User Manual

DM15/25 Integrator’s Manual 29 OF 38
6.4.3 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.
6.4.4 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.
0
-
+
0
R2
R2
R3
R3
R1
R1
VCC
C2
C2
C3
C3
0
-
+
0
R4
R4
R5
R5
C5
C5
<=1k
BIAS
>=2.2uF
ATMS
AGND
>=2.2uF
+
+