Digital Isolation solutions

In digital isolation we merely want to convey on and off information across a galvanically isolated barrier. Frequently there needs to be a transfer of DC power as well. I covered transfer of low wattage DC power in “Low Power Isolated Power Supplies”. Just to complete the trifecta of power, digital and analog isolation, I wrote a sequence of three blogs “Analog Isolation Techniques”: Analog Isolation Techniques, Part 1, Analog Isolation Techniques, Part 2 and Analog Isolation Techniques, Part 3.

Used to be that the only way you could get isolation in a digital system was to use optoelectronics or magnetics, especially if you include relays as part of the magnetic realm. Truth be told, optoelectronics needs an analog approach. The transfer of the signal is dependent on LED current and transfer ratio as well as a pullup resistor at the output. Optocouplers also age over time and suffer from thermal variations. Another issue may prove to be “dark current”. These are discussed in the Renesas document “Explanation of Photocoupler / Optocoupler Specifications”. Optoelectronics also haven’t benefitted much from the size dividend that has been derived from the move to surface mount technology.

Modifications have been made to existing technology and there are optically based devices that improve on the basic technology. Devices such as the 6N137 have been around for years where you only have to worry about the current limiting resistor to the LED at the expense of requiring a 3.3 or 5V supply at the output. My impression is that the leading proponent of high end optical isolation is Broadcom (via Avago via Agilent via Hewlett Packard). Start your search here (there are non-optical solution on this page as well; we will get to the alternatives in a moment).

There are both magnetic and capacitive methods of isolation, and sometimes it is not even obvious which technique is being employed.

Some of these devices allow for multiple channels, often back to back allowing bidirectional communication across the barrier. Texas Instruments has a broad offering that can overwhelm you. Just as overwhelming is the offering from Analog Devices. Smaller offerings are available from Maxim and NVE Corporation with their IsoLoop range. Silicon Labs has a wide offering although I find the selection table a little difficult to follow. Linear Technologies (technically Analog Devices also) also do a range of isolated transceivers.

With the ability for multiple channels several manufacturers have created products for specific market segments. There’s the granddaddy of the parts, Maxim’s MAX1490 which provides a one –chip isolated logic to RS485 interface (albeit with optoelectronics). The family has grown somewhat here. Maxim also produces a range of RS232 isolators.

Once the idea of integrated isolated communications devices took hold, many other manufacturers started making devices that not only address RS485 and RS232 but also SPI and I2C. Linear Technologies make a range, and I find several (like the LTM2887) especially interesting because, not only do they provide a power supply across the barrier, the power it is available for circuitry use beyond the isolator. Analog Devices support USB and SPI isolation whilst TI does RS485, CAN, and I2C.

And now we come to some interesting and unique parts. Silicon Labs do the Si838x family which takes 8 x 24VDC inputs and isolates and massages them (including debouncing) into a microcomputer compatible signals (parallel or SPI).

ST provides the ISO8200 device which is an octal high side output driver capable of sourcing 700mA with isolation.

The LTM9100 from Linear Technologies has a single isolated high side load driver switch for high voltages (1KV). It is possible to adjust the turn on ramp and current limit. It also includes feedback on the status of the output signal that can be read over the controlling I2C bus. It should be also be included in the list of drivers I covered in my ancient blog “Using MCUs: Intelligent Digital Power Outputs

At this point I have a conundrum. Is a device like an optical triac to be considered a digital device? I would argue that it is, but perhaps there is sufficient material to be gathered into a blog on driving AC loads, should there be any demand for said blog (hint, hint).

Let me finish with a real weirdo. I have played with this device for well over 30 years and it hasn’t changed at all. Broadcom still makes the HCPL3700. It has a Zener bridge at the input, so it can handle AC or DC along with a programmable sense voltage and they have thrown in hysteresis as well. I have managed to find uses for it in several projects especially where space is tight, but there have been many times when I have thought it was a solution looking for a problem (see my blog “A solution looking for a problem”).

There must be hundreds of devices that fit into the rather broad frame of reference of this blog. No doubt, I have missed and confused manufacturers and products. Please update me in the comment area below.

1 comment on “Digital Isolation solutions

  1. AubreyKagan
    November 12, 2018

     “Silicon Labs has a wide offering although I find the selection table a little difficult to follow”

    I originally wrote this blog around 15 months ago.I don't know if Silicon Labs has improved their product matrix since then or more likely, I was having a bad day. Either way this statement no longer holds true. In fact not only are all the details presented in the table, there is filter button marked “Display” on the upper right hand side which will allow you to zoom in on the features that are important.


    My apologies.

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