LTE Standard Module Series
EG915U_Series_Hardware_Design 38 / 81
4
Application Interfaces
4.1. Analog Audio Interfaces
The module provides one analog audio input channel and one analog audio output channel. The pin
definitions are shown in the following table.
Table 9: Pin Definition of Analog Audio Interfaces
⚫
AI channels are differential input 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.
⚫
The module's internal audio amplifier is configured as a class AB amplifier by default.
4.1.1. Audio Interfaces Design Considerations
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. Note that
the resonant frequency point of a capacitor largely depends on the material and production technique.
Therefore, you would have to discuss with your 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
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 (+)
