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Intermittent A/D converter

Discussion in 'Electronic Design' started by Richard Henry, Mar 27, 2007.

  1. To save power in a remote device, I want to turn on a circuit
    containing an A/D converter once every few seconds only. However, I
    can't find specs for how soon after powering up an A/D its outputs
    would be valid. I am interested in a low-power device (naturally), 8
    bits out, 4 MSPS or better, with max input voltage range about 2-3
    volts. Any suggestions?
     
  2. The very first part I looked at in your ball park was specd.

    http://www.analog.com/UploadedFiles/Data_Sheets/AD7276_7277_7278.pdf

    One microsecond from power down. What where you looking at?

    Best, Dan.
     

  3. Good find, except the serial interface means I will have to run some
    of the connected circuitry about 8 times faster than I want.
     
  4. tkirk

    tkirk Guest

    Based on your comment about the micro, do you actually need 4 MSPS?
    Are you powering up, taking a lot of samples, and then powering down,
    or are you just taking a few samples each time you power up? TI and
    Microchip both make some great little low power A/Ds if you only need
    10 bits or so and only a few KSPS.

    Often times, if you are also powering down your analog circuitry (op-
    amps and the like), they will take far longer to stabilize than the A/
    D itself. At the company I work for, we do extremely low power
    embedded designs, and have used this trick ourselves, completely
    turning off power to all of the analog circuitry including the A/D
    when not in use. It sometimes takes several tens of milliseconds for
    everything to re-stabilize after reapplying power due to the
    capacitances in the analog circuitry and the op-amp chips. The A/D
    usually has been powered up and ready long before the power to it has
    stopped ringing and fully stabilized enough to take accurate readings.
    Your best bet is to put an oscilloscope on the power pin and reference
    of the A/D and see first of all how long it takes that to come up and
    stabilize. Chances are that will actually be the deciding time for
    you, more so than the A/D spec or how long it takes to do a few
    samples before powering back down.

    Of note, if all you are powering down is the A/D, you might want to
    look into getting an A/D with a built-in sleep mode rather than
    completely pulling power. Several manufacturers such as Linear Tech
    are now making A/Ds with sleep mode currents less than 2 uA, and
    exiting sleep mode will almost always be faster than waiting for power
    to stabilize after turning it on.

    Good luck!

    Tim

    www.rogue-engr.com
     
  5. It's a remote battery-powered device that powers up, listens for a
    signal, then responds if addressed or goes back to sleep if nothing is
    there. The A/D is not needed if it just going back to sleep, only if
    it has to read a message and respond.

    I realized there would be some stabilization time after turning on the
    A/D. My difficulty is very few parts have a declared specification
    for it.
     
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