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What will be the SN65LVDS047 Driver Output with-out Receiver(SN65LVDS048 ) Connected ..??

Govind K
Intellectual 510 points
Other Parts Discussed in Thread: SN65LVDS047

Hello,

I am using SN65LVDS047 driver & SN65LVDS048 receiver to transmit my SPI signals to a longer distance. During my module testing i have observed the below Issue .

Test Condition: Input signal of frequency 4 MHz is applied to Driver  SN65LVDS047 input. Receiver is not connected. Suplly voltage 3.3V, Driver is enabled permanently (Pulled High). 

Observation: Both Dout+ and Dout- are giving pulses of 3.3V amplitude (but Inverted). When i terminate with 100 Ohms resistor (Receiver not connected), amplitude decreased to 1.5V. Please see attached wave forms. 

Is it a expected behavior ..?? As per my understanding, LVDS drivers are current mode drivers, so should not generate voltage with out termination resistor. Kindly Clarify.

Regards,

Govind.

  • 0
    TI__Mastermind 20820 points

    Hi Govind,


    The LVDS driver in the SN65LVDS047 is pulled to ground (Dout-) and Vcc (Dout+) internally so if the bus is left unterminated the output signal will swing full rail or at least be non-deterministic. By adding the termination resistor you are forcing the driver to a common mode point and forcing the output to give the proper levels. In the terminated case, what levels (Vocm and Vod) are you receiving? Are they in line with the TIA/EIA-644 spec?

  • 0 in reply to Michael Peffers
    Intellectual 510 points
    Hello Mr. Michael Peffers,

    Thanks for reply.

    The Voltage levels are:
    Vocm = 1.05 V
    Vod = 0.36 V

    Regards,
    Govind.