3
3.0 Applications
The CX28331/CX28332/CX28333 can be used in a variety of applications.
Figure 3-1 illustrates an example of three DS3 lines being terminated by the
CX28333. The data and clock are extracted and passed on to the framer chip for
further data manipulation and user interface.
It is important to employ high-frequency design techniques for the printed
board layout.
3.1 PCB Design Considerations for CX2833i
The CX28333 device is a triple LIU operating at frequencies up to 52.84 MHz.
The high-speed nature of the device calls for a careful design of the PCB using
this device. Some design considerations are outlined below.
3.1.1 Power Supply and Ground Plane
A unified power plane with properly placed capacitors of the correct size will
mitigate most power rail-related voltage transients. A properly placed bulk
capacitor, where the power enters the board, with noise-bypassing capacitors at
the power pins on the integrated circuits should be adequate. The noise-bypassing
capacitors must be able to supply all the switching current.
Ferrite beads are used with power rails to filter the high-frequency noise. For
every design, noise frequencies and levels are different. Therefore, whether beads
are necessary, and the effective frequency where they should operate, is difficult
to determine. It is a good idea to provision for ferrite beads on the boards.
The board trace from the CX28333 power supply pin to the noise-bypassing
capacitor should be minimized. Additionally, ground connections from the
ground plane to the CX28333 ground pins and the noise-bypassing capacitor
ground pins should be minimized.
A unified ground plane is the best way to minimize ground impedance. Most
of the ground noise is produced by the return currents and power supply transients
during switching. This effect is minimized by reducing the ground plane
impedance.
100985A
Conexant
3-1
CONEXANT [ CONEXANT SYSTEMS, INC ]
