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ISOUSB111: How to connect USB shell (shield) with isolated USB interface?

Samuel Compton
Prodigy 60 points
Part Number: ISOUSB111

Hi,

We have a custom board design that uses ISOUSB111 with a self-powered USB device to provide ground loop isolation between the USB host and the device. The device is not earthed; it is powered from a galvanically isolated DC power supply, that has only two prongs with no earth connection. We did not see any guidance in the ISOUSB111 datasheet on how to connect the USB shell. Anecdotally, the noise levels measured within the device go down with a capacitive connection between the USB shell the device's internal signal ground. Can anyone confirm if there is official guidance on how to connect the ISOUSB111 shield for self powered USB devices that use ISOUSB111 to isolate a USB data connection?

Thanks

  • +1
    TI__Guru 53025 points

    Hi Samuel,

    Thank you for reaching out.

    Most commonly, the shell is directly connected to the Chassis of your product especially if you product has a metal chassis. If connection to Chassis doesn't exist then the shell connection is not critical. You can probably have a high impedance connection of shell to system ground through an RC in parallel. Typical value for resistance is 1MΩ and cap can be between 100pF to 1nF. Thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 60 points

    Hi Koteshwar,

    Our application features a plastic enclosure, so there is no relevant chassis ground. Our concern is that the device is a mixed-signal system with a sensitive analog to digital acquisition channel. In most cases, the shell connection doesn't seem to make any difference to the acquisition channel noise floor. However in certain instances, providing a capacitive connection from the isolated USB data shell to the board signal ground improves the noise floor. Based on your guidance, we've tried a 4.75M resistor in series with a 100 nF capacitor and we're achieving consistent results.