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Original question:

ISOW1412: Questions on Schematic and Layout

ISOW1412: EMC requirement for MIL standard

first_name last_name
Prodigy 20 points
Part Number: ISOW1412

Tool/software:

Hello,

In the datasheet on page 37 I read: GND1 and GNDIO must be shorted directly

What is the background for this direkt connection?

I made a test to use common mode choke with capacitors instead of ferrite beads and capacitors. Additional I add 100pF between pin10 and pin11. In this case GND1 and GNDIO is not connected directly as in the datasheet requested. But the EMC results in the range of 25MHz are better as with ferrite beads.

Is there any risk to do it this way?

Best regards,

Manfred

  • 0
    TI__Expert 7585 points

    Hi Manfred,

    Thanks for reaching out. Please find my inputs below:

    The direct connection between GNDIO and GND1 it is recommended for the best performance of the device even in noisy environment where the noise can get coupled to GND2 pin(due to high impedance node with ferrite/CMC) and cause performance issue. 

    CMC can give better performance than Ferrite Beads but either of those should be on Supply negative and should not separate GNDIO and GND1 for the device.  You can refer ISOW1412 EVM Schematics of this connection with ferrite bead.

    Regards
    Varun

  • 0 in reply to Varun Kumar
    Prodigy 20 points

    Hi Varun,

    Thank you for your feedback. Which details of performance will be reduced when I do not connect GNDIO and GND1? Is the maximum possible output current of the DC/DC reduced or the maximum bit rate lower?

    When I measure with a multimeter between GNDIO and GND1 I have about 500k. For me it looks that the DC/DC circuit in the chip is an independent part like an external DC/DC converter. So the performance loss is not understandable to me. Can you explain the performance loss in more detail?

    Best regards,

    Manfred

  • 0 in reply to first_name last_name
    TI__Expert 7585 points

    Hi Manfred,

    The Performance issue can can come in extreme operating conditions like in Noisy Environment in terms of Bus Data Glitches and a higher/lower VISOUT Values.

    This Performance can vary from device to device which is why to eliminate this it is must that GNDIO and GND1 gets shorted on the PCB.

    Regards
    Varun