AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 2

• Manufacturer Part Number: AN2321
• Description: Designing for Board Level Electromagnetic Compatibility
• Manufacturer: Freescale Semiconductor / Motorola

AN2321/D
Elements of the
Electromagnetic
Environment
EMI source
Coupling path
2
A simple EMI model consists of three elements:
This is shown graphically in
EMI sources include microprocessors, microcontrollers, electrostatic
discharges, transmitters, transient power components such as
electromechanical relays, switching power supplies, and lightning. Within a
microcontroller system, the clock circuitry is usually the biggest generator of
wide-band noise, which is noise that is distributed throughout the frequency
spectrum. With the increase of faster semiconductors, with faster edge rates,
these circuits can produce harmonic disturbances up to 300MHz.
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.
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
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For More Information On This Product,
EMI source
Coupling path
Receptor
EMI source
Go to: www.freescale.com
(Reduce propagation efficiency)
(Reduce noise source level)
Control emissions
Figure 1. EMI Elements
Figure
Coupling path
1.
(Reduce propagation efficiency)
(Increase receptor immunity)
Control susceptibility
Receptor
MOTOROLA