User's Manual

Smart Machine Smart Decision

SIM868_Hardware_Design_V1.00 34 2016-06-20

4.7.2. Microphone Interfaces Configuration

Electret

Microphone

The lines in bold type should

be accorded to differential

signal layout rules

These components should

be placed to microphone

as close as possible

MICP

Module

MICN

ESD

10pF

33pF

10pF

Figure 29: Microphone reference circuit

4.7.3. Audio Electronic Characteristic

Table 15: Microphone input characteristics

Parameter Min Typ Max Unit

Microphone biasing voltage - 1.9 2.2 V

Working current - - 2.0 mA

Input impedance(differential) 13 20 27 KΩ

Idle channel noise - - -67 dBm0

SINAD

Input level:-40dBm0 29 - - dB

Input level:0dBm0 - 69 - dB

Table 16: Audio output characteristics

Parameter Conditions Min Typ Max Unit

Normal output

=32 Ω receiver - 15 90 mW

=8 Ω speaker - - 1080 mW

4.7.4. TDD

Audio signal could be interferenced by RF signal. Coupling noise could be filtered by adding 33pF and 10pF

capacitor to audio lines. 33pF capacitor could eliminate noise from GSM850/EGSM900MHz, while 10pF

capacitor could eliminate noise from DCS1800/PCS1900Mhz frequency. Customer should develop this filter

solution according to field test result.

GSM antenna is the key coupling interfering source of TDD noise. Thereat, pay attention to the layout of audio

lines which should be far away from RF cable, antenna and VBAT pin. The bypass capacitor for filtering should

be placed near module and another group needs to be placed near to connector.