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Reference Guide
3.1.2.5.
Audio Lines Connection
The modem supports one microphone input and one speaker output.
3.1.2.5.1.
The microphone inputs are connected in differential mode to reject common mode
noise and TDMA noise. The microphone inputs have already included biasing for an
electret microphone (0.5mA and 2V) and are ESD protected. An electret microphone
may be directly connected to these inputs allowing an easy connection to a headset.
The microphone gain can be adjusted by AT+VGT and the transmit digital gain can
be adjusted by AT+WDGT. Refer to documents the AT Commands Interface Guide
at
http://www.usr.com/support/3500
The following table shows the pin assignments of the microphone input.
Table 9.
Microphone Pin Description for USR3500
(Sub D
15-pin)
Signal
Pin #
4
CMIC1P
5
CMIC1N
Table 10.
Equivalent Circuits of CMIC
DC Equivalent Circuit
Table 11.
Electrical Characteristics of CMIC for USR3500
Parameters
DC Characteristics
AC Characteristics 200 Hz<F<4
kHz
Working voltage
(CMICxP-CMICxN)
Maximum rating voltage
(CMICxP or CMICxN)
Microphone
I/O
I/O Type
I
Analog
I
Analog
Z
AT+VGT*=3500dB
AT+VGT*=2000dB
AT+VGT*=700dB
Positive
Negative
Rev 1.0
for more information about these AT commands.
Description
Microphone positive input
Microphone negative input
AC Equivalent Circuit
Min
70
-0.9
Typ
Max
N/A
120
160
13.8
77.5
346
+7.35**
1/27/14
Interfaces
Unit
V
kΩ
mVrms
V
26
26
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