll be thankful optoisolator packaging is opaque.) Current goes in on one side, this causes current to flow on the other side, and there is no galvanic connection between input and output, so you can send a signal across a high-voltage differential. (If the package weren’t opaque and you could see the LED light up, that would also mean that you could shine light from outside onto the phototransistor and turn it on, which is not good, because the only time you want the phototransistor to turn on is when the LED turns on. You don’t see it because the light is inside the package, and it’s designed to shine onto the phototransistor and turn it on. ![]() On one side of the package, you put in current through the LED. These devices - and I’m tempted to be lazy and call them “optos”, but that sounds more like a cereal with Greek yogurt-covered raisins - are essentially just an LED and a phototransistor inside an opaque package. Today’s topic will be showing you some tricks for how to get more performance out of an optoisolator. I started out with circuit design articles, and I’ve missed it. It’s been a few months since I’ve rolled up my sleeves here and dug into some good old circuit design issues.
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