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How to provide analog power

Discussion in 'Electronic Basics' started by Markus Zingg, Nov 2, 2006.

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  1. Markus Zingg

    Markus Zingg Guest

    Hi all

    This is most likely a dumb question, but :)

    I do have a digital design requiereing 3.3 and 5v supply. One of the
    chips however is a mixed signal kind and requieres analog ground and
    analog power (also 3.3v). The external voltage I can use is 12v and
    apart from this mixed signal chip, I would have used two voltage
    regulators in series, one regulating down to 5 and from there to 3.3v.
    Overall power consumtion is < 500mW. Can I now simply use a third
    regulator to say regulate the 5v once more down to 3.3 and use it for
    the mixed signal analog supply pins? Is this the way this is usually
    done?

    TIA

    Markus
     
  2. Tim Auton

    Tim Auton Guest

    The purpose of the separate supply is to keep digital switching noise
    out of the analogue section. Voltage regulators alone generally aren't
    all that great at eliminiting this kind of noise, so just adding a 3.3V
    regulator to your 5V digital supply (which will have a good bit of
    digital noise) may not be the best solution.

    How much effort you need to go to depends on how clean you need your
    analogue supply, which depends on the size of the analogue signals and
    the accuracy you need. Can you give us any idea? If there's an ADC
    involved, the number of bits of resolution would be a useful indicator.

    Simply decoupling your existing 3.3V supply with an LC filter may be
    good enough, this is recommended in datasheets by microcontroller
    manufacturers and would be OK for, say, 3V signals and a 10-bit ADC. A
    series inductor from the 3.3V regulator to the analogue supply pin of
    your IC, and a capacitor between the analogue supply pin and ground.

    If that's not enough (say, you have a 16-bit ADC), then a separate 3.3V
    supply with its own regulator would be useful. If your 12V supply is
    fairly clean (no PWMed motors, no switching regulators...), run the
    regulator from that. If the 12V is particularly noisy, run the 3.3V
    regulator from your 5V. Either way, use an LC filter on the input to the
    3.3V regulator to keep the digital noise at bay.


    Tim
     
  3. Chris

    Chris Guest

    Hi, Markus. Start with these Analog Devices appnotes of general
    interest:

    AN-202: An IC Amplifier User's Guide to Decoupling, Grounding, and
    Making Things Go Right for a Change

    AN-345: Grounding for Low-and-High-Frequency Circuits

    AN-214: Ground Rules for High Speed Circuits

    http://www.analog.com/en/dcIndex.html

    Your question is kind of application specific, and everything depends
    on the details.

    If you've got analog ground sliding through digital ground, with trace
    inductances (high speed) and trace resistances (precision DC voltages),
    this can cause all kinds of problems. Or not -- it depends entirely on
    you app and what you're trying to accomplish.

    Feel free to post again with more information.

    Good luck
    Chris
     
  4. Markus Zingg

    Markus Zingg Guest

    Hi Tim (and Chris)

    Thanks for your reply(s). The mixed signal part is acutally an image
    sensor (should have mentioned that in my first post) and the ADC seems
    to be 10 bit (at least that's what I get out of it on it's digital
    end) . So, the LC filter should be sufficient - right?

    Markus
     
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