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3.2.2 Analog Audio without Hands free.
The D15 will drive the audio out/in in order to allow a voice call. In order to connect a Headset it
is needed to amplify the speaker and microphone channels,
An example for these drivers can be seen in Annex C EV board.
Other point to be taken in consideration is the Buzz noise existing in GSM phones due to the
transmission rate (217Hz). In order to minimise this noise the D15 have a separate ground for the
analog circuits.
Customer side
Digital Ground
PS
The main problem causing the Buzz is drops in the Ground line because the peak current during
TX mode. In order to minimize the Buzz the following acts should be done:
−
Use short Flat cable.
−
Connect the analog ground from the D15 to all the analog circuits in the customer application
without connecting them to the power supply ground.
−
all the capacitors to ground in the audio circuits should be connected to the analog ground.
−
Any reference voltage that may be used should have the external capacitor connected to the
analog ground.
−
The maximal audio In/Out levels from/To the D15 can be seen below.
d15 Modem Integration and Application Developers Guide version draft 0.53
Analog
Ground
Figure 12 – Ground connections
D15
Analog
Circuits
Logic
Circuits
Digital Ground
Page 31 of 181
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