Technical information

Chapter 2: Hardware Interface Description
December 15, 2009 G30 - Module Hardware Description 41
Most of the audio noise originates from the GSM transmit burst current surges (217 Hz TDMA
buzz), which appear on the main power supply lines and antenna, but also indirectly penetrate the
internal application's supplies and signals. The noises are transferred into the G30's audio circuits
through the microphone input signals and then are amplified by the G30's internal audio
amplifiers.
To minimize the audio noise and improve the audio performance the microphone and speaker
signals must be designed with sufficient protection from surrounding noises.
The following guidelines should be followed to achieve best audio performance:
• Reference the microphone input circuits to the G30 AGND interface signal.
• If using single-ended audio outputs, they should be referenced to the G30 AGND interface
signal.
• Keep the audio circuits away from the antenna.
• Use RF filtering capacitors on the audio signals, as described in Table 2-3.
• The audio signals should not be routed adjacent to digital signals.
• Isolate the audio signals by a surrounding ground plane or shields.
• Filter internal supplies and signals that may indirectly affect the audio circuits, from noises
and voltage drops.
Analog Ground
The G30 interface incorporates a dedicated analog ground contacts, AGND pads 52, 54 (of the 81
pin LGA interface), which are internally connected to the G30's ground. The AGND signal is
intended to provide a separate ground connection for the application's external audio devices and
circuits.
This signal provides an isolated ground connection directly from G30, which is separated from
the noisy digital ground of the application. It is recommended to connect this signal to analog
audio devices and circuits used by the application. Using a separate analog ground minimizes
audio noises and improves the audio circuit's immunity from external interferences.