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AM26LV31: Schematics implementation review

Albin Takami6
Prodigy 160 points
Part Number: AM26LV31

Hello Support,

Please review the attached implementation and let me know if it's correct.

Based on attached schematics, please estimate and let me know how long it would take you to thoroughly review the schematics.

Then please review the design.

Thank you & regards,

-albin

  • 0
    TI__Guru 51635 points

    Hi Albin,

    Termination is generally only used on the far-end receiver for RS-422 devices. If the CLK and SYNC lines are also terminated at the receiver end, the additional loading on the device may impact performance. The typical voltage for this device is specified at a load of 100-Ohm

    The termination resistance of 220-Ohm on the U140 device is a bit higher than usual. This value normally ranges from 80-Ohm to 120-Ohm depending on the cable. Is this value based off of the cable impedance?

    The RC filter network on AD_CLK may introduce some phase delay. This won't be significant at a slow data rates, but may impact higher rate signals. What bit rate is the device going to operate at?

    Regards,

    Eric

  • 0 in reply to Eric Schott1
    Prodigy 160 points

    Hi Eric,

    Re: "Termination is generally only used on the far-end receiver for RS-422 devices. "

    I found some application notes stating that the termination is needed on both ends for RS-422 devices, because of the bidirectional communication.

    Re: "If the CLK and SYNC lines are also terminated at the receiver end, the additional loading on the device may impact performance."

    Do you mean the transmitter end? typo? How would it significantly impact performance when the additional loading is seemingly small?

    Re: "The RC filter network on AD_CLK may introduce some phase delay. This won't be significant at a slow data rates, but may impact higher rate signals. What bit rate is the device going to operate at?"

    It works like anti-aliasing filter with cut-off at 151MHz. It should be fine for signals operating at 32MHz.

    Please advise.

    Thanks,

    -albin

     

  • 0 in reply to Albin Takami6
    TI__Guru 51635 points

    Hi Albin,

    Re: "I found some application notes stating that the termination is needed on both ends for RS-422 devices, because of the bidirectional communication."

    RS-422 are unidirectional (only one driver per bus) so there only needs to be termination at end of the cable near the last receiver. You may be thinking of the RS-485 standard which is bidirectional and is terminated at both the driver and the receiver. This application note may help.

    Re: "Do you mean the transmitter end? typo? How would it significantly impact performance when the additional loading is seemingly small?"

    Adding more resistance in parallel on the bus increases the current draw from the driver to maintain the differential voltage. This is the same reason that the number of receivers on each bus must be limited.

    Let me know if this clearifies.

    Regards,

    Eric