Efficient Use of Decoupling Capacitors

Discussion in 'Analog & Mixed-Signal Design' started by Abbas_BrainAlive, May 17, 2018 at 7:38 AM.

  1. Abbas_BrainAlive

    Thread Starter New Member

    Feb 21, 2018
    15
    3
    Hello all!

    I could not figure out yet why it is recommended to use different values of capacitors for decoupling in any (analogue/digital/mixed-signal) system; be it the decoupling of power pins, or the reference pins or any other pin.

    For suppressing transients, capacitors are needed, bypassing that pin to ground. Why is it recommended to use different values of capacitors, some small some large, for the decoupling? Isn't it so that the higher the decoupling capacitance, the more powerfully it suppresses the transients, along with RF and other noise? Wouldn't it be better that instead of instead of using different values of decoupling capacitors, we use all the capacitors of the same (highest) value, which would have a higher equivalent capacitance on the same amount of (precious) board space, which would ultimately lead to better transient performance per unit of board area?

    It would be really appreciable if someone could explain why these different values are used for decoupling, and what should be these values in a typical system?

    Thanks in advance.
     
  2. ericgibbs

    Moderator

    Jan 29, 2010
    4,464
    864
    hi ABA,
    This PDF shows how to select the 'best' capacitor for particular project.
    E
     
    shortbus likes this.
  3. dendad

    Well-Known Member

    Feb 20, 2016
    1,499
    371
    Some caps work best at various frequencies so a range of caps will bypass a wider range of frequencies.
    For instance, electros, because of their construction, have pretty poor high frequency response so an electro with a parallel ceramic that has a high frequency response is a better option. I generally use Electrolytics in the power supply and then Tantalum caps and ceramics in the works.
     
  4. danadak

    Active Member

    Mar 10, 2018
    641
    130
    ericgibbs likes this.
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