XEM3010: Layout of pins for LVDS/differential signals


Depending on how you do your cabling, the pin layout for the BRK3010 may not be suitable/usable for differential pairs, despite what it says on page 17 of XEM3010-um.pdf:
“The XEM3010 PCB layout and routing has been designed with several applications in mind, including applications requiring the use of differential (LVDS) pairs.”

Here’s why:
If you use standard twist-to-flat twisted-pair cabling (eg. from Amphenol or Hitachi) and crimp on a standard 40-pin connector, the (twisted) pair of wires for a given signal land on consecutive pins (eg. 1+2, 3+4, etc; numbering given according to the diagram on p. 23 of XEM3010-um.pdf and from examination of the silkscreen on the BRK3010 board itself).

If you look at pages 23-25 of XEM3010-um.pdf, you’ll find that corresponding pins for an differential pair (eg. U18+T18, ie. L16P-3 + L16N-3) end up on pins 16+18 of the J2 80-pin connector. These in turn end up on pins 15+17 of JP1 on the BRK3010 breakout board. When connected with standard crimp connectors, these two do not end up as part of the same twisted pair.

Unless I am mistaken, the pin layout on the BRK3010 doesn’t provide even a single pair of signals that ends up wired together on a standard connector.

This unfortunately means completely custom cables, or a short (hard-to-make!) hand-wired adapter plug/cable will be necessary for cables of any significant length (max usable length will depend on frequency).

Here’s my questions:

  • Is there any chance I’m mistaken? (… I would love for that to be the case).

  • Is there a chance that there will be a new rev of the BRK3010 board which flips adjacent differential signals from being adjacent in the “horizontal” direction to the “vertical” direction so that quick, simple, standard IDC connectors can be used?

Thanks very much.

The documentation states that the XEM3010 PCB is designed to help you with LVDS, NOT the BRK3010.

We do not recommend using the BRK3010 for LVDS usage. First, some connections are quite long. Second, the BRK3010 is not controlled-impedance; it is only a 2-layer board without even a ground plane.

If you are doing LVDS, we highly recommend you build a board to mate with the XEM3010, not with the BRK3010.