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Decoupling wisdom

Discussion in 'Electronic Design' started by Raveninghorde, Nov 16, 2012.

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  1. I read conflicting advice on decoupling.

    frequently I see suggestions that ICs etc should be decoupled by, for
    example, 1nF, 10nF and 100nF in parallel. Is this just a hang over
    from PTH days or is there still some advantage in doing this with
    surface mount X7Rs?
     
  2. Guest

    Three separated by only a decade is nuts. If you have power and
    ground planes one is usually overkill but I generally put one (100nF)
    per just to get the footprints on the board. Larger chips get more,
    as recommended by the manufacturer. Sometimes I'll add a couple with
    a SRF at the frequency of interest if I'm worried about EMI from the
    chip. Caps are cheap, deleting them from the BOM is even cheaper,
    adding them later isn't.
     
  3. Uwe Hercksen

    Uwe Hercksen Guest

    Hello,

    with SMD caps, you dont even pay for additional holes in the pcb.

    Bye
     
  4. Bill Sloman

    Bill Sloman Guest

    It never made much sense. If you need a lot of decoupling, a 10uF
    tanatalum is handy and has the advantage of having enough ESR to damp
    the resonance between the high capacitance and the significant loop
    inductance.

    Back when I was being ultra-careful, we bought 1nF porcelain microwave
    capacitors, which looked capacitative all the way up to 1GHz (IIRR)
    and used them to provide extra by-passing for GigaBit Logic's GaAs,
    but never went to the trouble of finding out if they made much
    difference.

    John Larkin doesn't think that it was worth the trouble, and he's the
    guy who's got recent hands-on experience with logic that runs just as
    fast Gigabit's GaAs did.
     
  5. Tim Williams

    Tim Williams Guest

    Do you put those capacitors, vertically, sideways, on additional boards,
    when you have to route a BGA with 200+ power pins? :)

    Tim
     
  6. Tim Williams

    Tim Williams Guest

    Hmm, possible. Back then, a 1nF was physically smaller than 100nF, so the
    ESL was lower.

    Today, they all fit into the same 0603, even 0402 at low voltages, so as
    soon as Xc --> 0, impedance is dominated by ESR and ESL, which are
    geometry defined. ESL is by length / width, while ESR is by electrode
    resistance (and some from dielectric losses).

    Simulating a crude backplane is illustrative. More caps can actually make
    things worse, with staggered values or otherwise.

    Aluminum polymer caps are generally a bad thing, because their super low
    ESR enhances the resonances due to ESL. Standard tantalums are perfect
    because they tend to damp things, as Bill said. Special low-ESL tantalums
    fall somewhere between alpoly and regular, probably on the low side for
    most purposes. (Save the low ESR caps for where it matters: power
    supplies with big peaky currents!)

    To give you some understanding of the system, the ground plane generally
    looks like a short circuit across its width. It has very low inductance
    between points, and a small capacitance to ground, which dominates at high
    frequencies (over 50MHz or so). The sheer existence of the ground plane
    dominates HF performance, and in many cases you might easily survive
    without any tiny caps at all!

    When you add a bypass cap, it pokes down to the planes with vias. The
    via-cap-via loop is on the order of 2-3nH. This is worsened with
    connecting traces. Via pairs, placed directly aside the pads, with
    minimal trace length, are best. Since Xc --> 0 at high frequencies, any
    cap looks like an inductance, which makes the ground plane's capacitance
    resonate -- *worsening* things!

    When you add a cap with ESR, it acts in parallel with the ground plane C.
    This resistance has to be connected with a sufficiently small ESL, so ESL
    must be very small on the damping components. Thus, you want to use a
    few, scattered about (where doesn't really matter!), and perhaps use extra
    vias with them.

    You could easily distribute 100nF's with 1 ohm chip R's to accomplish the
    same thing, but you might as well take the additional bulk capacitance in
    stride.

    When low noise is desirable (or required!), of course, you'll want a good
    bit of ESL between the noisy PSU and the ground plane. Ferrite beads are
    handy little buggers.

    Tim
     
  7. Guest

    Generally, at least one of the pads needs a via.
     
  8. Tim Williams

    Tim Williams Guest

    They're pretty amazing. The only downside, as far as I know, is they get
    steamed to death. In a manner of speaking. Specifically, inevitable
    moisture *ingress* degrades the dielectric, increasing leakage or reducing
    capacitance or voltage rating, or something like that.

    And as you've noted before, dielectric failure isn't usually a bad thing,
    they tend to self-heal at about twice rated voltage.

    Also, you don't have to pick a 2-3x safety margin on operating voltage,
    like you do with tantalums.

    Still, MTBFs in the >10khr range, for reasonable ambient temperatures
    (<60C, maybe <85C but I forget), at full rated ripple, are nothing to
    scoff at.

    Tim
     
  9. Bill Sloman

    Bill Sloman Guest

    Mine is dated 1989. And I was sentimental enough to ship it out to
    Sydney, despite the major clean-up when we moved countries.
    It was source-coupled logic, very like ECL, and equally greedy of
    power. The data book talks about depletion mode FETS, but says they
    were set up to produce enhancement/depletion mode FETs too. We mixed
    it with 100k ECL - they were compatible.
    Worked fine for us. Of course I'd taken the precaution of improving
    the control loop stability on the Cambridge Instruments/ Metals
    Research GaAs crystal pullers (used to produce 95% of the single
    crystal GaAs made in the West at the time) the previous year.
    Admittedly replacing a 741 with an op amp without pop-corn noise did
    take much inspiration.
     
  10. Robert Macy

    Robert Macy Guest

    might have been thinking about 'stitching' across a boundary. for
    those GPS, Bluetooth, Wifi, zigbee, or GSM or the multi Gbs boards
    which require surface GND planes - well sometimes.
     
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