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I need to isolate the SPI communication between two chips.

The bottleneck of the communication is on the slave, which apparently can handle up to 2MHz SPI clock frequency.

My idea is to do this isolation using optocouplers. On this first attempt, I used the 4n35. It was just not working, so I looked on the datasheet and I saw the chip takes 10 us to rise and 10 us to lower the signal (on the worst case). This makes its use pointless.

Can someone suggest me an optocoupler that can:

  • be fast enought for this purpose (2MHz, but 1MHz is also acceptable)
  • be easy to buy and not expensive

I searched over the internet for an optocoupler that met those requirements, but I couldn't find one that was most recommendable.

I'm open to suggestions on how to isolate the communication in a different way, if it's practical.

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    \$\begingroup\$ You tried to use the 4N35 then look at the datasheet!? \$\endgroup\$
    – Olin Lathrop
    Sep 28, 2012 at 22:50
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    \$\begingroup\$ @Olin - OK, that's a bit late, but at least he has heard about datasheets, and ultimately did look at them. That's more than you can say about some others :-) \$\endgroup\$
    – stevenvh
    Sep 29, 2012 at 18:10
  • \$\begingroup\$ @OlinLathrop I don't really let it clear. I had read the datasheet partially but the information about the timings went unnoticed because I had no knowledge about such restriction. Many responses suggest several different options. I'll try to get some of those chips before accepting an answer. Thank you! \$\endgroup\$
    – borges
    Sep 29, 2012 at 22:04

4 Answers 4

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Borges, there are isolated devices made specifically for digital isolation and even for SPI busses. Texas Instruments and Analog Devices both make these. It's capacitive or magnetic isolation instead of optical isolation. Here is a link to popular device:

ISO7241C @ TI.com

It's also available on digikey:

ISO7241CDW @ Digikey

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For high speeds other ways of coupling, like capacitive or magnetic, are often used instead of optical. There are 15 Mbps optocouplers, but most 4-channel types will not be 3+1.

The Si8441 from Silicon Labs is the cheapest 3+1 channel, > 1 Mbps isolator at Digikey.

enter image description here

The Si8441 can handle bit rates up to 150 Mbps. Alternatively you may use a 2+1 magnetic isolator for SCK, MOSI and MISO, and use a slower optocoupler for SS, but that solution will only be marginally cheaper.

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There are many different opto-couplers. Faster ones cost more money. That's the way it works. You can go to a reasonable distributor site like Mouser or DigiKey and look for opto-couplers, sorting or selecting by speed.

The 6N137 or one of its variants can do what you want, but again, you should look around to make the tradeoffs that matter to you.

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I've used the ADUM14xx series from analog devices for SPI isolation. They aren't strictly speaking "opto" isolators, they are magnetic, but provide similar levels of isolation. They are available in a variety of speeds as well, with the faster ones being more expensive.

http://www.digikey.com/catalog/en/partgroup/adum1400-01-02-series/15413

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