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Need Multiplexer Solution

Discussion in 'Electronic Basics' started by [email protected], Apr 13, 2005.

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  1. Guest

    Hello,

    For a project, I need to switch 5V DC through one of eight different
    resistors using the digital outputs of a small USB data acuisition
    device. I'm thinking of using a 74LS138 3 to 8 line decoder. Will this
    work? I need the resistance of the chip to be 50 Ohms or less, as
    anything higher will affect the project. I've come up with a design
    using a relay network, but if there is a better way to do it with a
    multiplexer chip, I really wnat to go that route. The resistances are
    50-300 Ohms, and the 5 VDC will be coming from USB power. Thanx.

    gm
     
  2. Guest

    I should add to this that the output will carry up to 20 milliamps
     
  3. Lord Garth

    Lord Garth Guest

    No, the 138 is not going to work for you. The 138 has active low outputs
    so the output is one of eight low also, TTL is a current sinking logic
    family so the 74LS138 will sink current to ground far more effectively
    than it can source 'something near' 5 volts as an output. You could
    use the 4022 counter since it has decoded outputs that are active high.
    It sounds to me like you really want a switch like the 4066. This old
    part has an on resistance of about 80 ohms. Worse would be the 4067
    which has a 125 ohm on resistance. There are much better analog
    switches but these are in surface mount packages and only one switch per:

    http://www.fairchildsemi.com/ds/FS/FSA1156.pdf
    or
    http://www.fairchildsemi.com/ds/FS/FSA4157.pdf

    Watch the power supply on these to insure they will operate on 5 volts,
    Have the type of switch configuration you need and have TTL compatible
    inputs. The two links above are very low resistance devices... .9 ohms
    and 1 ohm respectively. The first is a SPST N.O. switch while the second
    is a SPDT. Either will operate on a 5 volts supply. Be aware of the
    current
    limitations through each switch.
     
  4. Guest

    Thanks for the help.

    If I put my device between the 138 and ground, and the eight resistors
    between the "outputs" and the 5V, will this destroy the device? Just
    curious. But I think I may have a new solution, which is to use an
    octal line driver with the voltage source going from the outputs to
    ground. I can put my device between the ground pin and ground. I'll
    need eight lines to drive it, but thats OK.
     
  5. John Fields

    John Fields Guest

    ---

    You could use eight P-channel logic-level MOSFETS, and an HC138, like
    this:

    +5V>---+-----------------------------+--[R8]----S D--+---->OUT
    | | G |
    | +-----------+ | | |
    +-|G1 Y7-|O--------------|------------+ |
    A2>----|A2 Y6-|O-----------+ | |
    A1>----|A1 | | +--[R7]----S D--+
    A0>----|A0 | | | G |
    GND--+--O|G2A- | | | | |
    +--0|G2B- Y0-|O--+ +--|------------+ |
    +-----------+ | | |
    HC138 | | |
    . . .
    . . .
    . . .
    | | |
    | +--[R1]----S D--+
    | G
    | |
    +------------------------+
     
  6. Guest

    Excellent suggestion, thank you very much. Another question: Is there
    a digital potentiometer available that will carry a wiper current up to
    25 ma continuously? If so, I could replace all those discreet
    components with the pot, and get 256 resistor values instead of 8,
    which would be nice for this project. The output of my circuit will
    draw up to 20 ma of current at the lowest resistor value. I looked at
    the AD8400 and it will only allow a max current of 5 ma when the wiper
    is set at the lowest (50 ohm) setting. Is there another device similar
    to the AD8400 that is designed to carry more current? Thanks for your
    help.

    gm
     
  7. John Fields

    John Fields Guest

    ---
    What would _really_ have been nice would have been if that's what you
    said you wanted in the first place. However, if you can't find a pot
    you can replace the '138 with an HC594/595 and accomplish the same
    thing.
    ---
     
  8. Lord Garth

    Lord Garth Guest

    Here is some reading re digital pots:
    http://pdfserv.maxim-ic.com/en/an/AN1956.pdf

    Looks like the current is always going to be a problem with analog switches.
    Think about the voltage drop across a digital pot...that would be the
    current
    through the pot times the resistance of the pot. You might have to figure a
    way to buffer the signal. You should not allow the signal to exceed the
    power supply limits unless you want the magic smoke to appear.
     
  9. Rich Grise

    Rich Grise Guest

    This makes it actually sound like you're looking for a multiplying DAC
    and buffer.

    Is it a programmable _resistance_ you're actually looking for? Or are
    you making a divider or something, to give a programmable voltage or
    current?

    Good Luck!
    Rich
     
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