RH56D-PCI datasheet(42/60 Pages) ETC

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RH56D-PCI Modem Designer’s Guide

4-8

Conexant

1213

Conexant Proprietary Information

4.2.8 Modem Analog Interface

This is a critical portion of the layout. Failure to do this correctly will result in poor modem performance. The signals between

the transformer and the device are all analog signals.

Conexant devices that connect to the transformer have an analog area separate from the digital. Keep them separate.

Keep all digital signals away from the transformer and associated analog signals. Following is the list of critical analog

signals that should never get near digital signals.

Signal

Routing

TXA1

Route in close proximity to TXA2.

TXA2

Route in close proximity to TXA1.

RXA

Very sensitive; minimize trace length and its exposure to other signals, especially digital. Use a

12-mil trace.

MIC_V

Very sensitive; minimize trace length and its exposure to other signals, especially digital.

Very sensitive; place decoupling capacitor network as close as possible with the shortest path to

AGND pin. Only then, tie to a good local analog ground. Use 12-mil or thicker traces. Use ferrite

bead in AGND lead is necessary. Avoid loading by external circuits.

VREF

Place decoupling capacitor network as close as possible. Use 12-mil or thicker traces.

TELIN

TELOUT

Provide a separate local analog ground plane for the analog signals listed above.

Rotate the modem device(s) so that the routing to the transformer is as short as possible. That way the traces can be

short and the opportunity for digital signals to compromise the analog signals is decreased.

Do not put vias on these signals.

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