656x: splitting LVDS signals with ribbon cable

[mlloyd]

For my application, I need to test several devices. Each DUT has 5 input lines and 3 output lines. I'm thinking of running the LVDS signals from the 656x to an intermediate board. From there, I would run 8 signals to each of my DUTs using ribbon cable connectors.

Is this a really bad idea?

What would be your recommended way to talk to several different DUTs?

I greatly appreciate your help!

Regards,
mlloyd

[Erick]

Hi mlloyd,

Splitting an LVDS signal is not a good idea and there is no easy method for connecting multiple DUTs to one 656x board. The reason is because a very specific cable (12x infiniband) with exact impedance was created for the 656x board. This cable has to be 1 meter long in order for the signals to propagate appropriately at all frequencies. If the cable is longer then reflections will start appearing and the signal transmission will deteriorate, especially at high frequencies. Also, if the signal is split into multiple paths then there will be transmission line reflections introduced at the intermediate board, and if the cable length is longer then the impedance will not match and the signal will deteriorate further. I'm sorry there's no easy way to split LVDS signals. You can surely give your setup a try and if you're operating at the lower frequency range of the 656x board then it will probably work.

Hope this helps!

Erick D
NI Applications Engineer

[mlloyd]

Thanks for your advice. I was worried about splitting up the signals... All the documentation I've found has really stressed impedance matching. I guess I'll have to leave everything together and deal with having a larger connector!

Thanks again,
mlloyd

[Ryan_M]

Mlloyd,

There is a test fixture developed for the NI 656x device that routes from the controlled impedance bulk connector to individual SMA connectors.  The SMA 2164 (http://sine.ni.com/nips/cds/view/p/lang/en/nid/201705) was carefully designed to maintain signal integrity and channel to channel skew.  Using this fixture, you can easily use coaxial SMA cables to route each pair to your DUT.  You must take care, though, that any coaxial cables you use must be 50ohm cable and must have matched lengths within a pair.  Also, take note that the longer your cabled solution is, the higher chance there is for signal loss and reflections.  Since LVDS is terminated at the destination, any mismatch you have in cable lengths within a pair may propogate through your system.

I hope this helps.