Kontron COMe Starterkit Eval T2 - Manual (Page 110)

COM Express Interfaces
2.15.1.3. Routing Considerations
The implementation of proper component placement and routing techniques will help to ensure
that the maximum performance available from the codec is achieved. Routing techniques that
should be observed include properly isolating the codec, associated audio circuitry, analog power
supplies and analog ground planes from the rest of the Carrier Board. This includes split planes
and the proper routing of signals not associated with the audio section.
The following is a list of basic recommendations:
Traces must be routed with a target impedance of 55Ω with an allowed tolerance of ±15%.
Ground return paths for the analog signals must be given special consideration.
Digital signals routed in the vicinity of the analog audio signals must not cross the power plane
split lines. Locate the analog and digital signals as far as possible from each other.
Partition the Carrier Board with all analog components grouped together in one area and all
digital components in another.
Keep digital signal traces, especially the clock, as far as possible from the analog input and
voltage reference pins.
Provide separate analog and digital ground planes 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 inch wide.
Route analog power and signal traces over the analog ground plane.
Route digital power and signal traces over the digital ground plane.
Position the bypassing and decoupling capacitors close to the IC pins with wide traces to reduce
impedance.
Place the crystal or oscillator (depending on the codec used) as close as possible to the codec.
(HDA implementations generally do not require a crystal at the codec)
Do not completely isolate the analog/audio ground plane from the rest of the Carrier Board
ground plane. Provide a single point (0.25 inch to 0.5 inch 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 inch 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 Carrier Board ground. That is, no signal should cross the
split/gap between the ground planes, because this would cause a ground loop, which in turn
would greatly increase EMI emissions and degrade the analog and digital signal quality.
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
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