FPGA to 50 Ohm Coax Cable Impedance Matching

Discussion in 'General Electronics Chat' started by capucsc, May 4, 2009.

  1. capucsc

    Thread Starter New Member

    May 4, 2009
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    I have a digital output from an FPGA that I would like to drive over a 50 ohm coax cable. The pulse is +5V for 10 us. What is the proper way to match the impedance of the cable?

    Thanks,
    Chris
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    I'd start with a pseudo line driver, like a 74AC04 and a 51 Ohm series resistor. At the other end you can have a high impedance line receiver with a 51 Ohm resistor to GND.

    Then I'd rent a network analyzer and check it out. You will need to unlimber your checkbook for one of those however.

    If that is too rich for your blood then a TDR might fill the bill. That will tell you if reflections are a problem with your termination.
     
  3. capucsc

    Thread Starter New Member

    May 4, 2009
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    I should have noted one other thing:

    It would be best to isolate the FPGA I/O pin.
     
  4. capucsc

    Thread Starter New Member

    May 4, 2009
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    Thanks for the quick response Papabravo. Is there another solution that doesn't require the line receiver at the other end? This triggers another device and I would like to avoid an intermediate step on that end.
     
    Last edited: May 4, 2009
  5. Papabravo

    Expert

    Feb 24, 2006
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    How much cable are we talking about? Skimping on line drivers and receivers is usually penny wise and pound foolish for anything over about 6-18 inches. Trust me on this.
     
  6. capucsc

    Thread Starter New Member

    May 4, 2009
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    I understand the advantages of using a line driver / receiver. I only struggle with one thing: When I hook a 50 ohm coax into a pulse generator, it can drive the pulse easily over a 2 m cable. No receiver is require. Could a unity gain, non-inverting op amp to accomplish the same result the driver / receiver setup achieves? Or is there a method to recreate what is being done in the pulse generator?
     
    Last edited: May 5, 2009
  7. Papabravo

    Expert

    Feb 24, 2006
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    Depends on the opamp. A 741 won't cut the mustard. A TL081 might do the job, and a BUF634 would probably work as well. Why are you so resistant to using the part you require. If you test it with 2m of cable and it works: end of story. If you want to build a bunch of them,then you need to worry about how repatable the design is. Why take a chance?
     
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