GL865-DUAL V3 Hardware User Guide
1vv0301018 Rev.5 – 2013-08-05
Reproduction forbidden without written authorization from Telit Communications S.p.A. - All Rights Reserved.
Page 43 of 76
Mod. 0805 2011-07 Rev.2
10.
Audio Section Overview
The Base Band Chip of the GL865-DUAL V3 provides one input for audio to be transmitted
(
Uplink
) that can be connected directly to a microphone or an audio source.
The bias for the microphone is already provided by the product; so the connection can be
done in both following ways:
10.1.
MIC connection
The first connection (referred to as single ended) is preferable, since the residual Vmic noise
is fed into the input as common mode and therefore rejected, while the ground noise is
blocked by the high impedance of the microphone (electret mike is a current signal source). In
this situation we have to recall that the microphone is a sound to current transducer, so the
resistor turns the current into voltage and acts as a voltage source; 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-100Ohm-33pF RF-filter has to be doubled (one each wire).
1u
220n
220n
1K
2.2K
1u
÷10u
MODULE
-
+
INPUT
ADC
Vmic
MIC/AF_IN+
MIC/AF_IN-
AGND
Integral Ground Plane