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ISO7721: EMI over limit ISO7721

Prodigy 20 points

Replies: 3

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Part Number: ISO7721

During module-level EMC test, we found one board assembly with iso7721 failed at two frequency points.
To exclude the effect on the whole test, we tested the power module(AC-DC module) first, and it passed.
But when the suspect board is powered, and no input/output was connected to the board. Its EMI energy exceeded limit(EN55032 Class A) more than 10 dB in 414MHz and 830MHz.
I checked the iso7721 datasheet, Iguess the modulation scheme may cause the EMI over range.
please help investigate this issue.

  • Hi JKW,

    Sorry to hear about the issue. Since ISO7721 is only a digital isolator (with no power isolation in-built) the signal power levels involved are very low and we do not expect it to cause any radiated emissions issue in meeting EN 55032 Class A. We have tested it ourselves and we have many customers testing ISO7721 for EN 55032 Class B and they do not usually face any issues.

    Please see below a tech note that presents ISO7741 test results where the device is able to comfortably meet CISPR 32 Class B which is same as EN 55032 Class B.

    What is EMC? 4 questions about EMI, radiated emissions, ESD and EFT in isolated systems

    To be able to assist you better with the issue, please do share us more information about your application and the issue. Please share below information to help us debug, thanks.

    1. Let us know more details about your application and where exactly is ISO7721 used in your application.
    2. Please share schematic showing ISO7721 and devices connected to it on either side of device.
    3. Please share emissions test results plot along with your product test configuration.
    4. If possible, please do also share the test setup diagram showing various connections to your product in the emissions lab.

    Let me know if you have any other questions, thank you.

    Koteshwar Rao

  • In reply to Koteshwar Rao:

    Thanks for quick response,Koteshwar .

    Based on your information, I may switch to my focus onto another part on my board to find the root cause.

    But I would like to share some basic information about my issue. During the test, the board was no input/output connection only powered by AC-DC module.

     To exclude the effect on the whole test, we tested the AC-DC module first, and it passed. 

    it is the simple test setup of my test. the board assembled iso7721 has mainly two function-RS485 communication isolation(iso7721) and chirp signal amplication.

    below is the EMI test result with two freq point over range.

  • In reply to jkw:

    Hi JKW,

    Thank you for sharing additional information and for the emissions results plot. The spikes observed at frequencies 414MHz and 839MHz do not seem to be related to ISO7721. An easy and quick way to confirm this it to remove the isolator ISO7721 and replace it with shorts and then test for emissions. This is assuming the product can operate without isolation, only for testing.

    Once you complete the suggested test, please do let us know the results.

    I will go ahead and mark this E2E post closed, you will still be able to reply to this E2E thread once you have results. Thanks.

    Koteshwar Rao