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ISO1430: Bus Capacitance

Mark Philip Felipe
Genius 9705 points
Part Number: ISO1430
Other Parts Discussed in Thread: ISO1176, PROFIBUS, ISO7741, SN65HVD1176

Hi Team, we would like to confirm if we have specifications for bus capacitance.

Thank you in advance.

Regards,

Mark

  • 0
    TI__Genius 17100 points

    Hi Mark,

    Thank you for posting to E2E! ISO1430's switching characteristics on the bus driver are specified to a line resistance and capacitance of 54ohms and 50pF respectively.

    These specs don't have to be matched, but if they are exceeded, data rise and fall times will generally increase.


    Respectfully,
    Manuel Chavez

  • 0 in reply to Manuel Chavez
    TI__Genius 9705 points

    Hi Manuel,

    Thank you for supporting.

    Can I just confirm the 50pF is the same as CID - Differential input capacitance on the ISO1176? That is specified as 10pF maximum on the datasheet.

    Thank you.

    Regards,

    Mark

  • 0 in reply to Mark Philip Felipe
    TI__Genius 17100 points

    Hi Mark,

    You're welcome! Differential input capacitance is a different spec than what the device is meant to drive on the bus: the bus driver on ISO1176 is also specified to drive a line resistance and capacitance of 54ohms and 50pF.

    CID refers to the capacitance ISO1176 contributes to the RS-485 bus by connection between its A and B pins. A similar spec for ISO1430 is "unit load impedance," where ISO14xx devices contribute 1/8 Unit Load to the bus, so up to 256 of these devices could be driven on a bus using standard-compliant drivers.


    Thank you,
    Manuel Chavez

  • 0 in reply to Mark Philip Felipe
    TI__Expert 3482 points

    Hi Mark,

    Thank for clarifying the question! 

    The 50pF is the total capacitance that includes fixture and instrumentation capacitance during the measurement for the switching characteristics.

    The typical differential input capacitance for ISO1430 is 22pF.

    Regards,

    Alfred 

  • 0 in reply to Alfred Chong
    Prodigy 160 points

    Hi Alfred,

      The ISO1176 is often used for Profibus as the low Cid of 10pF means it works well with high speed networks where the total capacitance must be under 200pF.

      Does this mean the ISO1430 is not as suitable for Profibus? We are aware the ISO1176 has a 'stick bit' bug so we were hoping we could change to an alternative part but you don't seem to offer anything?

      Thanks,

    Stephen

  • 0 in reply to Stephen Hughes1
    TI__Expert 3482 points

    Hi Stephen,

    Thank you for posting your question on E2E!

    ISO1430 has incorporated system-level IEC-ESD and IEC-ETF protection logic. This eliminates the use of external protection logic and thus removes the external capacitance that is contributed by the external protection logic.

    An alternative solution is to use an isolator ISO7741 with a transceiver SN65HVD1176.

    Regards,

    Alfred Chong

  • 0 in reply to Alfred Chong
    Prodigy 160 points

    Hi Alfred,

      Thanks for the clarification. We'll look in to this and make a decision. Currently we use the ISO1176 without any external protection. I'm unsure if the unit has passed EMC testing in this configuration as I have only recently joined the company. If we do need to add this then the ISO1430 does look a viable option, I'm just slightly concerned that while the ISO1176 datasheet references Profibus the ISO14xx does not.

      Regards,

    Stephen

  • 0 in reply to Stephen Hughes1
    TI__Expert 3482 points

    Hi Stephen,

    Thank you for sharing your concern with us!

    The ISO1430 datasheet mentions that it is "PROFIBUS compatible at 5-V bus-side supply".

    Regards,