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ISO1430: Capacitances of A and B ports

Thomas Lipps
Prodigy 30 points
Part Number: ISO1430
Other Parts Discussed in Thread: PROFIBUS,

I had implemented the ISO1430 in our bus equipment expecting to meet the ProfiBus requirements regarding the very low connection capacitance of about 10pF to 25pF to the bus. But by bus driver circuit has failed the ProfiBus Test Scenario where the difference of over- and undershot within a test pulse periode has to be less than 10% of the pulse amplitude. In my case the spikes cover more than 40% of the pulse amplitude which indicates a much higher capacitive load. My measurement with an LCR Meter showed about 90pF. In the regarding area of my layout there should be no such high parasitic capacitance: The traces from connector to the ISO1430 are about 1 inch long, the biassing and terminating network was placed close to the ISO1430.

I did not find the capacitances for A / B pins in the datasheet. Maybe the ISO1430 contributes a relevant part of the 90pF?

  • +1
    TI__Mastermind 49445 points

    Hi Thomas,

    Thanks for reaching out and sorry to hear about the issue.

    The capacitance A and B with respect to the GND is not a datasheet spec, hence, I do not have an exact value to share with you. But we do expect them to be <10pF. Hence, I believe most part of 90pF is coming from external circuit and not by ISO1430.

    Let me know if you have any questions, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 30 points

    Hi Koteshwar Rao,

    thank you for your reply  -  10pFs are negligible  -  so I think I have to look for the main stray capacitance on my layout.

    Maybe the method measuring the clamp capacitance with an LCR meter was not ideal ...

    Have a good time,

    Thomas