EMC notes

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Coupling path
The simplest way noise can be coupled into a circuit is through conductors. If a wire runs through a noisy
environment, the wire will pick up the noise inductively and pass it into the rest of the circuit. An example
of this type of coupling is found when noise enters a system through the power supply leads. Noise carried
on the power supply lines are conducted to all circuits.
Coupling can also occur in circuits that share common impedances. For instance, two circuits that share
the conductor carrying the supply voltage and the conductor carrying the return path to ground. If one
circuit creates a sudden demand in current, the other circuit’s voltage supply will drop due to the common
impedance both circuits share between the supply lines and the source impedance. This coupling effect
can be reduced by decreasing the common impedance. Unfortunately, source impedance coupling is
inherent to the power supply and cannot be reduced. The same effect occurs in the return-to-ground
conductor. Digital return currents that flow in one circuit create ground bounce in the other circuit’s return
path. An unstable ground will severely degrade the performance of low-level analog circuits, such as
operational amplifiers, analog-to-digital converters, and sensors.
Coupling also can occur with radiated electric and magnetic fields which are common to all electrical
circuits. Whenever current changes, electromagnetic waves are generated. These waves can couple over
to nearby conductors and interfere with other signals within the circuit.
Receptor
All electronic circuits are receptive to EMI transmissions. Most EMI are received from conductive
transients, although some are received from direct radio frequency (RF) transmissions. In digital circuits,
the most critical signals are usually the most vulnerable to EMI. These include reset, interrupt, and control
line signals. Analog low-level amplifiers, control circuits, and power regulators also are susceptible to
noise interference.
To design for EMC and to meet EMC standards, the designer should minimize emissions (RF energy
exiting from products), and increase susceptibility or immunity from emissions (RF energy entering into
the products). Both emission and immunity can be classified by radiated and conductive coupling, as
shown in Figure 1. The radiated coupling path will be more efficient in the higher frequencies while a

conducted coupling path will be more efficient in the lower frequencies.

Component Packages
There are basically two types of packages for all electronic components: leaded and leadless.
Leaded components have parasitic effects, especially at high frequencies. The lead forms a low value
inductor, about 1nH/mm per lead. The end terminations can also produce a small capacitive effect, in the
region of 4pF. Therefore, it is usually the lead length that should be reduced as much as possible.
Leadless and surface mount components have less parasitics compared with leaded components.
Typically, 0.5nH of parasitic inductance with a small end termination capacitance of about 0.3pF. From
an EMC viewpoint, surface mount components is preferred, followed by radial leaded, and then axial
leaded

Resistors
Surface mount resistors are always preferred over leaded types because of their low parasitic elements.
For the leaded type, the carbon film type is the preferred choice, followed by the metal film, then the wire
wound.
The metal film resistor, with its dominant parasitic elements at relatively low frequencies (in the MHz), is
therefore suitable for high power density or high accuracy circuits.
The wire wound resistor is highly inductive, therefore it should be avoided in frequency sensitive
applications. It is best for high power handling circuits.
In amplifier designs, the resistor choice is very important. At high frequencies, the impedance will increase
by the effect of the inductance in the resistor. Therefore, the placement of the gain setting resistors should
be as close as possible to the amplifier circuit to minimize the board inductance.
In pull-up/pull-down resistor circuits, the fast switching from the transistors or IC circuits create ringing. To
minimize this effect, all biasing resistors must be placed as close as possible to the active device and its
local power and ground to minimize the inductance from the PCB trace.
In regulator or reference circuits, the DC bias resistor must be placed as close as possible to the active
device to minimize decoupling effect (i.e. improve transient response time).
In RC filter networks the inductive effect from the resistor must be considered because the parasitic
inductance of the wire wound resistor can easily cause local oscillation.

Capacitors
Selecting the right capacitor is not easy due to their many types and behaviors. Nonetheless, the
capacitor is one component that can solve many EMC problems. The following sections describe the most
common types, their characteristics and uses.
Aluminium electrolytic capacitors are usually constructed by winding metal foils spirally between a thin
layer of dielectric, which gives high capacitance per unit volume but increases internal inductance of the
part.
Tantalum capacitors are made from a block of the dielectric with direct plate and pin connections, which
gives a lower internal inductance than aluminium electrolytic capacitors.
Ceramic capacitors are constructed of multiple parallel metal plates within a ceramic dielectric. The
dominant parasitic is the inductance of the plate structure and this usually dominates the impedance for
most types in the lower MHz region.
The difference in frequency response of different dielectric materials mean a type of capacitor is more
suited to one application than another. Aluminium and tantalum electrolytic types dominate at the low
frequency end, mainly in reservoir and low frequency filtering applications. In the mid-frequency range
(from kHz to MHz) the ceramic capacitor dominates, for decoupling and higher frequency filters. Special
low-loss (usually higher cost) ceramic and mica capacitors are available for very high frequency
applications and microwave circuits.

Bypass capacitors
The main function of the bypass capacitor is to create an AC shunt to remove undesirable energy from
entering susceptible areas. The bypass capacitor is acting as a high frequency bypass source to reduce
the transient circuit demand on the power supply unit. Usually, the aluminium or tantalum capacitor is a
good choice for bypass capacitors, its value depends on the transient current demand on the PCB, but it
is usually in the range of 10 to 470μF. Larger values are required on PCBs with a large number of
integrated circuits, fast switching circuits, and PSUs having long leads to the PCB.
Decoupling capacitors
During active device switching, the high frequency switching noise created is distributed along the power
supply lines. The main function of the decoupling capacitor is to provide a localized source of DC power
for the active devices, thus reducing the switching noise propagating across the board and decoupling the
noise to ground.
Ideally, the bypass and decoupling should be placed as close as possible to the power supply inlet to help
filter high frequency noise. The value of the decoupling capacitor is approximately 1/100 to 1/1000 of the
bypass capacitor. For better EMC performance, decoupling capacitors should placed as close as possible
to each IC, because track impedance will reduce the effectiveness of the decoupling function.
Ceramic capacitors are usually selected for decoupling; choosing a value depends on the rise and fall
times of the fastest signal. For example, with a 33MHz clock frequency, use 4.7nF to 100nF; with a
100MHz clock frequency, use 10nF.
Apart from the capacitive value when choosing the decoupling capacitor, the low ESR of the capacitor
also affects its decoupling capabilities. For decoupling, it is preferable to choose capacitors with a ESR
value below 1Ω.

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