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50 Ohm, 5v driver needed

Discussion in 'Electronic Design' started by Norm Dresner, Oct 21, 2003.

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  1. Norm Dresner

    Norm Dresner Guest

    We need to drive a terminated by a load that's equivalent to 50 ohms with a
    pulse created by the TTL-compatible output of a PC counter/timer board.
    The hard part is that the pulse at the receiving end has to go from 0 to 5V
    (more or less). Standard 50 ohm TTL drivers (74S140) are only spec'd to
    produce a TTL-high of 2.5 volts and that's what I've observed; paralleling
    74128 (75 ohm drivers) doesn't do much better. The basic specs are 0-5v
    into 50 ohms with rise/fall times on the order of (but not much greater
    than) 20 ns. The board we're using has a 5v supply output so we can use
    that for any TTL(or equiv)-logic and a single voltage external supply to
    provide the higher voltage needed to get rail-to rail 0-5v output. We have
    on hand some 6v/1A regulated wall-warts which should be adequate.

    My prototype used a 7407/7417 (HV OC inverter) to drive a 2N2219 transistor
    operating from a 5.7v separate supply, and while this works, I think there's
    got to be a neater way. I'm hoping to find a single IC that will accept a
    TTL-compatible input and produce the necessary output, but so far I've
    failed to find anything. Since we're only producing single-digit
    quantities, cost is not a primary factor.

  2. John Larkin

    John Larkin Guest

    Just use an HC or HCT part. Paralleling several/all sections of an
    HC(T)04 should do what you want. AC-series parts are even a bit
    fiercer, but maybe *too* fast.

  3. Norm Dresner

    Norm Dresner Guest

    Umm.. NO. A 5v pulse into 50 ohms requires 100 ma and that's beyond
    the spec for even the 74HC(T)244 octal buffer.

  4. Perhaps a nice opamp, line National LMH6639 in SOT23?
  5. John Larkin

    John Larkin Guest


    Try it.

  6. Frank Bemelman wrote...
    There are many ways to drive 50-ohm lines, including a few rather
    attractive ones we mention in AoE. But let's reconsider the '244.
    Each of the eight sections has a roughly 50-ohm Ron for the pullup
    FET (see the SGS datasheet, also pullup is higher than pulldown).
    Eight in parallel should be under 6 ohms, marginally-suited for
    driving a single 50-ohm far-end termination to about 4.5V at 90mA.
    Various manufacturer's parts have Vdd and Vss supply-pin maximum
    ratings of 50 to 75mA (some Fairchild parts). OK, but will they
    work? They'll no doubt work just fine. But with a large number
    of parts over a long time period, perhaps some can be expected to
    have electromigration problems.

    As for myself, I like 50-ohm source termination with open-circuit
    destination termination. This only requires a few HC gates. But
    should I need to provide a zero-ohm source, 50-ohm destination
    termination, I'd consider a tc4427 mosfet driver IC, both outputs
    paralleled for Ron < 7 ohms (5V supply).

    Or I'd use my favorite AoE single-transistor solution (page 612).

    - Win

  7. Usually, not 5V are required, but a TTL_Hi level.
    So a Phillips 74F0037 would do the job.
    The 100mA required at 5V are an extremely bad choice,
    a better choice would be a conversion to ECL or LVDS
    and back at the other side.

  8. Rene Tschaggelar wrote...

    - Win

  9. Fred Bloggs

    Fred Bloggs Guest

    That is just a simple OC high current switch- use a Shottky B-C clamp to
    eliminate storage time delay. If the termination at the receiving end is
    to GND, but still a DC connection, then go with an pnp. OnSemi and Zetex
    have many candidates for the job-use the selector guide under
    "switching", they work:).

    Please view in a fixed-width font such as

    SD |
    | |
    | c +------------+
    RB | |/ | |
    o---/\/\--+------| | |
    |\ | |
    e | |
    | | |
    +--+ |
    | |
    --- ---
    gnd gnd
  10. oops, right, thanks.

  11. Norm Dresner

    Norm Dresner Guest

    There are many other choices, but they weren't mine. After the
    equipment was built and they wanted to build the production test set, I was
    called to write a major Windows Emulation program, a device driver for the
    counter/timer board, and to provide a way to get this signal from the
    program to the equipment. They have an optoisolator at their end that has a
    rather heavy current requirement as well as the 50 ohm termination. Based
    on their design, we need at least 4.6V across the termination to get the
    required current through the optoisolator and its series resistor.

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