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Decoupling
by William D. Kimmel, P.E.
and Daryl D. Gerke, P.E.
Decoupling problems are the source of numerous EMI problems.
The biggest problem with decoupling is due to lead inductance
in the decoupling capacitors. Interestingly, decoupling plays
a significant role in signal paths as well.
While the chip houses specify decoupling for Vcc droop, their
recommendations are woefully inadequate for emissions control,
which may demand two to three decades better decoupling than
is required to satisfy the chip.
Interlayer capacitance plays a dominant decoupling role above
1GHz, and a decreasing role down to about 200MHz, where the
interloper capacitance becomes ineffective and decoupling
capacitors take over. This is a critical frequency range for
decoupling, as poorly mounted capacitors fail to do their
job.
Let’s take a look at the decoupling problem, and some
of the things that can help.
ESL
Effective series inductance (ESL) is the culprit in many
of the decoupling problems we encounter.
The problem lies in the inevitable loop area associated with
the mounting of the capacitor. A typical 0603 SMT capacitor
will have 2nH of inductance – partially due to the path
in the vias, and partially due to the fact that the capacitor
sits above the nearest plane. 2nH of inductance will produce
an impedance of Z = 2pi*f*L = 12 ohms at 1GHz – hardly
the high frequency short we were hoping for. In practice,
the upper frequency performance in decoupling is driven almost
entirely by the lead inductance – at 1GHz, it makes little
difference if your capacitor value is 100pF or 10,000pF, as
long as the package size is the same.
Accordingly, we should look for the possible ways of reducing
the inductance in the path. There are three factors that are
significant in keeping the high frequency impedance down –
component type, mounting technique leads if any need to be
kept short and mounting location so that clads paths are as
short as possible. All of these need to be done correctly
or your decoupling will not be effective.
Selecting Low ESR Capacitors
The first rule is to select the right capacitor type. We
can do a better job of decoupling if we start with the right
capacitor, one with a low series resistance and inductance.
Electrolytic capacitors play no role in high frequency decoupling
– use them only for bulk storage.
The good news is that ceramic capacitors are about as good
as you can find. The material type is not important for decoupling
effectiveness – inductance is primarily driven by the
package size (and the mounting, discussed below). Some loss
factor is beneficial – it softens resonances.
Use the smallest capacitor package possible, consistent with
voltage requirements and capacity. A 1206 capacitor will nearly
double the series inductance over a 0603. Use reverse aspect
components, such as a 0306 instead of 0603. This can reduce
your inductance to nearly half.
Low inductance capacitors are available, such as feedthrough
capacitors. Where higher capacitance is needed, multipin capacitors,
with alternating voltage and ground pins can be used.
By the way, using multiple capacitor values tends to create
multiple resonances on-board. The common practice of using
a small/medium/large capacitor should be confined to paralleling
high frequency capacitors with electrolytics. If you must
use multiple values, keep the value closely spaced (say, a
factor of three) to reduce the stray resonance problem.
Mounting Technique
Next, we need to mount the capacitor for minimum inductance.
How can this be done? Start with simple geometry – the
ultimate goal is to reduce the effective loop area of each
capacitor. Increasing mutual inductance between via pairs
also helps. Here are some options:
1. Eliminate the trace between the via and the solder pad.
Place the via in the solder pad if you can, otherwise immediately
adjacent.
2. Use bigger vias or multiple vias. This reduces the effective
inductance – in the case of multiple vias, we are effectively
putting two inductors in parallel. A via for a 0603 capacitor
typically has 0.5nH of inductance, or 1nH per capacitor mount.
3. Locate the via inside the solder pad. This increases the
mutual inductance between the two pads. This is more feasible
with the larger size capacitors.
4. Feedthrough capacitors reduce inductance largely by using
additional solder pads, but there are also other benefits.
5. Use more capacitors. Simply put, if you double the number
of capacitors, you will halve the effective inductance.
Mounting Location
You can improve the effectiveness of the capacitors by mounting
them in the proper location. For two layer boards and multilayer
boards with widely spaced planes (>0.25 mm) the following
rules apply:
1. Mount the capacitor for minimum loop area, placing it near
the voltage supply pin or ground pin, whichever results in
the smallest loop area. For two-layer boards, use wide traces
to connect to the capacitor.
2. Use decaps at all critical locations, including Vcc/Gnd,
adjacent critical input and output pins (clocks, buses), at
headers and connectors and near bulk capacitors. The main
purpose is to provide not only a low impedance AC connection
between power and ground, but also to provide a low impedance
return path for the critical signals, including clock and
high speed data buses.
Where the planes are tightly spaced, positioning becomes less
important, and difficult to predict. The easiest and often
the best choice is to place the caps on a more or less uniform
grid, spaced 1/20 wavelength. Tap power/ground from the planes,
not from the capacitors.
Decoupling for Signal Integrity
We tend to think of decoupling as being just for Vcc droop,
but it plays significant role in maintaining a constant impedance
path and for containing emissions. The key is the return path
for the critical signal lines, especially the clock lines
and, to a lesser extent, address and data bus.
The goal is to contain the high frequency harmonics, namely
by keeping the signal/return path loop areas to a minimum.
We need to pay as much attention to the signal return path
as with the signal path itself. If the return path is interrupted,
there will be an impedance discontinuity, adversely affecting
signal integrity of high speed signal lines and energizing
the slot and increasing emissions.
One of the key issues in controlling impedances is to ensure
the path is constant along its entirety. You tend to think
of impedance control as being an issue with terminations –
the path continuity is assumed. This is a natural enough assumption
when driving a cable, with the source at one end and the load
at the other end. But with circuit boards, the path is usually
not a straight shot from source to load – the signal
may turn a corner or pass through a via. While the corner
is not usually a problem, the via may cause a considerable
problem. If the signal simply passes from one side of the
reference plane through the via to the other side of the plane,
the discontinuity is minimal. If the signal trace switches
to another reference plane, the return path is disrupted,
and the return currents will need to divert to the closest
path back to the original plane, either through a ground via
or a decoupling capacitor. In either case, the discontinuity
can be minimized by placing an adjacent via from ground to
ground, or by placing a decoupling capacitor adjacent to the
signal via. True, this is not a real good return path, but
it is better than nothing.
You also have discontinuities at the source and load chips.
Depending on the preferred return path (either ground or power
plane), the return current will need to go from signal to
ground or power pin at both ends. So a decoupling capacitor
at the critical input and output signal pins is important.
A similar situation exists at headers and connectors.
Summary
The key issue in effective decoupling is inductance. All
capacitors will resonate – it is your job to push the
resonant frequency as high as possible by selecting capacitors
with low inductance and mounting them for low inductance.
Positioning plays a significant role as well, especially for
the two-layer circuit boards.
And don’t forget about the impact on signal integrity.
Decoupling capacitors play an important role in return path
continuity.
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