Date: Mon Nov 13 2000 - 06:40:30 PST
My approach is to put bypass capacitors as close as possible to the power pins
on a chip. Whether we are using a multilayer card with ground and power planes,
or a double-sided card with ground gridding, we will almost always have more
connections to and more copper for ground than for any one supply voltage. Thus
the path from a bypass capacitor to ground is usually shorter and wider than the
path to a supply voltage. This results in:
* Faster response by the capacitor, due to shorter transit time in the
microstrip/stripline between the chip and the
capacitor(s)--about 1/6 ns per inch for FR-4 boards.
* More return-paths to the chip for the transient current, which reduces the
inductance, impedance, L * dI/dt drop, and
maybe radiated emissions by the magnetic fields of the various paths
partially cancelling one another.
* Smaller loop area for the transient currents, ditto.
Since transient currents are my major concern, I try to put the highest
frequency-response capacitors (typically 220pF NPO ceramics for clock and
phase-locked loops (PLL's)) right next to their corresponding power pins. Then
I put lower frequency-response capacitors (typically a 100nF X7R ceramic for
each power pin or cluster of power pins) as close as possible to their
corresponding power pins. Next I consider how to route traces/vias to bring
power and ground to the power-pin/ground-pin/bypass capacitor cluster. If I can
I will bring in power and ground right next to each other, but I don't worry too
much if the power and ground connections are on opposite sides of a cluster.
Depending on the complexity and package size of the chip, I will also put 1 to 4
bulk ceramic capacitors (typically 2.2uF Y5V ceramics) on each supply voltage
within 1 inch of its power pins, trying to "surround" the chip. If I have a
bunch of small chips in a small area, I may use just one bulk ceramic capacitor
for the entire clump.
Finally I put bulk aluminum electrolytic or tantalum capacitors (10uF and up):
* Near the power connector(s) to a card.
* Near power connectors to other cards/devices.
* Near the corners of the supply-voltage domain.
* Near "power hog" components, trying to have one within 2 inches of every
power pin on that supply voltage ("coffee
John Barnes Advisory
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