AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 25

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

Ground copper fills
Ground and power
planes in multi-layer
PCBs
Multi-power
requirements
Tracking Layout
Techniques
MOTOROLA
In some analog circuits, unused board areas are covered with a large ground
plane such that it provides shielding and improve decoupling. But if the copper
area is floating (i.e. not connected to ground), it may act as an antenna, and it
will cause EMC problems.
In multi-layer PCBs, it is preferable to place the power and ground planes as
close as possible on adjacent layers to create a large PCB capacitor over the
board. The fastest and critical signals should be placed on the side which is
adjacent to the ground plane, while non-critical signals placed near the power
plane.
When the circuit requires more than one power supply, the idea is to keep each
power separated by a ground plane. But on a single-layer PCB, multi-ground
planes are not possible. One solution is by running power and ground tracks for
one supply separated from the others (see
noise coupling from one power source to the other.
The following sections discuss some rules on PCB tracking.
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
For More Information On This Product,
Figure 23
Figure 23. Tracking Arrangement on Multi-Layer PCBs
Go to: www.freescale.com
PCB
capacitor
shows the typical multi-layer board track arrangement.
Bypass
capacitor
Figure 24. Multiple Power Sources
Star points
PART 3: PRINTED CIRCUIT BOARD LAYOUT TECHNIQUES
Non-critical signals layer
Critical signals layer
Ground plane layer
Power plane layer
Laminate
Laminate
Laminate
Decoupling
capacitor
7812
7805
Figure
Voltage regulators
24). This still helps to avoid
+12V
GND
GND
+5V
AN2321/D
25