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THS4131 Driver twisted-pair cable

Sulyn ZHANG
Expert 8700 points
Other Parts Discussed in Thread: THS4131

Dear all,

My ctm is now want to use our THS4131 to driver twisted-pair cable, the cable lenth is about 3m, as I know the twisted-pair cable is capacital load, so I suggest ctm to use a 100 ohm serial resistor to improve the stability. The question is is 100ohm resistor enough for this? Ctm hope to use smaller resistor, because this resistor will introduce a gain error because the load resistor is about 10kohm. So here need your kindly support for this, many thanks!

Sulyn 

  • 0
    TI__Mastermind 22825 points

    Hi Sulyn,

    The best arrangement, if you are dealing with relatively higher frequencies compared to the 3m cable length, to avoid reflections and unwanted distortion, is load and source termination (~100ohm). This would require 2x the gain from the THS4131 stage. Is that possible in this application?

    When terminated properly, the twisted pair would look most resistive with less concern about being a capacitive load. The series output resistance at the THS4131 source would help with isolation (as well as the help with reflection absorption).

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    TI__Expert 8700 points
    Hi Hooman,
    Many thanks for the support! The signal frequency at customer side is less than 5kHz, very low frequency. The 100 ohm resistor will work in such sitation? Customer want to use as less as 20ohm to reduce the gain error caused by the Serial resistor.
    Sulyn
  • 0 in reply to Sulyn ZHANG
    TI__Mastermind 22825 points
    Hi Sulyn,
    At 5kHz with 3m cable length, you don't need to worry about reflection of the signal.

    According to THS4131 Figure 33, 20ohm in series with the outputs is a good value to provide isolation if you leave the load end of the cable unterminated. If you try this with an unterminated (10kohm load) cable and have any instabilities, you could also try terminating the cable (in ~100ohm) and increase the THS4131 gain to compensate for the gain loss and retest.

    Regards,
    Hooman
  • 0 in reply to Hooman Hashemi
    TI__Expert 8700 points

    Hi Hooman,

    Many thanks for the support!!

    Sulyn