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Terminology - Decoupling or Bypass Capacitors?

Discussion in 'Electronic Basics' started by Mike Switch, Jul 12, 2003.

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  1. Mike Switch

    Mike Switch Guest

    I'm sure many on this newsgroup are aware of the importance of using
    capacitors from pwr supply to gnd on digital logic integrated
    circuits. I've heard them refered to as bypass caps, despiking caps
    and decoupling caps - but I'm not sure if decoupling is the correct
    terminology in this situation; from my understanding, decoupling caps
    are used to block DC in a circuit. Is this term mis-applied in this
    situation and if so, how did it come to such common usage?

  2. A decoupling capacitor is intended to act as a filter that keeps
    energy of some frequency range from one circuit section from leaking
    into and interfering with another circuit section. If you think of
    the rectifier ripple energy as a range of frequencies from one circuit
    section, then the energy storage capacitor in the supply is a form of
    decoupling capacitor. A bypass capacitor, while it may produce
    decoupling effects, is intended to act as a low inductance source of
    charge that supplies a circuit section or device that draws current
    from its supply in bursts, rather than in a steady way. Without this
    bypass, the inductance of the paths back to the supply storage would
    produce large voltages that would interfere with the operation of the
    circuit, whether or not this noise caused problems with any other
    circuit sections. So bypass caps are focussed on the needs of an
    individual customer, while decoupling caps block signals between two
    or more customers. The caps may be identical or even be involved in
    both tasks. The names (bypass and decoupling) refer to the tasks, not
    to different kinds of caps.
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