EMI REDUCTION TECHNIQUES ( 3 Page)
Cirrus Logic, Inc. 2000
(All Rights Reserved)
P.O. Box 17847, Austin, Texas 78760
(512) 445 7222 FAX: (512) 445 7581
CS4297A/CS4299 EMI REDUCTION TECHNIQUES
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.
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
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.
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