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Decoupling caps

Discussion in 'Electronic Design' started by Paul Burridge, Sep 10, 2004.

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  1. I recently bought 4,000 1uF SMD caps from e-bay. I can't recall why
    exactly, but they were cheap, anyway and you'd kick yourself if you
    needed to build a circuit that called for 3,500 1uF caps and you
    hadn't bought them!
    Now, seriously, one often sees values for RF bypass caps quoted
    typically at 0.1 or .01uF or similar value depending on whatever. I
    don't see any reason why those values shouldn't be regarded as
    *minimum* for the purpose of decoupling and I should be able to use my
    neat little 1uFs for any RF decoupling circuit cap. It can't cause any
    problems, can it? I assume they only bothered to specify .1 or .01 in
    the days when physical size had to be considered. What's the view of
    the panel?
     
  2. Clarence

    Clarence Guest

    You didn't say if these are Ceramic, or Tantalum. But assuming Ceramic they
    will work fine up to many MHZ. Probably not good above something like 100 MHZ
     
  3. James Meyer

    James Meyer Guest

    Obtain the full specs for your caps and use those specs in your favorite
    spice simulator to look at the equivalent impedance for the caps for the
    frequency of interest. You may find that the caps are excellent RF bypass
    capacitors or you may find that they look like inductors in series with
    resistors.

    There are good reasons why .1 and .01 uF caps are often used as bypass
    caps instead of 1.0 uF.

    Jim
     
  4. John Fields

    John Fields Guest

    ---
    Hmmm... sed, right?^)

    The reason is because once you start to get into high frequencies the
    inductance of the cap (leads, end terminations, _winding_) starts to
    matter and, generally, the higher the capacitance for a given family
    of caps, the higher the inductance of the cap. At low frequencies it
    doesn't matter much, but once you leave the slop of audio behind and
    things start to get important, you'll eventually get to a point where
    instead of soldering in bypass caps what you'll really be doing is
    soldering in peaking coils, where some really interesting things start
    to happen when you hit the self resonant frequency of the cap. Look
    it up. Especially, think about what an aluminum electrolytic with
    _coiled_ plates...
    ---
    ---
    Right. Like you really _need_ an 0.1, but since you can't get one to
    fit you'll settle for an 0.01 instead?

    That's kind of like if you were starving to death and someone handed
    you a shrimp instead of a lobster.
    ---
     
  5. John Larkin

    John Larkin Guest

    Why?

    John
     
  6. They are 0.001 cent cheaper and work as well.

    Best regards,
    Spehro Pefhany
     
  7. Guy Macon

    Guy Macon Guest

    Simple solution: just put ten of those 1uF caps in series, and you
    have the 0.1uF cap that everyone likes!
    (GUY runs away as everyone throws rotten fruit and rock at him...)
     
  8. John Larkin

    John Larkin Guest

    Oh. Right.

    John
     
  9. Jeff

    Jeff Guest

    Because, due to series inductance, etc, a 0.1uf cap is likely good for a few
    MHz, a 0.01uf cap good for a few tens of MHz, etc. Case size (1206, 0805,
    0603, etc) and type (SMD TH) have a huge play in this.
     
  10. What is the difference in series inductance between a 0603 1uF part
    and a 0603 0.1uF ceramic part?

    Best regards,
    Spehro Pefhany
     
  11. Boris Mohar

    Boris Mohar Guest

  12. John Larkin

    John Larkin Guest

    A little more precisely, a 0.1 uF cap is likely worthless below a few
    MHz, a 0.01uF cap is worthless below a few tens of MHz, etc.

    For a MLC of a given case size, series L is pretty much independent of
    capacitance. So the issue becomes, would you rather have, as a bypass
    or coupling cap, a 1 nF cap in series with 0.7 nH, or a 1 uF cap in
    series with 0.7 nH? At very high frequencies, they both reduce to 0.7
    nH.

    A 0.33uF 0603 surfmount cap is a very good coupling cap/DC block in a
    50-ohm system, to 10 GHz maybe.

    John
     
  13. Jeff

    Jeff Guest

    http://rocky.digikey.com/WebLib/Panasonic/Web data/ECJ Series.pdf

    Take a look at page 15, Only the impedance values are given, and it's for
    0805 caps. Note that the ceramic material also has a significant effect on
    impedance. Add in a little trace inductance, and the graphs will shift to
    the left, likely enough to get close to my ballpark #'s.

    http://www.speff.com
     
  14. John Larkin

    John Larkin Guest

    At high frequencies, the impedances converge. You *can* use the SRF
    notch to advantage in narrowband applications.

    John
     
  15. John Larkin

    John Larkin Guest

  16. Carl Ijames

    Carl Ijames Guest

    They don't say but they say many other things for your inference
    Doesn't Fig 5 say that the 1 uF's are better (or no worse) for bypassing
    everywhere except between maybe 30 and 80 MHz where a .001 uF has maybe
    4x lower impedance but the 1 uF is still under .5 ohm? I guess you
    could parallel a .001 and 1, but it looks to me like the 1 uF by itself
    would be just fine.
     
  17. Figures 4 and 5 on on page 3 give a pretty good idea (the latter for
    1206, but whatever).

    Best regards,
    Spehro Pefhany
     
  18. PaulCsouls

    PaulCsouls Guest

    Yes your probably right for decoupling applications but ceramic caps
    tend to get worse as they get bigger in value. The ones upto 1000pF
    are usually the NPOs, then you hit the X7Rs upto 0.1uF, as you
    approach 1uF and above you'll need to compomise down to Z5Us.
    Basically because you need funkier dielectrics for bigger capacitances
    in small packages. In decoupling it probably doesn't matter but if
    your designing a filter it helps to keep the caps small.

    Paul
     
  19. Ray Anderson

    Ray Anderson Guest

    For the most part, the partial inductance contributed by the ceramic MSD
    decap is fairly independent of actual capacitance value for a given case
    size. (i.e. .01uF, .1 uF and 1uF in a 0805 package have about the same
    inductance which is mainly determined by the height dimension). The
    actual mounted inductance is what really matters and is composed of the
    partial inductances contributed by the capacitor, by the escape
    geometries and vias, and the spreading inductance of the planes which is
    a function of the stackup thickness.

    If you have poor mounting technique then the partial inductance
    contributed by the capacitor itself can almost get lost in the noise.
    With proper attention to detail one can achieve a mounting inductance of
    300-400 pH. At this point the capacitor inductance is a significant part
    of the total loop inductance which, with the capacitor's nominal
    capacitance, will determine the parts SRF in use.

    Reverse geometry capacitors (0306 as opposed to the more common 0603 and
    0204 etc.) have considerably less mounted inductance than their more
    common counterparts for a couple of reasons. 1.) Less intrinsic
    inductance due to internal construction 2.) Lower mounting inductance
    due to the smaller current current loop encountered in the mounting process.

    Someone quoted an Agilent app-note near the beginning of this thread
    that addressed the measurement of capacitor inductance. Not to cast
    aspersions on Agilent, but the methodology and fixture they supply for
    the impedance meter as discussed in the app note does not produce very
    accurate or repeatable numbers (particularly ESR). There are much more
    accurate ways of doing the measurements by utilizing a VNA to do S21
    measurements.

    -Ray

    -Ray
     
  20. Clarence

    Clarence Guest

    There are many considerations for using a cap in a filter. I prefer film types
    at low frequencies, and Mica for RF. But there are always exceptions.
     
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