
PCB Design and Layout Considerations
iChip & iChip LAN Datasheet 9-1
9 PCB Design and Layout Considerations
9.1 Design Consideration
Good engineering practices must be adhered to when designing a printed circuit board
(PCB) containing the SocketModem module. Suppression of noise is essential to the
proper operation and performance of the modem itself and for surrounding equipment.
Two aspects of noise in an OEM board design containing the Conexant SocketModem
module must be considered: on-board/off-board generated noise that can affect analog
signal levels and analog-to-digital conversion (ADC)/digital-to-analog conversion
(DAC), and on-board generated noise that can radiate off-board.
Both on-board and off-board generated noise that is coupled on-board can affect
interfacing signal levels and quality, especially in low level analog signals. Of particular
concern is noise in frequency ranges affecting modem performance.
On-board generated electromagnetic interference (EMI) noise that can be radiated or
conducted off-board is a separate, but equally important, concern. This noise can affect
the operation of surrounding equipment. Most local governing agencies have stringent
certification requirements that must be met for use in specific environments.
Proper PC board layout (component placement, signal routing, trace thickness and
geometry, etc.), component selection (composition, value, and tolerance), interface
connections, and shielding are required for the board design to achieve desired modem
performance and to attain EMI certification.
9.2 PC Board Layout Guidelines
1. In a 2-layer design, all unused space around and under components should be
filled with copper connected to the board ground on both sides of the board, and
connected in such a manner as to avoid small islands. Isolated islands should be
avoided by connecting all grounds on the same side at several points and to the
ground plane on the opposite side through the board at several points. In a modem
design, connect the SocketModem DGND and AGND pins to the ground plane.
2. In a 4-layer design, provide an adequate ground plane covering the entire board.
In a modem design, SocketModem DGND and AGND pins are tied together on
the SocketModem. Do not split analog and digital ground planes.
3. As a general rule, route digital signals on the component side of the PCB and the
analog signals on the solder side. The sides may be reversed to match particular
OEM requirements. Route the digital traces perpendicular to the analog traces to
minimize signals cross coupling.
4. Route the modem signals to provide maximum isolation between noise sources
and noise sensitive inputs. When layout requirements necessitate routing these