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LVDS Transmitter out and Line Receiver output when data is not transmitted.

Arun Patil
Prodigy 60 points

When the LVDS transmitter is not transmitting data, what will be the output, the DO- and DO+ lines will have same DC voltage level as common mode voltage? And under this condition, will the receiver output be zero DC voltage as the inputs (IN+, IN-) are at same DC voltage levels (with respect to ground (meaning zero volt differential input)?  

  • 0
    Guru 316920 points

    Most LVDS transmitters cannot be disabled; they always output a valid state, depending on whether the input is high or low. When you have an LVDS transmitter that can be disabled, then in the disabled state, the outputs will have a high impedance, i.e., they will be floating. (The termination resistor at the receiver will force both lines to the same random voltage.)

    Most LVDS receivers do not have a fail-safe function; when the differential input is zero, they can output high, or low, or oscillate. Some receivers do have fail-safe inputs and output high in this case; for other receivers, you can add fail-safe pull-up/-down resistors to the bus.

  • 0 in reply to Clemens Ladisch
    Prodigy 60 points

    Does it mean that when LVDS transmitter is in disabled state (output high impedance), the receiver inputs are at same DC voltage (common mode bias, e.g. 2V) with respect to ground? If suppose the cable from Transmitter to Receiver is open, then what would be the DC voltages on Receiver inputs (with respect to ground) considering there is no fail-safe detection circuit incorporated? 

  • 0 in reply to Arun Patil
    Guru 316920 points

    The outputs are floating, i.e., the voltage can have any voltage.

    If the termination resistor is not disconnected, then both lines have the same voltage.

  • 0 in reply to Clemens Ladisch
    Prodigy 60 points

    That means, when the LVDS transmitter is in high impedance state or cable to receiver is open (both connections between transmitter outputs and receiver inputs), termination resistor at receiver input is intact (connected across inputs), then both the inputs at the receiver will be at same random voltage (noise) resulting zero voltage difference across those inputs and receiver output would be indeterminate if there is no fail-safe circuit incorporated, right?

  • 0 in reply to Clemens Ladisch
    Prodigy 60 points

    In this situation (either cable open or transmitter in high impedance state, termination resistor connected), if both inputs of the receiver are connected to Vcc through exact same resistor value (e.g. 680K. 0.1% tolerance) then both inputs will have same DC voltage (no current flows through termination resistor and the output of the LVDS receiver would still be indeterminate, right?

  • 0 in reply to Arun Patil
    TI__Mastermind 40656 points

    Hey Arun,

    Yes, this is correct- since both inputs have the same DC voltage, then the output of receiver will be indeterminate as the differential voltage is zero. To mitigate this issue, external biasing resistors can be used to provide a small bias to set the differential input voltage while the line is not driven. 

    See this app note for more details to prevent indeterminate states.

    Regards,

    Jack

  • 0 in reply to Jack Guan
    Prodigy 60 points

    Hello Clemen and Jack,

    Thank you both as this resolved my query and helped me to understand LVDS fail-safe biasing.

    Arun