AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 4

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

AN2321/D
PART 2:
COMPONENT SELECTION AND CIRCUIT DESIGN TECHNIQUES
Component
Packages
Resistors
4
Component selection and circuit design are major factors that will affect board
level EMC performance. Each type of electronic components has its own
characteristics, and therefore requires careful design considerations.
The following sections will discuss some common electronic components and
circuit design techniques for reducing or suppressing EMI.
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.
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.
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
MOTOROLA