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AC coupling SN65LVDS179 LVDS inputs

Stephen Petlak
Prodigy 60 points
Other Parts Discussed in Thread: SN65LVDS179

I'm looking to AC couple an LVDS signal pair going into the SN65LVDS179. We're running data at 155Mbps. I've seen different approaches for implementing the coupling and terminations. One method was to use 0.1uF or 0.01uF caps in series with the LVDS signals going into the LVDS receiver. A 100 Ohm termination resistor is placed across the pair on the receiver side of the caps. Then the juncture of 10K/10K resistor divider (one end pulled up to Vcc, the other end grounded) is connected to the "B" (negative) input of the receiver. There is no similar divider on the "A" (positive) input. This method is attractive because it uses fewer parts. Will this method work, or is there a preferred method (or one that uses fewer parts and still gives me AC coupling) considering the SN65LVDS179 is used in this application?

  • 0
    TI__Mastermind 20820 points

    Hi Steven,

    Typically in LVDS applications when there will be a termination resistor between the differential pair of a 100 ohms (or 50 ohms single ended) to reduce reflections in the system. When using the AC coupling method the common mode voltage need to be reestablished on the receiver side so on both the P and N portions of the LVDS inputs the will be a resistor divider to set the common mode voltage at 1.2V. Take a look at this document about LVDS and let me know if any more questions regarding implementing it in a design:

    http://www.ti.com/ww/en/analog/interface/lvds.shtml

    Regards,

    Michael Peffers

  • 0 in reply to Michael Peffers
    Prodigy 60 points

    Michael Peffers said:

    Hi Steven,

    Typically in LVDS applications when there will be a termination resistor between the differential pair of a 100 ohms (or 50 ohms single ended) to reduce reflections in the system. When using the AC coupling method the common mode voltage need to be reestablished on the receiver side so on both the P and N portions of the LVDS inputs the will be a resistor divider to set the common mode voltage at 1.2V. Take a look at this document about LVDS and let me know if any more questions regarding implementing it in a design:

    http://www.ti.com/ww/en/analog/interface/lvds.shtml

    Regards,

    Michael Peffers

    Hi Michael,

    Thank you for the response. So for each LVDS input I'll use a resistor divider consisting of 138 Ohms (to +3.3v) and 78.8 Ohms (to GND). The divider will set the receiver input bias to 1.2v, and the parallel equivalent for each divider will be 50 Ohms. So the two resistor dividers give me a 100 Ohm equivalent termination. Does this sound correct?

     

    Steve

  • 0 in reply to Stephen Petlak
    TI__Mastermind 20820 points

    Hi Steve,

    Yes Steve, this sounds correct. What is the intended end application for the SN65LVDS179? Is a single channel solution what you need for this application or can you benefit further from one of our multichannel solutions?

    Regards,

    Mike

  • 0 in reply to Michael Peffers
    Prodigy 60 points

    Michael Peffers said:

    Hi Steve,

    Yes Steve, this sounds correct. What is the intended end application for the SN65LVDS179? Is a single channel solution what you need for this application or can you benefit further from one of our multichannel solutions?

    Regards,

    Mike

    Mike,

    This is a single channel application. We've been using the SN65LVDS179 for some time now and just need to change from DC to AC coupling between systems.

    Regards

    Steve