Applicable to:
1- or 2-sided boards or multi-layer
boards that do not employ planes for power
distribution
General Guidelines
-
Provide at least one "local"
decoupling capacitor for each active device
and at least one larger "bulk" decoupling
capacitor for each voltage distributed on
the board.
-
Local decoupling capacitors
should be connected between the voltage and
ground pins of the active device. the area
of the loop formed by the capacitor/device
connection should be minimized.
-
Local decoupling capacitors
typically have nominal values of 0.001,
0.01 or 0.047 microfarads. Some active
devices may require several local
decoupling capacitors in order to respond
to a sudden demand for current. (See: "How
much decoupling capacitance do I
need?")
-
Bulk decoupling capacitors
should be located near the point where a
voltage comes on to the board. If the
voltage is generated on the board, the bulk
decoupling should be near the location
where it is generated.
-
Bulk decoupling capacitors
should be sized to meet the transient
current needs of the entire board. (See:
"How much decoupling capacitance do I
need?") Typically, bulk decoupling
capacitors have values equal to 1 - 10
times the sum of the values of the local
decoupling capacitors connected to the same
bus.
-
As a general rule, two local
decoupling capacitors with the same nominal
value are better than one capacitor with
twice the nominal value. Two capacitors
have a lower overall connection inductance
[1] and provide better high-frequency
filtering to the rest of the power bus
[2].
Examples
The figure below shows various
examples of good local decoupling capacitor
connections to boards without power planes.
References
[1] C. R. Paul, "Effectiveness of
Multiple Decoupling Capacitors," IEEE Trans. on
Electromagnetic Compatibility, vol. 34, no. 2, May
1992, pp. 130-133.
[2] T. Zeeff, T. Hubing, T. Van Doren and D.
Pommerenke, "Analysis of Simple Two-Capacitor
Low-Pass Filters," IEEE Transactions on
Electromagnetic Compatibility, vol. 45, no. 4, Nov.
2003.
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