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Seeking the solutions of high speed interconnection for the long distance transmission of 3.3v/24MHz

Discussion in 'Electronic Design' started by X.Y., Apr 18, 2007.

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  1. X.Y.

    X.Y. Guest

    Seeking the solutions of high speed interconnection for the long
    distance transmission of 3.3v/24MHz signals.

    We need to connect a CMOS image sensor and a FPGA chip. The distance
    between them is approximately 1 meter. The output signal of the sensor
    is 3.3v and 24MHz. However, the length of the original cable of it is
    only 1 centimeter, so it should be prolonged. If they are connected by
    a cable directly, the signal would attenuate greatly which will cause
    the system doesn't work. In my opinion, an interconnecting circuit
    should be added between the sensor and the FPGA chip to process the
    signal before and after the transmission. By now, the methods I have
    found are as follows:
    1, Using LVDS signal Transceiver and Receiver
    The drawback of the method is: there are more than 10 bits signals,
    which need many of this kind of chips. It will take too much place in
    PCB and it also cost much.
    2, Using chips of Serializer and Deserializer.
    Both Maxim and National Semiconductor have such chips such as MAX9247
    and DS90C241. It is a good method. Unfortunately the solution is fired
    by my tutor because these chips are expensive.

    My question is whether there are any other solutions? More cheap will
    be better. Thanks a lot.
  2. John  Larkin

    John Larkin Guest

    24 MHz isn't very fast. A 20-conductor ribbon cable (with alternating
    grounds) should be fine, and source termination sounds like a good
    idea. If the CMOS sensor drive strength is known to be weak, buffer
    each logic signal.

  3. Joel Kolstad

    Joel Kolstad Guest

    There are plenty of quad LVDS drivers and receives in 16 pin TSSOP packages...
    you'd need 3 of them. This is not exactly "a lot" of board space, is it?
    National Semiconductor has some that do 28 bits at a swat.
    Is it really that expensive compared to the cost of your sensor and your FPGA?
    It almost sounds as if your "tutor" has a particular solution in mind he's
    trying to get you to produce rather than letting you just meet some real specs
    (such as an *overall system* price) however you'd prefer.
    -- John's approach of just sending the data over a ribbon cable -- with
    interspersed grounds if you like, or even twisted-pair ribbon cable (although
    this is spendy) -- will probably work fine.
    -- Presumably you have an ADC for your CMOS sensor. How about replacing it
    with one containing a built-in LVDS interface? (e.g., AD9219)
    -- You might find it cheaper to use an "application specific" SerDes such as
    those meant for DVI (see, e.g., Analog Devices' "HDMI transmitter/receiver
    selection tables")
    -- Since it is a camera, you might use the Camera Link interface
    ( It is just LVDS,
    but the point here is to tell your tutor that -- at least if this is an
    industrial application -- if Dalsa and Pulnix and Coreco and others can afford
    LVDS, so can you. Using Camera Link has the significant upside of being "plug
    and play" with many commerial framegrabbers as well.

  4. X.Y.

    X.Y. Guest

    To John Larkin: Thanks for your reply. Does the ribbon cable with
    alternating grounds means that the cable should be connected to
    signal, ground, signal, ground...? And, I think the CMOS sensor drive
    strength is weak because it is used in a mobile phone and has a power
    of only 90mW. It's IOH is 8mA and IOL is 15mA. Then, should I use a
    buffer or bus transceiver chip on the output of the sensor?

    To Joel Kolstad: Thanks for your recommendations. Maybe my tutor just
    wants to shrink the cost. We do not have an ADC for the CMOS sensor.
    We just buy it and its output is already digital signal. Thank you!
  5. John  Larkin

    John Larkin Guest

    It would probably be OK without it, but if you have the room and the
    money, buffer it. If it drives the cable directly, you could also get
    ground bounce that affects the imager circuits. Buffered or not,
    source terminate (100 ohms in series with each live ribbon cable lead
    at the driver end) for miminum current glitching; that way, each
    driver pin sees a 200 ohm load. Don't terminate the far end.

  6. X.Y.

    X.Y. Guest

    To John Larkin: Thank you for your help. We had design a PCB board to
    test the method you told us and we could get it next Tuesday (April,
    24th). However, we do a simple experiment to have a simple qualitative
    analysis. We use a scope of 2.5G/S and 10GSa/S. The conclusion is
    fairly positive. The figures can not be posted here, so they are
    posted on my blog. So please visit the site:
    to see it. Thank you! If we have further information, we will also
    tell you.
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