User manual

Advantech SOM-Express Design Guide
Chapter 5 Carrier Board Design Guidelines 53
5.3.2.5 Grounding Techniques
Take care when grounding back panel audio jacks, especially the line in and
microphone jacks. Avoid grounding the audio jacks to the ground plane directly under
the connectors. Doing so raises the potential for audio noise to be induced on the
inputs due to the difference in ground potential between the audio jacks and the
codec’s ground point. Figure 5-14 provides an AC‘ 97 example.
Codec
AUXAL
MIC
Audio
Line in
Audio
Line out
MIC in
Analog Ground
AUXAR
SNDL
SNDR
AA
AA
A
100
p
100
p
100
p
100
p
100
p
bead
bead
bead
bead
bead
Route to Minimum
Figure 5-14 AC link Audio Layout Guidelines
5.3.2.6 AC link Stereo Microphone & Line In / Auxiliary In consideration
Back panel microphone input signal should be independent routed, and the ground
return paths should be isolated from the carrier board ground plane. Use a capacitor
to filter noise from the microphone bias net feeding all microphone jacks. Route
microphone traces as far away as possible from non-microphone trace and digital
traces. Audio designs that support up to 2 V RMS line input signals are
recommended, but not required. To support audio inputs up to 2 V RMS, designs
should implement a voltage divider network to effectively reduce the input level 6 dB
prior to reaching the codec.
5.3.2.7 Azalia Audio Jack Consideration
The Azalia audio jack connectors should be designed to support up to two analog
audio jacks, each of which can signal user connection to the operating system via
sense resistor and a programmable GPIO signals. Figure 5-15 shows the example of
sense resistor. Please see the codec specification to get more information.
Figure 5-15 Sense resistor examples