January 2001/ version 1.4
WMOi3 Integrated Modem
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4.1.10.24.1.10.2 MicrophoneMicrophone 1 Inputs 1 Inputs
The MIC1 inputs are differential and do not include internal bias. To use these
inputs with an electret microphone, bias has to be generated outside the WMOI3
modem according to the characteristic of this electret microphone. These inputs
are the standard ones used for an external headset or a handsfree kit. When using
a single-ended connection, be sure to have a very good ground plane, a very good
filtering as well as shielding in order to avoid any disturbance on the audio path.
The gain of MIC1 inputs is internally adjusted. The gain can be tuned from 30dB to
51dB.
The gain can be tuned using the AT+VGR command.
Differential connection
figure 11 : microphone 1 input
R1 = R4 = from 100 to 330 Ω
R2 = R3 = usually between 1KΩ and 3.3KΩ as per the
microphone characteristics
C1 = 22pF to 100pF
C2 = 47µF
R1 and R4 are used as a voltage supply filter with C2.
C1 has to be the nearest possible to the microphone. Microphone manufacturers
provide this capacitor directly soldered on the microphone.
2.8V
MIC1N
C2
C1
MIC1P
R1
R2
R3
R4
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