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TRS3122E: Serial Resistors Selection/Calculation for Rin

Part Number: TRS3122E

I have used a pair of the TRS3122E as the evaluation board and choose R6 and R7 with 50 Ohm.  During the common noise test - injecting 1000V, 100kHz signal into the GND of the TR3122E and the chassis ground, the communication is disrupted.  After change R6 and R7 back to 0 Ohm, the common mode noise immunity is much better.  Do you have any application note or suggestion on how to select the proper R6 and R7?  Or this is bus specific?  Is it possible to give an example on how to select the correct value for R6 and R7?  We could not even measure the waveform properly when common mode noise is being injected.

Thank you very much,

Frank W.

  • In reply to frank wong:

    Hi Frank,

    It seems like the images still aren't working, and it sounds these are needed in order to understand how R6 and R7 are connected.  Can you please try sending them again?  It should work if you click the "Insert Code, Attach Files and more..." link below the text input field.  Then you can use the "Insert/Edit Media" button on the toolbar above the test field.

    If that still doesn't work, is it OK if I contact you via the e-mail address used by your ti.com account?

    Max

  • In reply to Max Robertson:

    Hi, Max,

    The R6 and R7 is the serial termination resistor of the Rin 1 and Rin2.

    Thanks,

    Frank W.

  • In reply to frank wong:

    3005.Doc1.docx

  • In reply to frank wong:

    Hi Frank,

    How high were the resistors you tried? In conjunction with the input impedance of the RS-232 receiver (nominally 5 kOhm), these would cause some reduction in signal amplitude. Since RS-232 is a single-ended standard it doesn't have any inherent common-mode noise immunity (like a differential standard would, for example), and so larger signal amplitudes are helpful in improving signal-to-noise ratio.

    Max
  • In reply to Max Robertson:

    Thanks, Max.

    We tried 50 Ohm and what range of resistors you would recommended or we can just use 0 Ohm?

    Best regards,

    Frank Wong
  • In reply to frank wong:

    Frank,

    0 Ohms is the most common usage. Some customers choose to insert additional series resistance in order to limit currents under fault conditions (like short-circuits) or improve the impedance matching to a cable (which may be useful for very long-distance applications), but this is generally not required.

    Max

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