Advantech SOM-ETX Design Guide
5.3.2.2 MIC (microphone input)
The MIC (microphone input) is intended for a monaural microphone.
5.3.2.3 Amplifier
Because SNDL/SNDR (line-out signal) are true line-out signals and cannot drive a
low impedance speaker directly, it is recommended to add an extra audio power
amplifier to these signals on the baseboard.
In SOM-DB4400 baseboard, we use LM4880 audio power amplifier to meet these
requirement, which provides dual 250mW for each channel.
For applications that require a stereo microphone or higher quality microphone audio,
the same as SNDL.SNDR, an external microphone preamplifier should be
implemented on the baseboard and connected to the AUXAL/AUXAR inputs. If the
AUXAL/AUXAR inputs are connected to the audio output from a CD-ROM drive,
shielded cables should be used.
5.3.3 Layout Requirement
5.3.3.1 Analog Ground
The X1 connector has an audio ground pin (ASGND, pin 48) that should be
connected to an analog ground plane underneath the audio amplifier circuits or the
audio input/output jacks. This plane should be isolated from the digital circuitry
ground plane by a 60 to100 mil gap. Notice this analog ground plane should be
implemented at all layers underneath the audio amplifier circuits or the audio
input/output jacks (See Figure 5.13). The audio grounds from the CD-ROM audio
cable should be connected to ASGND rather than to digital ground to minimize noise.
SOM-ETX
AUXAL
MIC
Audio
Line in
Audio
Line out
MIC in
Analog Ground
AUXAR
SNDL
SNDR
A
A
A
A
A
100
p
100
p
100
p
100
p
100
p
bead
bead
bead
bead
bead
Route to Minimum
Figure 5-13 Audio Layout Guidelines
5.3.3.2 Digital and Analog Signals Isolation
Analog audio signals and other digital signals should be routed as far as possible
from each other. All audio circuits require careful PCB layout and grounding to avoid
picking up digital noise on audio-signal lines.
74
Chapter 5 Carrier Board Design Guidelines
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