Specifications

Rev 1.0 1/27/14 26
26
Reference Guide
Interfaces
3.1.2.5. Audio Lines Connection
The modem supports one microphone input and one speaker output.
3.1.2.5.1. Microphone
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 for more information about these AT commands.
The following table shows the pin assignments of the microphone input.
Table 9. Microphone Pin Description for USR3500
(Sub D
15-pin)
Pin #
Signal
I/O
I/O Type
Description
4
CMIC1P
I
Analog
Microphone positive input
5
CMIC1N
I
Analog
Microphone negative input
Table 10. Equivalent Circuits of CMIC
DC Equivalent Circuit
AC Equivalent Circuit
Table 11. Electrical Characteristics of CMIC for USR3500
Parameters
Min
Typ
Max
Unit
DC Characteristics
N/A
V
AC Characteristics 200 Hz<F<4
kHz
Z
70
120
160
kΩ
Working voltage
(CMICxP-CMICxN)
AT+VGT*=3500dB
13.8
mVrms
AT+VGT*=2000dB
77.5
AT+VGT*=700dB
346
Maximum rating voltage
(CMICxP or CMICxN)
Positive
+7.35**
V
Negative
-0.9