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When should decoupling capacitors be used.

Discussion in 'Electronic Basics' started by hybrid_snyper, Jan 17, 2007.

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  1. Hi all,

    A quick question when and where should i use decoupling capacitors.
    Circuits that i have built at uni have always been giving to us and
    there are often these decoupling capacitors but it is never be
    explained why they are used and when they should be used. Could anybody
    shine a light on this subject?

    Thanks

    Wayne
     
  2. DJ Delorie

    DJ Delorie Guest

    While there are meny exceptions and special cases, in general,
    decoupling capacitors are added on each power pin on each IC. Their
    purpose is to provide "local burst power" for the chip. Example: if a
    chip switches its output in 1nS, there's going to be a sharp spike of
    current needed by the chip to do that change. At those speeds, the
    wires leading to the chip act like inductors, which block those kinds
    of spikes. The capacitor provides the short burst of current needed
    to accomodate that current spike. It also keeps the spike from
    "spreading" to other parts of the circuit (it's like an LC low-pass
    filter).

    So, in general, an 0.1uF cap at each power pin of each chip. Keep
    them as close to the pins as possible, keep lead length as short as
    possible. Sprinkle a few 1uF caps around the board to decouple the
    board itself, and a few 10uF caps near where the power comes into the
    board to act as "bulk" capacitors.

    For particularly fast chips, you'd put an SMD 0.01uF cap right at the
    power leads, too.
     
  3. By sprinkle you mean put them anywhere as well as the source pin on the
    ICs. Will this not affect the circuit in other ways.
     
  4. PeteS

    PeteS Guest

    The whole point of decoupling is to decouple noise and droop. This is a
    simple subject that has complicated issues and is covered regularly.

    Search s.e.b, s.e.d and comp.arch.embedded for the subject for a number
    of threads.

    Cheers

    PeteS
     
  5. DJ Delorie

    DJ Delorie Guest

    Well, I did say "in general" but yeah, I meant in addition to power
    pin caps.

    In my case, for example, with two-layer boards I use large copper
    fills on the back for the power and ground nets. I like to put one
    "sprinkled" 1uF cap for each copper region, so if there's only a tiny
    bridge connecting it, it will still have its own bulk decoupling.

    The idea being that, as you group things together into larger units,
    you decouple that unit with the next size up cap. 0.1uF per pin, 1uF
    per group, 10uF per board, etc. Using different size caps also avoids
    problems with ringing, since you're avoiding having a "tuned circuit"
    due to everyting having matched capacitances.

    Each step decouples a higher frequency than the previous, and provides
    power to the next.
     
  6. Gotcha, didnt know about grouping and using larger capacitors. Thanks
    for that
     
  7. Brian Ellis

    Brian Ellis Guest

    There is a pretty good article on that at
    http://www.seattlerobotics.org/encoder/jun97/basics.html
    As stated earlier, sometimes it is used as a current pool. When current
    passes through a pc trace, it will generate a voltage (due to it's
    impedance). You use the decoupling capacitor to short this out (or bypass it
    to ground), so that it won't inadvertently toggle another circuit. Some
    circuits are more sensitive than others to these types of noise. Also, some
    types of circuits will generate more noise than others. There are rules of
    thumbs to go by, but sometimes it is more of an art (developed through years
    of experience).

    Brian Ellis
     
  8. Brian Ellis

    Brian Ellis Guest

    Here are a few things that will help decoupling problems:

    Ground returns are very important. Keep the ground returns for power
    circuits separate from the low level sensitive circuits ground returns. The
    lower the number of components on each ground return, the better. The wider
    the ground return, the better. When possible, have each ground return go
    back to the ground of the main filter capacitor. The shorter the ground
    return, the better.

    Counters and low level logics are usually more sensitive to noise, so
    generally need more decoupling and better ground returns. Counters will also
    generate more noise on the ground return.

    Keep high power, analog and digital circuits as far away from each other as
    possible. If they are all on the same circuit board, then their grounds must
    be run very carefully (usually with a lot of decoupling).

    I'm sure the others can add a lot to this.

    Brian Ellis
     
  9. feebo

    feebo Guest

    prolly gonna get shot to bits on this one but here goes...

    on the power rails of *anything* switching... as close to the power
    pins of whatever it is you are decoupling as possible.

    In a perfect world, logic is a nice clean 0-1-0 swing, but in reality
    they are still analogue device and with TTL especially, the inrush
    currents when the transistor saturate can cause dips and spikes on the
    supply rails... very short, but they can get picked up by other
    devices and cause malfunctions of your circuit. the correct D-cup
    ahould remove normal glitches like this - you can still get rogue
    chips or batches that will wallop the power whatever realistic
    capacitor you use - these should be located and junked but this is
    usually as part of test dammit! >:eek:(

    OK.. I know this is a tad extreme, but you can't really have too many
    and with 10nf at $0.002 (in suff. qtys) if you have room you should do
    it.

    Old style TTL was really horrible and used to spike the rails all the
    time. I rememeber s100 bus boards with rows and rows of the stuff and
    every one had a 10nF at the end - you can get capacitors that fit the
    standard 14 & 16 pin (more?) DIL footprint and fit underneath chips...
    not sure if this is a good idea - obviously the density goes up but
    it's more hassle if you get a failure.
     
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