User Guide

GM47/48 Design Guidelines
BA/SEM/MSC 02:0006 Rev PA1 15
will be used by audio accessories like handsets or Hands Free
equipment.
• Portable Hands Free - This state activates a different
amplification factor in the GM47 and activates a microphone bias
level of 2V in ATMS when a call is in progress.
This is the default state at power-on.
.
Audio Circuit Electrical characteristics
All sources must be AC-coupled except the Portable HandsFree
microphone, which shall be DC-coupled in order to supply DC current
to the Portable HandsFree microphone. AC coupling prevents incorrect
biasing or damage of the ATMS input. The capacitor must have a value
greater than shown below to avoid attenuation of low frequencies.
Application driving impedance (0.3 – 3.5 kHz)
≤ 300 Ω
AC coupling capacitance
≥ 1 µF
Module input impedance (0.3 – 3.5 kHz)
> 50 kΩ
Low frequency cut-off. (-3 dB)
300 Hz ± 50 Hz
High frequency cut-off. (-3 dB)
> 3500 ± 50 Hz
Hands-Free
mode
0V
Output DC bias level
Portable
Hands Free
Mode
2.0V ± 0.1V
Additional Gain in Portable Hands Free Mode 28.5 dB
Table 1. ATMS Levels. Audio Levels
3.5.2 Advanced Portable Hands Free Functionality
This functionality consists in the detection of a push button press
connecting ATMS to AGND for a certain period of time. This will
create a change in the microphone DC bias level.
Microphone bias current at 2V
1
< 0.3 mA
Activated
2
< 100 Ω
DC impedance for push button
Deactivated
3
> 10 kΩ [TBC]
Portable Handsfree electrical data
1
Bias current >0.7mA will cause false indication of button activation (Threshold is 100mV).
2
2V x ( 100 Ω / ( 2700 Ω + 100 Ω )) = 71 mV. Threshold is 100mV.
3
Lower impedance will reduce bias on microphone.