Application Note
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 commonmode radiation. Note that 49.1 MHz is 2 times the
CS4297A internal clock frequency of 24.576 MHz.
The key to reducing common-mode emissions is to
add decoupling (called shunting) at the audio I/O
connectors. The shunt capacitors must be connected 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 values are typically selected for an impedance of
100 ohms at 100 MHz. The ferrite bead and capacitor circuit create a low pass filter to attenuate frequencies above 1.6 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 result 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
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Additionally, the following PCB layout checklist
includes generally accepted practices for reducing
both differential-mode and common-mode radiated
emissions, while maintaining audio quality.
Copyright  Cirrus Logic, Inc. 2000
(All Rights Reserved)
MAY ‘00
Connect analog and digital ground together
with a 1/16 inch trace under the CS4297A. A
direct connection between analog and digital
ground will reduce the differential-mode radiation and improve the EOS (Electrical Overstress) capabilities of the CS4297A.
Construct a "clean" chassis ground on the PCB
around the I/O connectors, and connect the I/O
ground to the system frame ground.
Connect chassis ground to digital ground in a
quiet area, away from the CS4297A.
The ferrite bead and decoupling capacitor combination shown in the block diagram on page
one of this document, form a low-pass filter to
remove the common-mode voltages. The de-
coupling capacitor must be terminated to a
clean (free of digital noise) I/O chassis ground.
A separate analog ground return path between
the I/O connectors and analog ground plane
must be maintained to reduce loop areas.
Chassis and analog planes should be identical
on all layers, and the gap or "moat" between
planes should be 1/8 inch to prevent coupling
between planes. The absolute minimum moat
spacing is 1/16 inch. Do not overlap digital and
analog ground planes.
Never route digital traces or digital planes under the analog or I/O chassis ground areas. Analog components should be located over analog
planes and digital components should be located over digital planes.
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