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SN65MLVD082: The Termination resistor used for the chip at receiver should be 100 ohm or 50 ohm

Rajesh khanna
Intellectual 690 points
Part Number: SN65MLVD082

Hi,

Please specify the termination resistor to be used at receiver side.

We are using an FPGA to receive the signal from LVDS driver of same make and receiver also we have same make.

What is the termination resistor across the P/N line.

Regards,
Rajesh khanna

  • 0
    TI__Genius 14981 points

    Hi Rajesh,

    A multipoint bus requires terminations on both ends of the bus with 100 ohm resistors nominal. If there is any bus loading that changes the effective characteristic impendence of the bus, consider selecting termination values to try to better match the characteristic impedance of the bus. (but selecting 100 ohms simplifies the design).

    This can be understood by this TI MLVDS training video: LVDS | TI.com

    Also, this article is a resource: New TIA standard enables multipoint LVDS - EDN

    Regards, Amy

  • 0 in reply to Amy
    Intellectual 690 points

    HI,

    thank you for your swift response.

    Can you also tell what happens if we use 49.9 ohms.

    Regards,
    Rajesh khanna.

  • 0 in reply to Rajesh khanna
    Intellectual 690 points

    HI,

    sorry to ask additional question, we have used 49.9 ohm in our hardware we find some unreliable behavior from our side.

    sometime data gets decoded while randomly it is not able to decode information. we operating at 10 Mbps.

    same chip we have used in other projects where 100 ohm used we have not observed any problem.

    Regards,
    Rajesh khanna

  • 0 in reply to Rajesh khanna
    Guru 281760 points

    The termination resistor should match the trace/cable impedance, and that is 100 Ω in your application.

    A unidirectional connection uses a single resistor at the receiver. A multipoint bus has termination resistors at both ends, so the load that the transmitter sees is two resistors in parallel, i.e., 50 Ω. However, you are still using 100 Ω resistors. You must never use a 50 Ω or 49.9 Ω termination resistor.

  • 0 in reply to Rajesh khanna
    TI__Genius 14981 points

    Hi Rajesh,

    You are welcome, good questions! If 50 ohm is used to terminate both ends, the load is seen as 50 ohm in parallel with 50 ohm, or a total of 25 ohm load. 

    If the differential impedance of the bus is significantly different than this, many reflections will occur in the system and degrade signal integrity. 

    If the only difference in the two projects you mentioned is the termination resistor value, I would suspect multiple reflections are occurring on the bus.

    See this TI video (minute 1:08 for further reference: Correct Termination of LVDS and MLVDS)

    Regards, Amy