User Manual

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USERS MANUAL

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4/198 17-LXE 108 566 Uen PA6

The bias can be tapped directly from the resistor voltage divider, but the amplifier will

make the bias more stable and less susceptible to noise.

3.4.3 Analog ground vs. AGND

The AGND signal output from the module is not a ground. It is an analog reference,

which is connected to the main ground used by the module in one place inside the

module. It should not under any circumstances be used as a ground or connected to a

ground in the application.

AGND must be treated as a signal. Together with ATMS and AFMS it creates a semi

differential interface. The analog ground shall be used as ground plane for the analog

circuitry of the application. It should not be connected to the AGND signal output from

the phone module.

3.4.4 Microphone path

An application using the analog audio interface must re-reference the signal from its

own internal BIAS to AGND received from the module.

The figure below shows an example of a microphone implementation.

The microphone should preferably be connected to its pre-amplifier differentially, which

will minimize noise picked up along the way from the microphone to its amplifier. If the

impedance is the same on both microphone lines, and the lines are run in parallel, the

same amount of noise is picked up on both lines. This noise is then removed in the

differential amplifier stage.

VCC

<=1k

BIAS

>=2.2uF

ATMS

AGND

>=2.2uF