AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 24

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

AN2321/D
Ground track in
double-layer PCBs
Guard ring
PCB capacitor
Fast circuit and slow
circuits
24
On a double-layer (double sided) PCB, the ground grid/matrix arrangement is
preferred for digital circuits because this arrangement can reduce ground
impedance, ground loops, and signal return loops. As with the single-layer
PCBs, the width of the ground and power tracks should be at least 1.5mm.
Another scheme is to have a ground plane on one side, the signal and power
line on the other side. In this arrangement the ground return path and
impedance will be further reduced and decoupling capacitors can be placed as
close as possible between the IC supply line and the ground plane.
The guard ring is a grounding technique that can isolate the noisy environment
(e.g. RF current) outside the ring, because there is no current flowing through
the guard ring in normal operation (see
On a multi-layer board, a PCB capacitor is created by the thin laminate
separating the power and ground planes. On a single-layer board, this
capacitive effect is also achieved by running the power and ground traces in
parallel. The advantages of the PCB capacitor is that it has a very high
frequency response and low series inductance that is evenly distributed along
the plane or trace. In effect, it is an evenly distributed decoupling capacitor on
the whole board. No single discrete component has these characteristics.
High speed circuits should be placed closer to the ground plane while the
slower circuits can be placed close to the power plane.
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
For More Information On This Product,
Ground plane also
acts as guard ring
against noise.
PLL filter network
Power supply decoupling
Go to: www.freescale.com
Crystal
Figure 22. Guard Ring
Figure
VCC
22).
GND
MOTOROLA