User manual

Advantech SOM-Express Design Guide
54 Chapter 5 Carrier Board Design Guidelines
5.3.3 Layout Guidelines
5.3.3.1 General Board Routing Recommendations
! Special consideration must be given for the ground return paths for the analog
signals.
! Digital signals routed in the vicinity of the analog audio signals must not cross the
power plane split. Located analog and digital signals as far as possible from each
other.
! Partition the 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 away as possible from the
analog input and voltage reference pins.
! 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
the voltage coefficient, temperature coefficient, and noise are not factors.
! Located the crystal or oscillation close to the codec.
! The AC link / Azalia trace impedance from codec to SOM-Express should be 55 #
± 15%.
5.3.3.2 EMI Consideration
Any signals entering or leaving the analog area must cross the ground split through
bead in the area where the analog ground is attached to the main motherboard
ground. That is, no signal should cross the split/gap between the ground planes,
which would cause a ground loop, thereby greatly increasing EMI emissions and
degrading the analog and digital signal quality.
5.3.3.3 Azalia Layout Guidelines
Figure 5-16 ~18 show the AC_SDIN, AC_SDOUT,AC_SYNC,AC_BITCLK,AC_RST#
topology.
Table 5-11 ~ 13 show the routing summary.
SOM-Express
Audio Codec
(down)
L1 L2
R1
Carrier board
Figure 5-16 Azalia – AC_SDIN Topology
Table 5.11 Azalia – AC_SDIN Routing Summary
Trace
Impedance
Azalia Requirements Trace
length
Series
Termination
Resistance
Signal
Length
Matching
55 #
+/- 15%
4 on 7 (stripline)
5 on 7 (microstrip)
L1= 1“ – 11“
L2= 0.5“
R1= 33 # N/A