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ISO7741-Q1: EMC issue

Part Number: ISO7741-Q1

Hi Experts,

My customer is using ISO7741F for SPI isolaiton. They use MCU to set the EN1 pin Low and High at a certain frequency(low frequency). In this application, customer found the RE test failed at around 180MHz. If trying to fix the EN1 pin constant High, they can pass the  EMC test. Could you please help to explain why switching EN1 pin frequently will induce EMC problem at ~180MHz? Any method can help to improve here? Thank you.

  • Hi Arie,

    Thank you for posting to E2E! At which frequency is the MCU toggling the EN1 pin of the ISO7741F? Are there any additional changes in the circuit or system when this is done?

    Please share a schematic of the circuit and confirm there are no hardware changes between the passing and failing RE tests. If you would like to share the schematic privately, feel free to send me a Request to Connect and Personal Message.

    Thank you!
    Manuel Chavez

  • Hi Arie,

    My apologies for not mentioning this sooner, but below is some context for the questions above and some follow-up questions.

    Since they transmit low power signals across the isolation barrier, radiated emissions from digital isolator devices are generally low, and we don't expect issues meeting many emissions requirements. This is confirmed in our customer's tests by the RE test passing when EN1 pin is a constant HIGH. In the tests where EN1 is toggled LOW and HIGH, emissions might increase due to power dissipation changes by the isolator or the devices that are powering it.

    In addition to the information requested above, please also share the passing and failing emissions measurements plots if possible. If you would not like to share any of this information on this public forum, feel free to reach out to me directly as mentioned.

    Thank you,
    Manuel Chavez

  • Hi Arie, all,

    This conversation has been moved offline, and an update will be posted later if necessary.

    The emissions fail around 180MHz could be a harmonic of the frequency used to switch EN1, which would be confirmed by comparing passing vs. failing emissions data.

    When EN1 is switched from LOW to HIGH, a transient occurs in the device’s Vcc1 pin to power the device’s internal circuits as well as supply any output signals and loads that are HIGH. This repeating transient could be contributing to the emissions failure, and if it is the case, a series resistor or ferrite beads could be added to the power supply net to reduce the magnitude of the transient. Below is a diagram of current flow into ISO7741-Q1 when EN1 is switched HIGH:

    It’s also possible in this case that the MCU is contributing to emissions through the data line controlling EN1. To confirm this, we will evaluate whether holding EN1 LOW causes the circuit to pass emissions tests.

    Thank you,
    Manuel Chavez