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Power supply bypassing, again

Discussion in 'Electronic Design' started by [email protected], Jun 12, 2007.

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  1. Guest

  2. Rich Grise

    Rich Grise Guest

  3. Guest

    I don't know, but his "simulation" of what's required for our product
    looks a lot like the example in the PDF. I wonder if I can have a
    business model like that?
     
  4. John Larkin

    John Larkin Guest


    "Every capacitor has a narrow frequency band where it is effective as
    a decoupling capacitor."

    What a doofus. His example, top of page 8, says it all.

    As I mentioned, we use three or four identical, 0.33 uF maybe, bypass
    caps per FPGA per supply voltage, 9-12 caps total for a Spartan 3. It
    always works.

    John
     
  5. Guest

    Yup, I can't say I follow his logic. The effectiveness of a decoupling
    cap also depends on the current required. Seems to me at least. If I
    need 1A at 10MHz or 1mA at 10MHz, I'd guess the same value cap won't
    be doing the same thing.
    Yes but I'd say that depends heavily on what you're trying to do with
    the thing. If you're strictly in the "digital" domain I suppose it's
    enough, but if you're looking at the outgoing waveforms of a LVDS
    transmitter, surely the quality of the power supply is paramount.
    Perhaps a "1" will always be seen as a "1" by a digital receiver
    expecting the worst, but what if you're looking at it with a high-
    bandwidth analog scope? Betcha the waveforms won't look the same
    depending on the amount of power decoupling. This is our problem.
     
  6. John Larkin

    John Larkin Guest

    Look at Figure 7. It shows the six caps as having monotonically
    greater impedances at 1 GHz, with huge differences, even though the
    esl's aren't very different for the four ceramic caps. That is crazy.

    An 0603 cap has an inductance that changes little with capacitance
    value. So above the srf, the z's will be about the same, regardless of
    value, converging at high frequencies. That's not what he shows.

    Just use biggish 0603 caps, 0.33 uF maybe, and tight power/ground
    planes. Four caps per supply maybe.

    This lunatic is suggesting something like 150 capacitors PER FPGA. How
    can you route around 300 (or 600!) vias? He should be committed to a
    nice institution where he can get the help he needs.

    John
     
  7. Guest

    I don't like how he makes it look like it's the planes demanding the
    current. He shows cute little current loops looping around the cap and
    the plane... Shouldn't he also include the path in to the device doing
    the actual demand? Include the bond wires and die? But that would blow
    his idea out the door, since typically you can't really know what's
    going on in there, therefore you can't get a ESL number.
    I had to. Use 3.5/3 rules to route between vias. It wasn't such a big
    deal.
    I've taken up kayaking. Since I'm mostly a cyclist, my legs get a rest
    and I feel that I remove far more frustration by rowing, since my
    upper body is comparatively weak so I feel it more. Plus I get to do
    and see something different!
     

  8. He's got that "quantity" column... I think each line is the *combined*
    (parallel) impedance for each of e.g. 32 x 2n2, 16 x 22n, 8x220n,
    etc. If you look at the intercepts on the left hand "inductance" axis,
    and assume he means "impedance", then it works out about right.

    I can sort of make sense of it if you assume his capacitance values
    are the largest practical in each package. He is trying to build a
    ~1000uF capacitor that has ~0.01 ohm impedance over a wide frequency
    range.
     
  9. John Larkin

    John Larkin Guest

    Imagine that Zetacap Inc makes a 10 farad, 0603 ceramic cap. Its SRF
    will be about 2 KHz. Is it useless as a bypass above that?


    We did one gadget that used a Xilinx FPGA, with our bypassing style.
    One signal path goes in/out four passes. Net jitter on the final
    outputs is below 30 ps RMS.

    But if my competitors have to go to 10 layers and 3 mil traces and use
    1000 caps per board, why should I object?

    John
     
  10. This is exactly why I went through the exercise of trying to
    come up with effective ways to layout bypass capacitors
    feeding chip pads. This articles examples of isolated caps
    tied just to planes is silly and pointless.
     
  11. Guest

    At what bit rate? Can you divide that into random and deterministic
    jitter numbers?
    Because we can do 3 ps jitter? :) Relax, we're not competitors!
     
  12. Joerg

    Joerg Guest

    Are you guys still simulating that stuff? Seriously, I'd stop that and
    get the layout rolling. Much of that staggered cap stuff is voodoo.
    Doubling everytime you go to the next value down etc., oh man...

    Yes, it can help to provide a 3300pF in parallel to a low cost 0.1uF cap
    but that's about it IMHO.
     
  13. John Larkin

    John Larkin Guest

    It was mostly deterministic, in the sense that one clock was clearly
    modulating the prop delay of the timing paths that run on a completely
    different clock.
    3 ps with ECL is easy. CMOS is trickier, if only because Vcc modulates
    prop delay.

    John
     
  14. Jon

    Jon Guest

  15. John Larkin

    John Larkin Guest

    That is getting closer to reality, but it still ignores the elephant
    in the kitchen, namely the planes themselves.

    John
     
  16. Guest

    Well the planes are full of holes (literally) once you add the vias
    for the Xilinx approach, which IMHO makes them less ideal and less
    negligeable.
     
  17. Joerg

    Joerg Guest

    ROFL! Great. Almost like some "modern" software. You fire up the
    greatest of all programs only to find out that it didn't leave enough
    RAM to do anthing with it.

    Anyhow, heck, I'd just lay the thing out and see what shakes out.
    Probably you could have had several layouts done for the time and money
    that went into this whole study.
     
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