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LVDS Decoupling

Discussion in 'Electronic Design' started by woko, Apr 12, 2006.

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  1. woko

    woko Guest

    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
     
  2. 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
     
  3. John_H

    John_H Guest

    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.
     
  4. Joerg

    Joerg Guest

    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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