AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 23

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

Grounding
Techniques
Ground track in
single-layer PCBs
MOTOROLA
Shunt trace and guard trace are effective methods to isolate and protect critical
signal traces such as the system clock from a noisy environment. In
the shunt trace or the guard trace are placed along with the critical signal trace
in the PCB. The guard not only isolate the magnetic flux coupling from other
signal traces, but also isolate the critical signal from coupling to other traces.
The difference between the shunt trace and the guard trace is that the shunt
trace does not need to be terminated (connected to ground), but the guard
trace must be connected to ground at both ends. To further reduce the
coupling, a number of vias to ground can be added at intervals on the guard
trace on multi-layer PCBs.
Grounding techniques apply to both multi-layer and single-layer PCBs. The
objective of grounding techniques is to minimize the ground impedance and
thus to reduce the potential of the ground loop from circuit back to the supply.
On a single-layer (single sided) PCB, the width of the ground track should be
as wide as possible, at least 1.5mm (60mil). Since the star arrangement is
impossible on single-layer PCBs, the use of jumpers and changes in ground
track width should be kept to a minimum, as these cause change in track
impedance and inductance.
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
Shunt trace
For More Information On This Product,
Magnetic flux
Go to: www.freescale.com
Figure 21. Shunt and Guard Traces
3W
Grounded at
driver end
Guard trace
PART 3: PRINTED CIRCUIT BOARD LAYOUT TECHNIQUES
Guard trace
H
Critical
Signal trace
W
W
PCB laminate
W
W
Grounded at
destination end
W
Figure
AN2321/D
21,
23