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For a 2-row connector intended to interface a cable to a PCB, which method is preferable for twisted pair LVDS signals?

Horizontal Pairs:

LVDS1-    LVDS1+    LVDS2-    LVDS2+

Vertical Pairs:

LVDS1-     LVDS2-

LVDS1+    LVDS2+

I would have thought the difference would be inconsequential but a PCB designer mentioned that the horizontal configuration offers better signal integrity. We were in a group conference call so I didn't really have an opportunity to ask him to explain himself. Is he correct that horizontal pairs are preferable to vertical pairs?

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  • \$\begingroup\$ We generally want each cable of a single pair to have the same length. If the target device has a single-row connector (e.g. TFT LCD display) then horizontal pairing can be a bit better. Please note that the topic is not only about length matching. For further info please refer to TI's awesome guide named "Hi-Speed Interface Layout Guidelines" \$\endgroup\$
    – Rohat Kılıç
    Commented Oct 3, 2018 at 22:04
  • \$\begingroup\$ If you are using IDC flat twisted pair cable, then the +/- signals are only twisted together in the vertical case. This assumes LVDS1+ is pin 1, LVDS1- is pin2, etc. \$\endgroup\$
    – crj11
    Commented Oct 4, 2018 at 0:32

1 Answer 1

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Low skew and good impedance to gnd for 100 Ohms is desired.

follow this for LVDS applications.

  • Connectors must have low skew with matched impedance over entire spectrum.

  • Ground pins should be placed between differential pairs.

  • Connectors with same length leads should be selected for lower skew and crosstalk.
  • Both polarities of the same differential pair must be adjacent
  • End pins of the connectors should preferably be grounded and must used for high-speed signals.
  • Any unused pins should be properly terminated to avoid high Z ingress.

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  • channel SMT CM chokes or ferrite flat strips are critical as cable length increases due to mismatched CM Z and SMPS CM conducted noise on supply or ground return between endpoints for better signal integrity at highest speeds due to CMRR and better crosstalk bypass. Often laptops just use AWG30 with AWG48 strands to make flexible insulated twisted pairs and bundle inside braided shield to folded display. Others use FPC cable.
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  • \$\begingroup\$ When you remarked that "both polarities of the same differential pair must be adjacent", does it make a difference if they're horizontally adjacent or vertically adjacent for a 2-row connector? \$\endgroup\$
    – pr871
    Commented Oct 4, 2018 at 12:44
  • \$\begingroup\$ Is there no skew in path length and does it have gnd between pairs? If so, ok \$\endgroup\$
    – D.A.S.
    Commented Oct 4, 2018 at 15:10

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