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AC75 Hardware Interface Description
Strictly confidential / Preliminary
3.15.2.3
Line Input Configuration with OpAmp
Figure 26 shows an example of how to connect an opamp into the microphone circuit.
C
K
~
C
K
C
F
Figure 26: Line input configuration with OpAmp
The AC source (e.g. an opamp) and its reference potential have to be AC coupled to the
MICPx resp. MICNx input terminals. The voltage divider between VMIC and AGND is
necessary to bias the input amplifier. MICNx is automatically self biased to the MICPx DC
level.
The VMIC voltage should be filtered if gains larger than 20dB are used. The filter can be
attached as a simple first order RC-network (R
gain are applied the filter is not necessary.
Consider that if VMIC is switched off, the signal voltage should be limited to Vpp = 0.5V and
any bias voltage must not be applied. Otherwise VMIC can be switched on permanently by
using AT^SNFM=,1. In this case the current consumption in SLEEP modes is higher.
If desired, MICNx via C
instead of connecting it to the reference potential for differential line input.
AC75_HD_V00.202
VMIC
R
VMIC
R
A
MICPx
R
GSM module
A
MICNx
V
Bias
AGND
can also be connected to the inverse output of the AC source
K
Page 67 of 120
R
= typ. 47k
A
R
VMIC
C
= typ. 100nF
k
C
= typ. 22µF
F
V
= typ. 2.5V
MIC
V
= typ. ½ V
bias
and C
). If a high input level and a lower
VMIC
F
s
= 470Ohm
= 1.25V
MIC
27.04.2006
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