Cirrus-logic AN165 Manuel d'utilisateur télécharger pdf

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Cirrus Logic, Inc. 2000

P.O. Box 17847, Austin, Texas 78760

(512) 445 7222 FAX: (512) 445 7581

http://www.cirrus.com

AN165

Application Note

CS4297A/CS4299 EMI REDUCTION TECHNIQUES

1. INTRODUCTION

The CS4297A and CS4299 AC ’97 audio codecs

are based on a new and faster fabrication process,

and certain precautions in the analog I/O circuitry

may be required to prevent 49.1 MHz common-

mode radiation. Note that 49.1 MHz is 2 times the

CS4297A internal clock frequency of 24.576 MHz.

2. DESCRIPTION

Common-mode radiation is the result of undesired

voltage drops due to voltage differentials in the I/O

ground system. The CS4297A clock transitions re-

sult in a rush of current that may induce the digital

ground plane to "bounce". When external speaker

or microphone cables are connected to the audio

system, any common-mode voltage potential on

the analog ground will drive the cables (antennas)

and radiate electric fields. The magnitude of the

electric fields are the result of the I/O cable length

(the antenna) and the magnitude of common-mode

current.

3. EMI SHUNTING CIRCUIT

The key to reducing common-mode emissions is to

add decoupling (called shunting) at the audio I/O

connectors. The shunt capacitors must be connect-

ed to a "clean" (free of digital noise) I/O ground.

The recommended EMI shunting circuit is shown

in the following diagram, and includes ferrite beads

and 1000 pF NPO capacitors. The ferrite bead val-

ues are typically selected for an impedance of

100 ohms at 100 MHz. The ferrite bead and capac-

itor circuit create a low pass filter to attenuate fre-

quencies above 1.6 MHz.

Additionally, the following PCB layout checklist

includes generally accepted practices for reducing

both differential-mode and common-mode radiated

emissions, while maintaining audio quality.

MAY ‘00

AN165REV1

1 2 3 4

Résumé du contenu

Page 1 - Application Note

Page 2 - 4. PCB LAYOUT CHECKLIST

AN1652 AN165REV14. PCB LAYOUT CHECKLIST• Connect analog and digital ground togetherwith a 1/16 inch trace under the CS4297A. Adirect connection betwe

Page 3 - • Notes •

• Notes •

Page 4

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Cirrus-logic AN165 Manuel d'utilisateur

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