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AM26LV32E need termination resistors?

jladuca
Intellectual 2625 points
Other Parts Discussed in Thread: AM26LV32E, AM26LV31E, AM26LV31

For the AM26LV32E, do you need to add external termination resistors to the board?  Unclear from the datasheet and looking for some guidelines, thanks

  • 0
    TI__Mastermind 28265 points

    Except for low data rates or very short lines you will require a 100 ohm termination resistor at the receiver input to eliminate reflections on the line.

    regards, Thomas

  • 0 in reply to Thomas Kugelstadt
    Guru 13040 points

    Hi all,

    Appending a question to this thread as it seems reasonable to put related subjects together...

    I have this project where I link two boards communicating via RS422 (am26lv32e with am26lv31e). It is a uart communication at 921600bps, and cables are cat5e at a maximum of 4 meters.

    The termination resistors adopted are 100ohm on each channel of the receivers. This translates into a current consumption of 4x33mA, ~130mA for four channels. Quite high for this project, quite high in a world where we celebrate that a microcontroller can work at 35mA instead of 40mA...

    Anyway, does anyone have experience with using higher resistors? Maybe all the way up to 1K or 470r? Would it be stable? Or do you have other suggestions for out next spin of boards, with more efficient components for our case?

    Cheers.

    Bruno

  • 0 in reply to Bruno Saraiva
    TI__Guru*** 145336 points
    Bruno,

    Yes, I have tried higher resistors and reflections do be come an issue.
    I suggest turning off the AM26LV31 when not transmitting.

    Here is an app note with wave forms with different termination methods.
    www.ti.com/lit/an/slla070d/slla070d.pdf

    Another solution is to get a slower driver.
  • 0 in reply to Ron Michallick
    Guru 13040 points
    Ron,
    Thanks a lot for the info, the application note is quite good indeed. I had looked at it on the first stages of the project, but it makes much more sense to revisit it when you know what you are looking for!
    We will try some 470r resistors and see what happens, while keeping 921600 for now. I have room to come down to 230400, not less than that at this moment. Will post the results later on when we have something solid to share.
    Regards
    Bruno
  • 0 in reply to Bruno Saraiva
    TI__Guru*** 145336 points
    Bruno,

    Source termination is also an option. The idea is to add series resistors to the driver output pins so that the driver plus resistors adds to 100 ohms. The signal will reflect at the open receiver then travel back the the driver and will be stopped at the driver because the impedance matched. At 4 meters and 70% velocity factor gives a cable travel time of 19nS or 38nS to go both ways. I expect the receiver signal to be very good at your data rate.

    In this case don't put termination at the receiver. With 100 ohm termination, 1/2 the voltage of the driver will be lost.
  • 0 in reply to Bruno Saraiva
    Guru 13040 points

    Folks,

    Just got reliable results with 470 ohm termination. Below is a quick print of both A and B channels on the receiver, and for as long as I tested I got zero errors on the incoming counter.

    I'm not saying this should be an official solution, just sharing the results so far. Hope this comes useful to others in the future.

    Cheers,

    Bruno