> > Lawrence Butcher <lbutcher@parc.Eng.Sun.COM> wrote:
> > Imagine that I build a 4 layer board. Imagine that there were two
> > chips on it, labeled U1 and U2. Imagine that I route the board
> > strictly manhatten style. All horizontal wires are on top above the
> > ground plane, and all vertical wires are on the bottom below the power
> > plane.
> > _______________
> > | |
> > | U1 ------* |
> > | | |
> > | | |
> > | | |
> > | U2 |
> > |_______________|
> > Normally, I would put bypass caps under U1 and bypass caps under U2.
> > I would cosy them up so that there was minimum distance between the
> > caps and the power supply pins on the chip.
> > Consider the image currents running on the power and ground planes.
> > An image current will sit directly under each wire. But that current
> > will have a hard time following the wire through the via, because it
> > would have to hop from the ground plane to the power plane.
> > It seems clear that a capacitor might be needed at that via site to
> > give the current in one plane a chance to hop to the other. Even
> > though there are no components nearby.
> > Intuition rarely substitutes for calculation. Question: Is this true?
> > How much capacitance? How does that vary if there are 40 wires instead
> > of 1? How does the number change with frequency?
> Dr. Frank Yuan <email@example.com> replied:
> First, the image current on power or ground plane sit directly under the
> trace will run in opposite direction as the trace current. Therefore
> U1 send trace current out, the image return current on the plane flow
> right back towards U1, and go through bypass caps under U1 to power
> This is why bypass caps should be, as you said, very close to
> pins on the chip. Seems to me no capacitors are needed at the via site
> over the board just for return path. However, de-caps may be need evenly
> over the board to supress power/ground noises (SSN) on the planes.
> Again accurate calculation helps a lot in answer those questions. An
> simulation tool call AC_GRADE from Viewlogic (Formerly Quad Design)
> does just that in analyze power and ground planes and the effects of
> decoupling capacitors.