Re: [SI-LIST] : bypass cap question (long, simple)

Haruny Said (harunys@haselect.com)
Sat, 31 Jan 1998 11:05:05 -0500

Lawrence,

after I read you message a couple of times I decided what you said does
make sense. I look at the problem as follows:

You are connecting two transmission lines together, one side of the
lines are connected by the via, the other side of the lines are connected
via the inter-plane capacitance, Ci, between the power, ground and other
planes.

This connection will cause an impedance mismatch because Ci will be in
series with the two lines. For the sake of argument let's say you want to
keep the mismatch within 10% of the characteristic impedance of the line.
If the capacitive impedance of Ci at the low frequency is more than 10% of
Zo then you need to add a capacitor at the via. Otherwise a capacitor isn't
required.

I would choose the capacitor as follows: the capacitor's impedance (taking
into account series L and R etc) in parallel with Ci would need to be less
than 10% of Zo at the low and high frequencies, and in your example I would
take the high frequency as being 500MHz - 1GHz, i.e. 5 - 10X 100MHz.

To calculate Ci I would take into account the area around the via whose
radius
is given by the propagation time that is equivalent to the rise time of the
signal.

The 10% is plucked out of the air, the value you choose would depend on
how much mismatch your design can tolerate.

If there are 40 wires then it gets a little more complex, it depends on
how the wires are placed but I think the same argument still applies.

Does this make sense, or does anybody strongly disagree? I know there is a
lot of rule-of-thumb in this answer but I don't think there is a perfect
solution.

If you want I can sketch something on paper to make it clearer and fax it
to you.

At 12:58 AM 1/31/98 -0800, Lawrence Butcher 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?
>
>
>The above illustrates a real problem. I am building an 8-layer board,
>with a tentative stackup of:
>
>1 Horizontal
>2 GND Plane
>3 V33 Plane
>4 Vertical
>5 Horizontal
>6 V5 Plane
>7 GND Plane
>8 Vertical
>
>My component placement places all of the 3 volt components above the
>midline of the board, and all of the 5 volt components below the midline.
>
>Therefore, there are NO bypass caps from the 5V plane to ground in the
>top half of the board, and NO bypass caps from the 3.3V plane to ground
>in the bottom half of the board.
>
>A trace running horizontally on layer 5 in the top half of the board
>will have an image current running on the V5 plane, and that current
>has no way to get to the ground plane at a via site. Same for traces
>running on layer 4 in the botton half of the board.
>
>
>I want to add about 1 cap per square inch (about 50 more bypass caps).
>Half will be between V5 and ground in the top half of the board, and half
>will between V33 and ground in the bottom half of the board.
>
>These components are there only to deal with my paranoia about image
>currents. They would be placed even though there are already tons of
>bypass caps in the same area, but exclusively to only one power plane.
>
>My coworkers have doubts. They especially don't like 50 more caps when
>there are no nearby chips connected to the power planes I am concerned with.
>
>Most of my signals are changing at 100 MHz, but there is a bunch of 33 MHz
>activity running around. (Fast, for me).
>
>Is this a non-existent problem, or a real one? Comments?
>
>Lawrence
>
>
>
>
Haruny Said
H.A.S. Electronics, Inc.
33 Boston Post Rd West
Suite 270
Marlborough
MA 01752

Email: Harunys@haselect.com
WWW: http://www.haselect.com
Phone: (508) 624 6227
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