In and Out – or – Why Have a Capacitor? A.

In and Out – or – Why Have a Capacitor? A.
R A Q ’ s
p e c i a l
d v e r t i s i n g
e c t i o n
Strange stories from the call logs of Analog Devices
In and Out – or – Why Have a Capacitor?
Q. Why do ICs need their
Contributing Writer
own decoupling capacitors?
James Bryant has
been a European
Applications Manager
A. To keep the HF in and the
HF out. (This is not a claim that
capacitors dance the Hokey
There are two important
reasons why every integrated circuit
(IC) must have a capacitor connecting
every power terminal to ground right at
the device: to protect it from noise which
may affect its performance, and to prevent
it from transmitting noise which may affect
the performance of other circuits.
Power lines acting as antennas can pick
up high-frequency (HF) noise, which can
couple by electric fields, magnetic fields,
electromagnetic fields, and direct conduction from elsewhere in the system. The
performance of many circuits is impaired
by the presence of HF noise on their supplies, so any HF noise which might be
present on an IC’s supply must be shorted
to ground. We cannot use a conductor
for this as it will short circuit dc and blow
fuses, but a capacitor (usually in the 1-nF
to 100-nF range) blocks dc while acting as
a short circuit for HF.
1 cm of wire or PC track has ~8 nH inductance (5 Ω at 100 MHz), which is scarcely a
short circuit. A capacitor acting as an HF
short circuit must have low lead and PC
track inductance, so each supply capacitor
must be located very close to the two terminals of the IC it is decoupling. It is also
important to choose capacitors with low
internal inductance—usually ceramic ones.
Many ICs contain circuitry which generates HF noise on their supply. This noise
must also be short circuited by a capaci-
with Analog Devices
since 1982. He holds
a degree in Physics
and Philosophy from
the University of Leeds.
He is also C.Eng., Eur.
Eng., MIEE, and an FBIS.
In addition to his passion
tor across the supply before it can corrupt other parts of the system. Again, the
length of leads and PC tracks is critical;
not only do long leads act as inductances
and make the short circuit less than perfect, but long conductors act as antennas,
transmitting HF noise to other parts of the
system by means of electric fields, magnetic fields, and electromagnetic radiation.
It is therefore very important that every
supply terminal of every IC should be connected to its ground terminal (or terminals,
which must all be joined together with
broad, low inductance PC tracks so that
there is no resistance or inductance to
prevent them all behaving as a single lowimpedance unipotential star point) with a
very low inductance capacitor.
for engineering, James
is a radio ham and holds
the call sign G4CLF.
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Decoupling Capacitors
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