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cables for digital signals

Discussion in 'General Electronics Discussion' started by masudasim, Jan 2, 2013.

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  1. masudasim

    masudasim

    6
    0
    Aug 25, 2012
    Hello all

    i am using FPGA board with ADC Ic and i have to provide clock signals (>40 MHz) from FPGA to ADC and digital output signals from ADC to FPGA.

    What type of cables should i select for digital clock/data?

    How much length of above cable i can use or in other words how far FPGA board can be placed from ADC board without clock distortion?

    Please help me

    Thanks
     
  2. GreenGiant

    GreenGiant

    830
    6
    Feb 9, 2012
    you should be able to use just regular solid or multi core wire, 18/20 AWG or so, and being a digital signal it is less susceptible to noise/distortion. As long as you have a clean output you can go pretty far before there are issues. Its all dependent upon the voltage and current output capabilities, the higher the current capabilities the farther that you can go before you have too much loss over a long distance.
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,301
    2,738
    Jan 21, 2010
    I would recommend that whatever you do, that you buffer the clock on the ADC board using a schmitt trigger or similar.

    I the cable is of any significant length, you may need something more exotic.

    It would also be wise to use (at least) a twisted pair or perhaps a coaxial cable to prevent the radiation of the 40Mhz signal.

    As to how far -- I would recommend "as short as possible"

    If this is a clock signal, it's likely to have fast rise and fall times, and that means harmonics right up into (perhaps) GHz if you have fast logic. This seriously increases the chance of radiating something.
     
  4. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    You haven't mentioned if you're talking about inches, feet, yards, meters or whatever. When a high frequency signal has to travel over distance it should be via a transmission line. UTP and coaxial cable are good choices but neither constitute a "Transmission Line" unless the source impedance and the load impedance match the "Characteristic Impedance" of the line.

    Chris
     
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