Chapter 3 – Electrical Characteristics
Multi-Tech Systems, Inc. MMCModem GPRS Developer’s Guide (S000445D)
21
Audio Electrical Interface
Two different microphone inputs and two different speaker outputs are supported. The
connection can be either differential or single-ended but using a differential connection in
order to reject common mode noise and TDMA noise is recommended.
Microphone Inputs for MIC2
The MIC2 inputs are differential ones. They already include the convenient biasing for an
electret microphone (0,5 mA and 2 Volts). This electret microphone can be directly connected
on these inputs. The impedance of the microphone 2 has to be around 2K. These inputs are
the standard ones for a handset design while MIC1 inputs can be connected to an external
headset or a hands-free kit.
The gain of MIC2 inputs is internally adjusted. The gain can be tuned from 30dB to 51dB. The
connection to the microphone is direct. The gain can be tuned using the AT+VGR command.
MIC2N
MIC2P
C1
C1 = 22pF to 100 pF
33 pF recommended
Microphone Input Drawing for MIC2
C1 has to be the nearest as possible to the microphone. Microphone manufacturers provide
this capacitor directly soldered on the microphone.
Microphone Input Pin Description for MIC2
Signal
Pin #
I/O
I/O Type*
Description
MIC2P
16
I
Analog
Microphone 2 positive input
MIC2N
18
I
Analog
Microphone 2 negative input
* See “Operating Conditions” in section on the 50-pin Connector Description.
