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Original question:

ISOW7841: High EMC emissions at 64, 207 & 316MHz

ISOW7841: Ferrite bead doubt - EMI issue

Marco Bevilacqua
Prodigy 40 points
Part Number: ISOW7841

Hi, I have doubt regarding the ferrite bead marked as optional in your emi mitigation suggestion schematic.

What is their function?Do they reduce emission or perform other functions?Is the CMC the only necessary part in order to reduce emissions?

If I need to reduce EMI only on the isolated side of the ISOW7841 can I mount the CMC only on the isolated side of it?

thanks

  • 0
    TI__Guru 51930 points

    Hi Marco,

    Thank you for reaching out to us, please allow me address your questions.

    Marco Bevilacqua said:
    What is their function?Do they reduce emission or perform other functions?Is the CMC the only necessary part in order to reduce emissions?

    The function of ferrite beads (FB) are similar to CMCs in this application, both offer attenuation for high frequency noise / radiations. The difference lies in the amount of attenuation offered and as you can see in their respective datasheets, the suggested CMCs offer about 10 times higher attenuation compared to the FBs. Hence, CMCs are more effective than the FBs. That said, if one only needs a small improvement in emissions then FBs might also suffice to meet their requirements.

    Since the source of radiations is DC/DC converter switching, we recommend that CMCs are used on power supply lines (VCC/GND) as suggested in my previous post. The I/O lines only see some coupled noise and hence, they are not going to radiate as much as the power supply lines. Since the improvement that can be achieved by using FBs on I/O lines is relatively small, we have marked it as optional.

    The FB offers attenuation only for high frequencies while a resistor offers attenuation for all frequencies. Hence, they both work the same for high frequency noise but the resistor is going to slow down your data rise/fall times as well. But if the datarate requirement is not very high and you are okay with slower rise/fall times then you could use the resistors instead of FBs and save on cost.

    Marco Bevilacqua said:
    If I need to reduce EMI only on the isolated side of the ISOW7841 can I mount the CMC only on the isolated side of it?

    Please note that the radiations are common-mode noise, i.e., they are present between GND1 and GND2. When the device or end-equipment is tested for radiated emissions, the emissions measurement equipment is going to pick-up this common-mode noise between GND1 and GND2. Using CMC on only one side of device is not going to suppress / block common-mode emissions, you might only see half the improvement. To see good emissions improvement, we recommend that CMCs are used as recommended in my previous post.

    I hope these points answer your questions, thanks.


    Regards,
    Koteshwar Rao