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ISOW7842: More Radiated Emissions Observed

JANAKIRAM NAGARURU
Prodigy 35 points
Part Number: ISOW7842
Other Parts Discussed in Thread: ISO7842, ISOW7741

Hi TI's,

We are using the ISOW7842FDWE part in our design for RS485 Signal Isolation purpose. while performing Radiated Emission test we are observing more emissions from this IC. 

While performing Whole device: 

In Horizontal Axis: 

    

In Vertical Axis Emissions: 

Schematic Image: 

When observing Emissions using snipping probe Emissions observed :

When IC Mounted Emissions Observed:

When IC Not Mounted Emissions Observed:

Please let me know how i can reduce the emissions from this device. 

Regards,

Janakiram 

  • 0
    TI__Mastermind 49545 points

    Hi Janakiram,

    Sorry to hear about the issue and thanks for reaching out and sharing the results as well as schematic.

    I see that you are trying to test the product for CISPR 32 Class B limits and it is currently failing these limits by about 15dB. The emissions can be reduced significantly when common-mode chokes (CMC) are used on both input and output supplies as suggested in the below E2E post and described in the following application note.

    https://e2e.ti.com/support/isolation-group/isolation/f/isolation-forum/806938/isow7841evm-isow7841evm

    https://www.ti.com/lit/slla368

    Since this requires redesigning of the PCBs for accommodating CMCs, I would recommend you to instead consider using our second generation ISOW device ISOW7741 in place of ISO7842. ISOW7741 has significantly better radiated emissions performance compared to ISOW7842 without requirement any CMCs and also offers many other advantages. Please see below E2E link for radiated emissions results of ISOW7741 from the EVM. It also provides comparison of ISOW7741 against ISOW7842 on other performance parameters.

    https://e2e.ti.com/support/isolation-group/isolation/f/isolation-forum/985520/isow7741-emc-inquiry

    I also see that you only have 3 channels used from ISOW7842 as seen in your schematic, hence, I believe ISOW7741 should also meet your number of channels requirements for your application. I am also sharing below additional best practice guidelines document to make sure you achieve the lowest emissions results.

    https://www.ti.com/lit/slla561

    Let me know if you have any questions for any of the information shared above. Thanks.


    Regards,
    Koteshwar Rao