High Frequency Measurements
Douglas C. Smith
Address: P. O. Box 1457, Los Gatos,
Technical Tidbit - May 2007
Placement of Parallel Capacitors on a Printed Wiring Board
(for troubleshooting or a quick fix)
Circuit Board With Added Part Stacked Over Existing Capacitor
Abstract: Capacitors are
sometimes added in parallel to an existing capacitor on a printed
wiring board to improve the high frequency characteristics of the
existing capacitor for troubleshooting purposes or for a temporary fix.
Ways of mounting the added capacitor are discussed and recommendations
made. The most popular mounting method is shown to be ineffective for
some cases, especially at frequencies of hundreds of megahertz and
: Figure 1 shows a
part mounted on top of an existing capacitor. Mounting a smaller
capacitor in parallel with an existing one this way is easy to do in
the lab and therefore such added capacitors are often mounted this way.
The reason for mounting an extra capacitor in the first place is to
improve, that is lower, the impedance of the original capacitor at
frequencies of hundreds of megahertz and higher. The additional
capacitor used is smaller in value in the hope that it has lower
inductance than the larger part on the board leading to a lower
impedance of the parallel combination.
There is a problem with this approach though. Imagine removing the
original capacitor on the bottom, but leaving its end caps to support
the added capacitor on top. No one would stand a capacitor off the
board like this on purpose! The smaller capacitor on top and its long
connections to the board (the end caps of the original capacitor) would
form a much larger loop above the board than the original capacitor
did. The result is that the added capacitor would have much higher
inductance than the original capacitor, making the added capacitor
ineffective at hundreds of megahertz and higher.
Recently, I was working with a client that had mounted a small
capacitor on top of a larger one just this way in an attempt to improve
the RF radiated immunity of an analog circuit at 800 MHz. The added
capacitor had little effect. We mounted the same additional capacitor
alongside the original capacitor and a significant improvement was
noted. Figure 2 shows the added part from Figure 1, moved to the
side of the original one in a way similar to what we did in the lab.
This is a much better way to mount an additional small capacitor in
parallel with a larger one as it minimizes the effective inductance of
the added capacitor. For capacitors that have one side connected to a
ground plane, the performance of the added capacitor can likely be
further improved if it can be connected directly to the ground plane
instead of relying on the mounting pad of the original capacitor, but
this takes a little luck.
Circuit Board With Added Part Mounted to the Side of Existing Capacitor
In general, one should try to mount an added capacitor next to, not on
top of, an existing capacitor during troubleshooting or rework of a
board. The lower inductance that results can significantly improve
Additional article on this website related to this topic is:
- February 2000, Measuring Capacitor Self-inductance
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Copyright © 2007 Douglas C. Smith