AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 22

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

AN2321/D
Decouple Local
Supplies and ICs
RF Current in
Reference Plane
Trace Separation
Guard and Shunt
Traces
22
Localized decoupling can reduce noise propagating along the supply rail. The
use of large bypass capacitors connected at supply entry to the PCB will help
as a low frequency ripple filter and potential reservoir for sudden power
demands. In addition, decoupling capacitors should be connected between
power and ground at each IC, as close as possible to the pins. This helps to
filter out switching noises from the IC.
Whether with the reference ground plane on multi-layer PCBs or ground traces
on single-layer PCBs, a current path is present from the load back to the power
supply source. The lower the impedance of the return path the better the EMC
performance of the PCB. Long return paths can create mutual coupling
because of the RF current from load to source. Therefore, the return path
should be as short as possible, the loop area as small as possible.
Trace separation is used to minimize the crosstalk and noise coupling (by
magnetic flux coupling) between adjacent traces on the same PCB layer.
The 3W rule states that all signals (clocks, video, audio, reset, etc.) must be
separated between traces, edge-to-edge as shown
minimize magnetic coupling, place reference grounds near critical signals to
isolate other noise being coupled onto the signal lines.
In clock circuits, local decoupling capacitors are very important to reduce noise
propagating along the supply rail. But the clock lines also need protection from
other EMI sources, otherwise jittering clock signals will cause problems
elsewhere in the circuit.
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
Non-critical
Signal
W
Figure 20. Trace Separation
2W
Critical
Signal
W
2W
Non-critical or
differential pair signals
W W W
Figure
20. To further
MOTOROLA