GE866-QUAD Hardware User Guide
Reproduction forbidden without written authorization from Telit Communications S.p.A. - All Rights Reserved.
Page 48 of 82
Mod. 0805 2011-07 Rev.2
10.
Audio Section Overview
10.1.
MIC Connection
The bias for the microphone has to be as clean as possible; the first connection
(single ended) is preferable since the Vmic noise and ground noise are fed into the
input as common mode and then rejected. This sounds strange; usually the
connection to use in order to reject the common mode is the balanced one. In this
situation we have to recall that the microphone is a sound to current transducer, so
the resistor is the current to tension transducer, so finally the resistor feeds the
input in balanced way even if the configuration, from a microphone point of view,
seems to be un-balanced.
If a "balanced way" is anyway desired, much more care has to be taken to Vmic noise
and ground noise; also the 33pF-100
Ω
-33pF RF-filter has to be doubled (one each
wire).
100
n
100
n
1
K
2.2
K
1
u
10
0
33p
F
33p
F
÷10
u
Vmic
MIC
+
MIC
-
GN
D
÷10
K
2÷5
v
TELIT MODULE
-
+
INPU
T
AD
C
+
-
