Terminology - Decoupling or Bypass Capacitors?

[Mike Switch]

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?

Mike

 

[John Popelish]

Mike Switch wrote:

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?

Mike

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.

--
John Popelish