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Digital and Analog power decouplings

Discussion in 'Electronic Design' started by linnix, Mar 20, 2007.

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  1. linnix

    linnix Guest

    Considering the three uCs:

    1. Lmi - no decouplings
    2. Avr - power decouplings
    3. Nxp - power and ground decouplings

    Question #1:
    Can I assume that Lmi has internal decouplings between Digital and
    Analog circuits?

    Question #2:
    Is ground decoupling (in addition to power) really necessary for 10
    bits Analog?

    Question #3:
    If I have to choose one, would it be better to:
    1. Power to Digital and filter to Analog
    2. Power to Analog and filter to Digital
  2. Joerg

    Joerg Guest

    I don't know this series but how would they internally decouple? On a
    chip you can't create any caps to write home about. I guess they assume
    that the circuit designer maintains good bypassing.

    No. I never split ground unless it must be done for safety reasons such
    as patient isolation. But this is my own humble opinion and almost
    guaranteed to set off a huge political debate within the next few
    milliseconds ;-)

    #1 is the prevalent method. But it depends a bit on what consumes the
    most. So if you had analog stuff that uses 500mA and one uC that
    consumes 2mA then it would be #2, of course. Actually, I am doing that
    right now on a circuit that is 95% analog but needs a bus interface that
    consumes very little. On this one the analog rails must be super squeaky
  3. linnix

    linnix Guest

    Unless they have very good fab process to build L & C on chip.
    Or they didn't think it was necessary.
    Or a design oversight.
    This could potentially push me over to the NXP.
  4. Joerg

    Joerg Guest

    L and C on a chip both cost oodles of real estate and for decoupling
    purposes they would be prohibitively expensive. Usually the only L and C
    you can find on chips would be on RF circuits in the GHz range and then
    you'd be talking about very few nanohenries and a few picofarads (but
    usually even femtofarads).

    Well, I wouldn't expect stand-alone performance from an AD converter on
    a uC.

    Most certainly not. Typically more of a compromise because people often
    want the lowest possible pin count and also because a larger package
    will cost more.

    Always welcome.
  5. linnix

    linnix Guest

    Without power filtering, 10 bits might be meaningless.
    But there are four power pins and four ground pins, they could have
    spare one for analog power. Avr has 2 digital power pins, 1 analog
    power pins and 3 ground pins.
  6. Joerg

    Joerg Guest

    With good bypass caps I've seen 12 bits work just fine.
    Then they could indeed have split the power pins. Who knows, maybe the
    converter is in an area on the chip where it wouldn't have helped much.
    But that's hard to say without mask pictures.

  7. Eeyore

    Eeyore Guest

    It's virtually essential in audio gear where clocked digital circuitry is also
    present. You simply can't allow the digital supply current to flow in your nice
    clean audio ground.

  8. Joerg

    Joerg Guest

    Even there I never did it and I have designed rather noise sensitive
    Doppler systems where you mix right down to baseband for I/Q audio
    detection. I did encounter a few system that others had designed with
    split grounds. Most didn't perform. Others miraculously did (not perfect
    but the marketeers liked it enough) but then failed the EMI cert, big time.
  9. Rich Grise

    Rich Grise Guest

    Just from lurking here for some years, I've come to realize that the
    consensus seems to be that split planes aren't needed, and sometimes
    more trouble than they're worth.

    What's needed is reasonable care when designing the power circuitry. Call
    it "intelligent design", maybe. :) What you need to do is watch where
    your return current paths go, and see to it that they don't actually share
    any copper (or as little as possible).

    Sometimes just hanging out can be an education. :)

  10. krw

    krw Guest

    L's cost oodles. C's are done on chip all the time. All of our
    whitespace was filled with decoupling C's. Of course caps are added
    to the packages too. When you're switching many amps...

    The analog PLLs had separate AVdd pins too.
  11. Joerg

    Joerg Guest

    Especially right before the next elections :)))
  12. Joerg

    Joerg Guest

    But you won't likely get enough pF to write home about. When a bunch of
    comparators are clanging around in an ADC there needs to be some serious
    capacitor close by to take the brunt.

