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Few Question on Digital isolators

Hello,

My customer has a few questions:

  1. A DigiKey search shows that the TI digital isolators use the capacitive coupling technology.  There are others (e.g. magnetic).  Do you know the advantages and disadvantages of capacitive coupling versus magnetic coupling, and why I would chose the TI capacitively-coupled digital isolators as opposed to another technology?
  2. Based on my understanding of how this works, there is an internally-generated, high-frequency signal that is modulated based on the input logic level (even the edge-based devices have an internal clock).  Is there any information within the data sheets or other documentation that identifies what those frequencies are?  In our application, we will be dealing with other RF interfaces that need to avoid interference from on-board devices that could cause problems.  If the digital isolation devices operate in those frequency bands, we may not be able to use them.

Regards,
David

  •  

    Hi David,

    Thanks for using the Digital Isolators Forum!

    1. Yes that is correct, TI uses capacitive digital isolator technology. I'll paste a section of a recent analog applications journal article that sums up this technology's advantages:

    Recent advances in capacitive digital isolators place them at the forefront of technology. These new isolators offer higher isolation performance, long-term reliability, increased channel integration, higher data rates and precision timing performance, better quality package mold compound, wider packages (14.5-mm creepage/clearance), and CMTI exceeding 100 kV/µs. Combining these features enables new applications, reduces system cost, and allows end-equipment manufacturers to push the performance envelope of their solutions.

    Texas Instruments offers the ISO78xx family of reinforced digital isolators and the ISO585x and ISO545x families of reinforced isolated IGBT gate drivers. These isolators offer features and capabilities that can solve difficult isolation problems. These isolators have a working voltage of up to 1500 VRMS, are rated for 40 years, have surge voltage capability of 12.8 kV, and withstand a temporary overvoltage of 5700 VRMS. They offer high data rates of up to 100 Mbps with low skews and part-to-part variations and CMTI exceeding 100 kV/us. They also use material group I mold compound and are available in industry-leading wide packages.

    2. Correct, in the on-off keying (OOK) based architecture there is a high frequency oscillator. (Here’s a link to the Digital Isolator Design Guide which describes OOK, edge based architectures, and some other useful terms more in depth). If you look in the conceptual block diagram, you’ll also note that this is spread spectrum to keep emissions low.

    We have done internal testing on our parts and have found that the radiated emissions are very low, especially compared to some of our competition, but depending on your application this could potentially still be an issue. We’ve found that many of these issues can be fixed after the team gives a schematic and layout review, which we would be more than happy to do for you. But in general our isolators are very quiet. For example, ISO7741 passes CISPR22 with a good deal of margin. Perhaps you could share a block diagram and some more information about your specific application and we can give you a better idea of whether or not these high frequencies should be of concern.

    Please let me know if I’ve answered your question or if you have any additional questions.

    Best regards,

    Dan

  • Great Response. Thank you.
  • Dan,

    Is there frequency range you can provide for the spread-spectrum block so we can understand the general frequencies that are used?

    Regards,
    David

  • In reply to David Cuevas:

    David,

    We generally don't disclose the frequency of the spread-spectrum block on the public forum. I'll contact you offline and we can continue the discussion there.

    Best regards,
    Dan

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