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UA9637A: Termination for both input and output

Shinya Sawamoto
Expert 5260 points
Part Number: UA9637A

Hello,

I have a couple of questions about UA9637A termination.

1) Regarding the differential input, it looks that an external input termination has to be added, is it correct? If yes, is the right value 50ohm?

2) Regarding the single-ended output, it looks that it has 50ohm impedance at high level output. When the output has a long routing, should we add 50ohm termination on the load termination to prevent unexpected signal reflection?

Thanks in advance.
Shinya Sawamoto

  • 0
    TI__Guru 60242 points

    Sawamoto-san,

    1) In general, yes, it is important to include termination resistance on a signal line, whether it be differential or single-ended, input or output. The idea is that you match the impedance of the driver/receiver. Typically the resistor chosen is equal to the characteristic impedance of the cable, and/or the impedance of the driver or reciever. In most RS422 applications (what this device can be used for), 120 ohms is chosen as the termination impedance. If your physical bus length is short and/or the data rate is not over 10kbps, termination won't be totally necessary. Do you know what to expect for bus length, cable impedance, and data rate?

    2) It'll depend on the data rate as well, but adding a termination resistor can help with reflections as you said. However, the value resistor used should be in the thousands of ohms range. The output driver capability would be significantly affected by a 50 ohm termination. Unless you plan on using a high data rate, I'm not sure termination will be necessary.

    What kind of application will this be used for?

    Regards,

  • 0 in reply to Eric Hackett
    TI__Expert 5260 points
    Thanks for the detail explanation, Eric.
    I may send you an email when more discussion is needed.

    Regards,
    Shinya Sawamoto
  • 0 in reply to Shinya Sawamoto
    TI__Guru 60242 points
    Sawamoto-san,

    Just to reiterate, 50 ohm termination would be too much for termination. Something in the 2k - 4k range is appropriate.

    Regards,