Product specifications

Design Guide - VT82C694X Apollo Pro133 with VT82C686A
Preliminary Revision 0.5, November 19, 1999 Appendix B - Audio Codec and Game/MIDI Port Layout
Guidelines
105
Technologies, Inc.
We ConnectWe Connect
B.2.3 Routing Guidelines
Routing to VDD, VREF, AFILT and FILT capacitors
All high frequency decoupling, reference high frequency decoupling and filter capacitors must be routed on the same layer as the
codec. This is done to reduce inductance in the supply, reference and filter networks. Inductive loops in these networks can be a
coupling mechanism for high frequency noise. High frequency noise can then be aliased down into the audio band during the A/D
or D/A process.
Audio Input/Output Routing
Keeping traces short can decrease inductance and help avoiding magnetic coupling. This allows a straight-line route and also
allows for shielding with ground traces. Regions between analog signal traces should be filled with copper; the copper fill should
be shorted to the analog ground plane. This will help to reduce high frequency interference by creating a capacitance coupling
mechanism from signal to ground.
AC’97 Link Routing
The AC’97 link signals should be as short as possible. If the signal traces need to be long certain pre-cautions should be made to
reduce ringing and signal reflections. Clock signal such as BIT_CLK should have a series resistor close to the codec. Other
clocks like the 24.576MHz clock should have series resistor by the clock source. If a 24.576MHz crystal is used series resistance
is not required. This series resistance in the clock lines will help to reduce reflections. A shunt capacitor should also be used on
these clock lines to help reduce ringing. Preferably the capacitors should be placed close to the clock source but should be
electrically connected to the opposite side of the series resistor. It is also recommended to place series resistors on the SDATA_IN
and SDATA_OUT lines. The resistors should be placed close to the signal source.
The use of ground trace shields may also help to reduce noise coupling and radiation from the digital link. Therefore, regions
between digital signal traces should be filled with copper. The copper fill should be shorted to the digital ground plane. Therefore,
excellent shielding through capacitive coupling can be achieved.
Referring to Figure B-5, the AC’97 link signals should come in from the lower side of the audio codec to keep them far from the
analog signals.
Game/MID Signals Routing
The Game/MIDI resistor-capacitor components should be placed on the analog Game/MIDI ground (GND_MIDI) and as close to
the Game/MIDI port as possible.
Referring to Figure B-6, the Game/MIDI signals should come in from the lower side of the Game/MIDI port to keep them far from
other analog signals.