    Once when I redesigned a pre-amp on an RF chip I could have really,
    really used a fraction of a pF to get rid of a spike. So I went to the
    guy that was in charge of chips at that client and politely asked
    whether I can have that little corner lot over there. "Nope".

    That is a smart thing to do.
  13. Filter to the lower energy/smaller node, which normally means option 1.

    When you say decouplings, do you mean Separate Analog/Power Pins ?

    What's needed depends also on if you use the internal Vref, or external
    Vref, or do relative to Vcc.

    A well specified device will give Power Supply Rejection ratios, for
    the analog sections.
    Some spec differing LSB errors, on the physical pin locations -
    expect the channels nearest GND pins to be better.

    More important, is does the vendor specify a MAX, or only typicals?
    ( and at what Vcc's )
    The better suppliers will spec their ADCs properly, the companies that
    'add the tickbox', have lower standards.

  14. linnix

    linnix Guest

    Yes, need separate pins to have external filter.
    No external Vref either.
    I think they just forgot to hire an analog consultant before making
    the chip.
  15. krw

    krw Guest

    Lotsa nFs (don't remember exactly) in reverse biased junctions. The
    slew rate of the comparitors will be less so you would need quite a
    bit more C.
    There was no white space on the chip. Any whitespace we had was
    automatically wired as decoupling. Warnings were generated if C got
    below some threshold.
    Learned (and relearned) the hard way. Some parts called out a low-
    pass filter on AVdd. I could never figure out why each part was
    different (other than different engineers).
  16. I have to agree with Eeyore. The common current paths and the ground
    loops can be a big problem in the audio. It requires careful attention,
    and it doesn't take much to ruin the performance. One solid ground plane
    can't be used as the power and signal return and the zero reference at
    the same time, unless all of the signals are differential.

    There is a technique to place the most noisy (or the most sensitive)
    parts of the schematic on the isolated island of the ground plane. All
    connections are done through the one bridge to this island. Thus the
    EMI, the noise currents and the ground bounce are restricted to the
    area. I had to do it that way a couple of times; however this technology
    is the overkill in most of the practical cases, and it can do more harm
    then benefit if used unproperly.

    Vladimir Vassilevsky

    DSP and Mixed Signal Design Consultant
  17. John Larkin wrote:

    This is not about what is right or wrong. This is about the numbers. The
    stray impedance is in mOhm/nH range. This can or can not be a problem,
    depending on many other issues at the system and board levels.
    I have the application where I have to pick the narrowband signals from
    ~50nV with the dynamic range up to 150dB. There is also DSP, LCD, SMPS
    and audio output. It would be difficult to accomplish that with just one
    ground plane.

    Vladimir Vassilevsky

    DSP and Mixed Signal Design Consultant
  18. John  Larkin

    John Larkin Guest

    I do NMR gradient amps with nanovolt shunt amps, uP, fpga's, switching
    regs, display drivers, and power output stages all on a single ground
    plane. Noise is a few PPM from 0 to 50 KHz, and the noise is all

    I just did a combination gradient amp and thermocouple-based
    temperature controller on a single board, single ground plane, and it
    worked fine first time. Passed UL/CE/FCC, too.

  19. Eeyore

    Eeyore Guest

    I should hope so !

    With out without pre-compliance testing btw ?

  20. Rich Grise

    Rich Grise Guest

    I do NMR gradient amps with nanovolt shunt amps, uP, fpga's, switching
    regs, display drivers, and power output stages all on a single ground
    plane. Noise is a few PPM from 0 to 50 KHz, and the noise is all thermal.

    I just did a combination gradient amp and thermocouple-based temperature
    controller on a single board, single ground plane, and it worked fine
    first time. Passed UL/CE/FCC, too.

    Yeah, but you're a Leprechaun, and so have magickal powers. ;-)

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