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MLVDS connectors for space applications, and a few questions about LVDS

Mike Valvo
Intellectual 470 points

hello, I'm building some hardware for space applications that involve a MLVDS data bus.  I read the most recent LVDS  owners manual, and it emphasized selecting connectors and cable for 100-ohm characteristic impedance.  It's a low speed application (2Mhz) and I'm looking for 1-4 signal pairs in the smallest package I can get.  There's a slideshow on Digikey that recommends some 3M products.  

Can anyone here recommend some PCB connector and cable candidates?

other questions:

1.  does the 100-ohm characteristic impedance only apply to the cable (the wire pairs), or does it also apply to the connector?

2.  In the LVDS owners manual, they mention AC coupling the signal pairs with inline capacitors, but they don't go into any detail about it.  Does anyone have any info or examples?  wouldn't inline capacitors disrupt the LVDS standard?

thanks

  • 0
    TI__Mastermind 43795 points

    Mike,

    The characteristic impedance will apply to the connector and the cable itself.  Generally MLVDS is used across a backplane interface.  The traces are 100 ohm impedance and the connectors are also 100 ohm impedance.  The connector impedance is driven by much higher speed signals which would not work well if the connector and PCB impedance were different.

    The inline capacitors are not usually used in LVDS applications.  For LVDS interconnects the LVDS transmitter is designed to provide and maintain the common mode voltage on the link.  Inserting the AC capacitors disrupts this connection.  Some LVDS inputs can handle this disruption.  It is good practice to re-establish a DC common mode voltage on the input side of the AC coupling capacitors.  This can be achieved via a voltage divider or similar means to secure the common mode within the LVDS input specification.

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    Intellectual 470 points
    Lee, how would I determine the characteristic impedance of a PCB connector? after looking at a few products, it doesn't appear to be available in the connector's spec.

    also, between the driver and receiver, can the characteristic impedance of the line be measured? if yes, then how?
  • 0 in reply to Mike Valvo
    TI__Mastermind 43795 points

    Some connectors will list signaling types - if "differential" or "LVDS" is listed, the connector is very likely a 100 ohm connector.  With the lower speed and edge rate of MLVDS, the connector impedance will not be critical to system operation.  MLVDS edge rates are > 1ns long which minimizes the connector impact on signal quality. 

    Most connector vendors have a support line for models and other information.  I would ask them directly.

    Measuring the line impedance is typically done with a TDR (time domain reflectometry).  These are usually incorporated into an oscilloscope, but it can also be a stand-alone piece of equipment.

     

    Lee