AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 21

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

Segmentation
MOTOROLA
Comparing the above values with component parasitics, these can be
neglected, but the sum of all the tracks may exceed component parasitics.
Therefore, the designer must take this into consideration.
The following are some general guidelines for PCB layout:
Segmentation is the use of physical separation to reduce the coupling between
different types of circuit, particularly by the power and ground tracks.
Figure 19
segmentation technique. In the ground plane, the unmetallized moat is used to
isolate the four ground planes. The L and C provide filtering for each part of the
board, coupling between different circuit power planes is reduced. High speed
digital circuits need to be placed near the power supply inlet because of their
higher transient power demand. The interface circuit may require ESD and
transient suppression components and circuits. For the L and C, it is better to
use different values of L and C filter components instead of one large L and C
because they can provide different filtering characteristics for different circuits.
Designing for Board Level Electromagnetic Compatibility
Power
source
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Increase the separation between tracks to minimize crosstalk by
capacitive coupling.
Maximize the PCB capacitance by placing the power and ground in
parallel.
Place sensitive and high frequency tracks far away from high noise
power tracks.
Widen ground and power tracks to reduce the impedance of both power
and ground lines.
VCC
GND
shows a typical example of separating four different circuits using the
VCC
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Figure 19. Separated Function Blocks
Unmetallized moats
Digital circuit
PART 3: PRINTED CIRCUIT BOARD LAYOUT TECHNIQUES
GND
Analog circuit
DC circuit
Interface circuit
AN2321/D
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