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logic level converter

Discussion in 'Electronic Design' started by Martin, Aug 29, 2003.

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  1. Martin

    Martin Guest

    Hi,

    in my actual design I must provide an interface which allows the customer to select
    between 1.8V, 2.4V, 3.3V CMOS and 3.3V, 5V TTL logic level.
    For this, I need a chip, which allows me to transform the selected levels to my
    internal 3.3V logic level.

    Who knows such a chip?

    Regards
    Martin
     
  2. Jim Thompson

    Jim Thompson Guest

    Seem like a high speed comparator should do the job... just vary the
    threshold. Maybe AD8561 or LT1016?

    ...Jim Thompson
     
  3. Fred Bloggs

    Fred Bloggs Guest

    Yeah!- LT1016 is an exceptionally good recommendation for him:)<wink>
     
  4. Sebasto

    Sebasto Guest

    http://www.semiconductors.com/pip/GTL2000DGG.html

    Sebasto
     
  5. Jim Thompson

    Jim Thompson Guest

    Fred, You don't like the LT1016? I've not used it myself, but the
    data sheet looks pretty good.

    ...Jim Thompson
     
  6. Fred Bloggs

    Fred Bloggs Guest

    Even J.Williams complains about decoupling and layout for that thing-
    the most oscillation-prone comparator in history- so beware.
     
  7. John Larkin

    John Larkin Guest

    A fabulous comparator is an LVDS receiver. The good ones are
    blindingly fast, have zero input current, work down to ground, are
    very well-behaved, and are dirt cheap relative to (compared to?)
    official "comparators."

    John
     
  8. Jim Thompson

    Jim Thompson Guest

    Yup, I forgot about those... did some designs for Fairchild Portland
    in 2000/2001.

    ...Jim Thompson
     
  9. Tim Shoppa

    Tim Shoppa Guest

    The threshold for a 1489 receiver is programmable by a resistor per
    receiver. The customer could swap a DIP resistor pack to change levels.
    Propogation and rise/fall time are pretty lousy by modern fast logic
    standards. Is that workable?

    Some manufacturers have two 1489 receivers, the standard one with a
    typical hysterisis of 0.2 or 0.3V and a "noise-immune" one with a
    hysterisis of closer to 1V. You probably want the smaller hysterisis.

    Tim.
     
  10. Jim Thompson

    Jim Thompson Guest

    Define "differential". Logic swing and common-mode?

    ...Jim Thompson
     
  11. Bill Sloman

    Bill Sloman Guest

    At Cambridge Instruments the LT1016 comparator was very popular - for
    its speed it was remarkably easy to keep stable. As with any fast
    comparator, you needed analog experience to get the layout right, and
    you had to decouple carefully - we had boards with LM311's that were
    prone to oscillation - but the LT1016 was less troublesome that most
    of the stuff we had been using before it came out.

    I had been using the AMD Am685 some years earlier and had graduated to
    the AD9685 by the time the LT1016 had gotten popular with my
    colleagues, and had had no problems with oscillations, but balanced
    ECL outputs do make it easier to produce a stable layout.

    Come to think of it, the AMD Am685, which seems to date from 1972,
    would seem to be a good candidate for a classic chip, were it not for
    the fact that it proceeded to evolve into the Plessey SP9685, the
    Analog Devices AD9865 and the Signal Processing Technologies SP9867.
    Maxim have a version as well.
     
  12. Have you considered the MAX3372 to 3393 family?.
    These are a whole family of logic level converters, with the levels used,
    set by the voltages supplied to each side of the chip. If you arrange a
    crude D-A converter, with buffer, so that you can program the voltage level
    (to 1.8, 2.4, 3.3 & 5v), then feed this to one side of the chip, with 3.3v
    fed to the other, it should do the translation necessary.

    Best Wishes
     
  13. Jim Thompson

    Jim Thompson Guest

    If I'm reading the data sheet correctly VL <= VCC, so the 5V input to
    3.3V output cannot be arranged by simple power supply variance.

    ...Jim Thompson
     
  14. John Larkin

    John Larkin Guest

    A National DS90C402 (dual LVDS-to-TTL) is $1.20, which is pretty good
    for two 3.5 ns comparators. The DS90LV012A is a single, 1.8 ns typ,
    $0.75. There are no ttl-output "comparators" anywhere near that fast,
    and even the 4 ns stuff is expensive. If your sigs are within their
    common-mode range, these are hard to beat.

    Of course, if you have nanoseconds to spare, an LM339 gets you four
    comparators for 9 cents... 2.25 cents per comparator.

    John
     
  15. Nico Coesel

    Nico Coesel Guest

    I'm trying to modify a P6464 pattern generator probe from a Tektronix
    Pattern generator to be used in a DAS9200 mainframe or TLA510
    mainframe.

    The outputs are probably an emittor from a transistor, but the output
    current is limited to a few milli amps. These outputs are intended to
    create any logic level at the output. I can set the common mode point
    -more or less- wherever I want with resistors. But there is a catch...
    the lower the value, the smaller the 'swing'. The higher the resistor
    value, the capacitance comes into play and limits the operating
    frequency.
     
  16. Nico Coesel

    Nico Coesel Guest

    Hmm, the DS90C032 (quad comparator) looks okay and is available from
    my usual supplier (Farnell). My eye must have slipped over it. I hope
    it's sensitive enough though. The sources of the signals I want to
    convert are not very good at driving capacitive loads. But I'll order
    some samples and give it a go. I've been testing on a breadboard so
    far and it's time to put something on a PCB to do some proper
    measurements at higher frequencies.
    I already looked into that but the LM339 is too slow.
     
  17. Andy Peters

    Andy Peters Guest

    Do your interface in a Xilinx or Altera FPGA that lets you select the
    I/O type. You'll need multiple FPGA loads, as the I/O type is part of
    the configuration. Seems like it's the easy way to go.

    -a
     
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