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ISO35 EMI Filtering

James Lockridge19
Genius 14485 points
Other Parts Discussed in Thread: ISO35, PROFIBUS

Hello Team,

I am working on a design for an aircraft which uses the ISO35 for RS-485 communication. I want to reduce EMI from this device. Are there any potential issues with putting a filter on the transmission output of the ISO35 (e.g. extra line capacitance, lowered load impedance)?

  • 0
    TI__Intellectual 2200 points

    Hi James,

    Sorry for the delay in this reply. I think this should be ok - common mode chokes are used in RS485 and CAN networks for emissions reduction. Also, given the slow speed and rise/fall times of ISO35, the parasitics of the common mode filter should be less of a problem.

    I am checking with RS485 experts if they have particular recommendations for the right filters. Meanwhile, if you can share the details of the filter/choke you are planning to use, over email if you like, we can comment further.

    Rgds,

    Anant

    (anantkamath@ti.com)

  • 0 in reply to Anant Kamath
    TI__Intellectual 2200 points

    Hi James,

    Were you able to get more information from the customer on how they plan to implement the filter? Here's some more information and suggestions from our interface experts, which could be useful to the customer. Hope this helps.

    "

    In general, there is no specification for capacitive loading on the bus for RS-485.  The Profibus standard specifies a “maximum station capacitance” for various data rates, where the “station” includes capacitance from everything on a given node (transceiver, board parasitics, transient protection, etc).  This is the only instance I can think of where there would be such a constraint.

     Also, in some conditions inductive flyback from the choke can result in high transients appearing at the transceiver pins.  Therefore you often see clamping diodes between the choke and the transceiver pins.

     This app note lists a bunch of part numbers, specifically in reference to CAN, but I don’t see why most would not also be suitable for RS-485.

    http://www.ti.com/lit/an/slla271/slla271.pdf

    "

    Rgds,

    Anant

     

  • 0 in reply to Anant Kamath
    TI__Genius 14485 points
    Anant,
    Thanks for the further information! The customer has not told me more about their filter. I will pass on the information you sent me.