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

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.

Reply to
X.Y.
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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.

John

Reply to
John Larkin

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

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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.

---Joel

Reply to
Joel Kolstad

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!

Reply to
X.Y.

Yes.

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.

John

Reply to
John Larkin

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:
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to see it. Thank you! If we have further information, we will also tell you.
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X.Y.

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