Chapter 3 – Electrical Characteristics
Multi-Tech Systems, Inc. MMCModem GPRS Developer’s Guide (S000445D)
22
Microphone Inputs for MIC1
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 MMCModem according to the
characteristic of this electret microphone. These inputs are the standard ones used for an
external headset or a hands-free 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.
C1
C2
R1
R2
R3
R4
Vcc analog power supply
2.8 V
MIC1P
MIC1N
Differential Connection
Microphone Input Drawing for MIC1
R1 = R4 = from 100 to 330
.
R2 = R3 = usually between 1
Κ
and 3.3
Κ
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.
Microphone Input Pin Description for MIC1
Signal
Pin #
I/O
I/O Type*
Description
MIC1P
20
I
Analog
Microphone 1 positive input
MIC1N
22
I
Analog
Microphone 1 negative input
* See “Operating Conditions” in section on the 50-pin Connector Description.