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SN65LVDS32B: Adding 100 Ohms across multiple LVDS receivers

Carlo
Expert 7715 points
Part Number: SN65LVDS32B

Hi E2E,

As mentioned in the application report SLLA031A, the resistor divider circuit is used in front of each LVDS channel that will receive 422 signals. In a multidrop configuration, the termination is usually only on the last receiver. Is there a loading issue by adding 100 Ohms across multiple LVDS receivers?

Regards,
Carlo

  • 0
    TI__Guru 64326 points

    Hi Carlo,

    Yes, in a multidrop application other 100 ohm differential loads should not be added and this can cause issues with signaling levels and reflections (at higher frequencies).  

    https://e2e.ti.com/blogs_/b/analogwire/posts/get-connected-lvds-for-multipoint-applications 

  • 0
    Prodigy 40 points

    Hi Malik,  You may have answered part of my intended question but I didn't do a good job in what I was asking.  I am trying to use a multidrop configuration driving multiple LVDS receivers with one TTL Driver.  If you were to look at TI application report SLLA031A you will see that the last sentence of the second paragraph  states that "This circuit is used in front of "EACH" LVDS channel that will receive 422 signals".  Its the "EACH" part that I am uncertain about.  In the depiction there are two 45.3 Ohm (R1 & R2) and two 5.11 Ohm (R3 & R4) resistors used as dividers to bring the TTL level down to LVTTL.    I think the better question I could have asked was how can I configure a multidrop differential architecture using TTL driver and multiple LVDS receivers.  Thanks, Wayne

  • 0
    Prodigy 40 points

    Hi Malik,  You may have answered part of my intended question but I didn't do a good job in what I was asking.  I am trying to use a multidrop configuration driving multiple LVDS receivers with one TTL Driver.  If you were to look at TI application report SLLA031A you will see that the last sentence of the second paragraph  states that "This circuit is used in front of "EACH" LVDS channel that will receive 422 signals".  Its the "EACH" part that I am uncertain about.  In the depiction there are two 45.3 Ohm (R1 & R2) and two 5.11 Ohm (R3 & R4) resistors used as dividers to bring the TTL level down to LVTTL.    I think the better question I could have asked was how can I configure a multidrop differential architecture using TTL driver and multiple LVDS receivers.  Thanks, Wayne

  • 0 in reply to Wayne Saunders
    TI__Guru 64326 points

    Hi Wayne,

    The" each" here is assuming a point to point topology for each receiver not multidrop. In a multidrop you want to have only one 100 ohm differential termination this includes equivalent networks. One termination scheme should be sufficient here however I am not sure if this has been done with a 422 differential driver before so I do have some pause. How many receivers our being used? We want to make sure that single driver will be able to provide enough loop current to create the correct LVDS signaling levels for the multiple receivers.

  • 0 in reply to Malik Barton57
    Prodigy 40 points

    Malik,  Number of receivers would be 4.  The length to furthest receiver is likely to be less than 2 Ft.  Signal is not likely to be any greater that 10khz. Could the termination be placed at the driver side to prevent reflection back into the receiver? 

  • 0 in reply to Wayne Saunders
    Prodigy 40 points

    Sorry meant to say " Could the termination be place at the driver side to prevent reflections back into the driver? "

  • 0 in reply to Wayne Saunders
    TI__Guru 64326 points

    Its actually best to place the termination closest to the receiver or at the far-end, in general. I recommend placing the resistor divider network closer to cable connector. This gives reflections time to dissipate back to the direction of the driver.