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Level shifting a clock

Discussion in 'Electronic Design' started by Spatch, Mar 15, 2007.

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  1. Spatch

    Spatch Guest

    I need to level shift a clock that toggles between 0 and 3.3v to a
    clock that toggles between -7v and +7v. I recall that you could use a
    voltage comparator to do this, however I can't seem to find a
    schematic in any application notes. I have plenty of Linear Tech
    LT1011 comparators in my toolbox and was hoping I could do something
    with them.
     
  2. James Beck

    James Beck Guest

    How fast of a clock?
    Could you get away with using a "standard" RS-232 level shifter?
    They are a dime a dozen.

    Jim
     
  3. Lionel

    Lionel Guest

    What's your maximum frequency? If it's within the usable range of your
    comparators, you can simply compare your clock signal to ~1.6V from a
    voltage divider (or a red LED!) & get the result you want. Jim Beck's
    suggestion of an RS232 output converter should work fine up to a
    couple of MHz, & is pretty cheap. If the frequency is higher than
    that, you could use a proper logic level converter, or a push-pull
    transistor output stage.
     
  4. Scott

    Scott Guest

    Lionel,

    Thanks for responding. The max frequency is 100khz...well within the
    bandwidth range of the LT1011. I actually breadboarded a circuit that
    did as you suggested. I ran my 100khz clock into the + input of the
    LT1011 and tied the (-) input to a voltage divider (1.65v). The LT1011
    has floating transistor output. I have tied the collector to +7v and
    the emitter is connected to -7v. I would expect the output to toggle
    +/-7 volts, however this is not the case. The output swings to the
    +7v , but not below ground. Relativley simple circuit to breadboard so
    I doubt I made a mistake.

    Incidently, the RS232 translator that Jim Beck suggested is a great
    idea, however most RS232 transceivers invert the outputs. I will have
    to look a little closer at the LT1011's output stage. Thanks for the
    input.
    -Scott
     
  5. Lionel

    Lionel Guest

    Hm. Looking at the datasheet, it looks like the chip is designed to
    interface between split-supply analog & logic-levels. You might get
    the full output swing by wiring it up for a 14V single supply instead
    of a +/- 7V split supply. (Looks like you need a pull up resistor on
    the output as well.)
    My pleasure, glad I could help.
     
  6. Jim Thompson

    Jim Thompson Guest

    My MC1488 will do nicely to convert TTL/CMOS levels to +/- whatever,
    up to +/- 10V.

    ...Jim Thompson
     
  7. Lionel

    Lionel Guest

    <reads futher> Okay, looks like I was on the right track. Check the
    schematic on the very last page of the datasheet. To get the full
    voltage swing, you need to connect *both* pins 1 & 4 to your -7V
    supply, with a pullup resistor from pin 7 to +7V.
     
  8. Lionel

    Lionel Guest

    Yep, plus they're cheap, easy to use, & easy to find. The one thing to
    watch out for with the OP's 3.3V inputs is that he might need a pullup
    resistor in the input of the 1488.
     
  9. Jim Thompson

    Jim Thompson Guest

    Nope. My input is classic DTL... threshold is precisely two Vbe's.

    ...Jim Thompson
     
  10. Lionel

    Lionel Guest

    Well then, no problem. (Seemed likely, but I was too lazy to check the
    datasheet.) ;^)
     
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