Chapter 2: Hardware Interface Description
January 1, 2006 Module Hardware Description 29
Figure 2-17 shows the microphone circuit and Table 2-6 gives the microphone specifications.
Figure 2-17: Handset Microphone Circuit
Important: The microphone circuit design depends on the type of microphone device.
A series capacitor is required in case a passive microphone is used, or the
application provides a separate bias voltage to an active microphone circuit.The
internal G24 biasing circuit may also be used with an active microphone, which
corresponds to the microphone port specifications.
Headset Microphone Port
The headset microphone port is designed for use with, but not limited to, a headset audio device.
It is located at pin 57 on the G24 interface connector, named HDST_MIC.
It is designed as a single-ended input and should be referenced to the G24 analog ground.
The microphone input includes all the necessary circuitry to support a direct connection to a
headset microphone device. It incorporates an internal bias voltage of 2.1V through a 2.2kÙ
resistor, and has an impedance of 1kΩ.
Table 2-6:
Handset Microphone Port Specifications
Parameter Conditions Min Typ Max Unit
Input Voltage No load
AT+MMI CG= 0
2.0 V
PP
Gain Programmable in
1 dB steps
031dB
AC Input
Impedance
1kΩ
Bias voltage R
BIAS
= 2.2 kΩ
I
BIAS
= 1 mA
22.12.2V
Bias Current 1 mA
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