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DS90UB933-Q1: Whether LVDS need to add a 50R resistor to pull down

Amelie Zheng
Genius 10865 points
Part Number: DS90UB933-Q1

Hi Team,

If LVDS+ and LVDS- are used as differential signal output, the power supply and signal are separated, and LVDS+ and LVDS- are differential signals. Do customer need to add a 50R resistor to pull down on the schematic circuit? Or LVDS+ does not need to add pull-down, as shown below

  • 0
    Guru 317060 points

    LVDS (i.e., not a single-ended coaxial interconnect) uses 0.1 µF capacitors on both lines, and no pulldown; see sections 8.1.3 and 8.2.2 of the datasheet.

  • 0
    TI__Mastermind 28400 points

    Hi Amelie,

    The DS90UB933 device can be configured for coaxial (single-ended) or STP (differential) operation, depending on the cable that the customer wishes to use in their design.

    1) In the coaxial design, DOUT+ requires a 0.1uF AC coupling capacitor and no pulldown resistor. DOUT- requires a 0.047uF AC coupling capacitor and a 49.9-Ohm resistor to ground. There is no need for another 49.9-Ohm resistor on the DOUT+ trace. See Figure 8-4. Coax Application Connection Diagram in the UB933 datasheet for more details.

    2) In the STP design, DOUT+ and DOUT- both require a 0.1uF AC coupling capacitor. There are no resistors on the traces. See Figure 8-8. STP Application Connection Diagram in the UB933 datasheet for more details.

    Since the customer seems to be using the coaxial design to connect the SER and DES, they need to remove R86. The DOUT+ trace in the coax configuration only requires a 0.1uF AC coupling capacitor. If they wish to use differential signals, please refer to the STP example instead.

    Best,

    Justin Phan