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4.9.2
AC'97 Routing
To ensure maximum performance from the codec, proper component placement and routing
techniques are required. These techniques include properly isolating the codec, associated audio
circuitry, analog power supplies, and analog ground planes from the rest of the motherboard. This
includes plane splits and proper routing of signals not associated with the audio section. Contact
your vendor for device specific recommendations.
Following are the basic recommendations:
•
Special consideration must be given for the ground return paths for the analog signals. If
isolated ground planes are used, pin B2 on the AMR connector should be used as an isolated
ground pin and should be connected to an isolated ground plane to reduce noise in the analog
circuits. The AMR designer and motherboard designer should jointly address any EMI issues
when implementing isolated grounds.
•
Digital signals routed in the vicinity of the analog audio signals must not cross the power plane
split lines. Analog and digital signals should be located as far as possible from each other.
•
Partition the board with all analog components grouped together in one area and all digital
components in the other.
•
Separate analog and digital ground planes should be provided, with the digital components
over the digital ground plane, and the analog components, including the analog power
regulators, over the analog ground plane. The split between the planes must be a minimum of
0.05" wide.
•
Keep digital signal traces, especially the clock, as far way from analog input and voltage
reference pins as possible.
•
Do not completely isolate the analog/audio ground plane from the rest of the board ground
plane. There should be a single point (¼" to ½ " wide) where the analog/isolated ground plane
connects to the main ground plane. The split between the planes must be a minimum of 0.05"
wide.
•
Any signals entering or leaving the analog area must cross the ground split in the area where
the analog ground is attached to the main motherboard ground (i.e., there should not be any
signals crossing the split/gap between the ground planes). Doing so will cause a ground loop.
This will greatly increase EMI emissions and degrade analog and digital signal quality.
•
Analog power and signal traces should be routed over the analog ground plane.
•
Digital power and signal traces should be routed over the digital ground plane.
•
Bypassing and decoupling capacitors should be close to the IC pins, or positioned for the
shortest connections to pins, with wide traces to reduce impedance.
•
All resistors in the signal path or on the voltage reference should be metal film. Carbon
resistors can be used for DC voltages and the power supply path where voltage coefficient,
temperature coefficient or noise are not a factor.
•
Regions between analog signal traces should be filled with copper, which should be
electrically attached to the analog ground plane. Regions between digital signal traces should
be filled with copper, which should be electrically attached to the digital ground plane.
•
Locate the crystal or oscillator close to the codec.
•
Figure 4-22
a codec on the motherboard and an AMR connector. Two routing methods are provided for the
AC'97 interface: the tee topology and the daisy-chain topology. The AC'97 link signals can be
routed using 5 mil traces with 5 mil space between the traces. NLX routing recommendations
will be provided in a future revision of this document.
®
Intel
810A3 Chipset Design Guide
and
Figure 4-23
show the motherboard trace lengths for an ATX form factor with
Layout and Routing Guidelines
4-19
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