Service Guide

LTE Standard Module Series
EG915U_Series_Hardware_Design 40 / 90
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 11: 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 of receiver.
⚫ The module’s internal PA is configured as Class AB 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 GSM900. 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 (+)