LVDS Decoupling

Apr 12, 2006 3 Replies
W

Hi,



we use LVDS to transfer videosignals from a our camera (SN65LV1021) to a our framegrabber (SN65LV1212) over a cat5e compatible cable. The pixelclock is 27Mhz with leads to about 324Mb/s. The problem is that cable and camera are in a pretty "dirty" emc environment, motors and that sort of stuff. To get rid of the emc disturbance we have to isolate the chassis of the camera from the rest of the machine, so that the shield can run from the framegrabber over the cable to the camera, and we do not have ground (shield) loops.



The idea would be to make it more enthernet-like, to introduce transformers into the LVDS signals to have the cable decoupled. Do you think this could work and improve our design?



regards,



Wolfgang Kopp



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R

If the common mode exceeds the specification of LVDS, then you could magnetically isolate the camera. There are magnetic couplers such as the ADuM1400 from Analog Devices. Some are specified for 100MBit. Put these into the parallel path to get rid of the common mode.

Rene

Ing.Buero R.Tschaggelar - http://www.ibrtses.com & commercial newsgroups - http://www.talkto.net
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J

Make sure you have a DC connection to ground (can be ferrite filtered). Use a ferrite around the LVDS *pairs* to let the full differential signal through but significantly reduce the common mode noise. LVDS is designed to handle significant common mode interference so I'm surprised that you have a problem. If the common mode interference takes the differential signal outside the common mode range, the differential ferrites should help.

When I dealt with noisy differential signals, you could get surface-mount ferrites that ran two signals through one core. Whether they're specified as 100 ohm differential impedance may be a whole different issue.

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J

Hello John,

You can buy common mode chokes that are geared towards CAT-5. Murata and Toko would be a couple of companies that come to mind.

Regards, Joerg

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