Notes On Audio Interface Design; Microphone Interface Circuit - Quectel EC200U Series Hardware Design

AIN
⚫
AIN channel is a differential input channel, which can be applied for input of microphone (usually an
electret microphone).
⚫
The AOUT channel is a differential output with a built-in power amplifier. The default configuration of
power amplifier is Class AB and the maximum driving power is 500 mW for 8 Ω load. When PA is
configured as Class D, the maximum driving power is 800 mW for 8 Ω load.

3.14.1. Notes on Audio Interface 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, you would have to discuss with their capacitor vendors to choose the most suitable
capacitor for filtering out high-frequency noises.
The severity degree of the RF interference in the voice channel during GSM transmitting largely depends
on the application design. In some cases, EGSM900 TDD noise is more severe; while in other cases,
DCS1800 TDD noise is more obvious. Therefore, a suitable capacitor can be selected based on the test
results. The filter capacitors on the PCB 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 at other connection points.
In order to decrease radio or other signal interference, RF antennas should be placed away from audio
interfaces and audio traces. Power traces cannot be parallel with and also should be far away from the
audio traces.
The differential audio traces must be routed according to the differential signal layout rule.

3.14.2. Microphone Interface Circuit

The reference circuit of the microphone interface is shown in the figure below:
EC200U_Series_Hardware_Design
LOUDSPK_N
MIC_P
MIC_N
74 AO
75 AI
77 AI
LTE Standard Module Series
output (+)
Loudspeaker differential
output (-)
Microphone analog input (+)
Microphone analog input (-)
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