LTE Standard Module Series
EG915U-EU_Hardware_Design 38 / 81
4
Application Interfaces
4.1. Analog Audio Interfaces
Table 11: Pin Definition of Audio Interfaces
⚫
AI channels are differential output channels, which can be applied for input of microphone (usually an
electret microphone is used).
⚫
AO channels are differential output channels, which can be applied for output receiver.
4.1.1. Audio Interfaces Design
It is recommended to use the electret microphone with dual built-in capacitors (e.g. 10 pF and 33 pF) for
filtering out RF interference, thus reducing TDD noise. The 33 pF capacitor is applied for filtering out RF
interference when the module is transmitting at EGSM900. Without placing this capacitor, TDD noise
could be heard. The 10 pF capacitor here is used for filtering out RF interference at DCS1800. Please
note that the resonant frequency point of a capacitor largely depends on the material and production
technique. Therefore, customers would have to discuss with their capacitor vendors to choose the most
suitable capacitor for filtering out high-frequency noises.
The filter capacitors on the PCB board should be placed as close to the audio devices or audio interfaces
as possible, and the traces should be as short as possible. they should go through the filter capacitors
before arriving other connection points.
In order to reduce radio or other signal interference, the radio frequency antenna should be far away from
the audio interface and audio traces. The power traces should not be parallel to the audio traces, and
Pin Name
Pin No.
I/O
Description
MIC_N
119
AI
Microphone analog input (-)
MICBIAS
120
PO
Bias voltage output for microphone
SPK_P
121
AO
Analog audio differential output (+)
SPK_N
122
AO
Analog audio differential output (-)
MIC_P
126
AI
Microphone analog input (+)
