EGS5 Hardware Interface Description
3.15 Audio Interfaces
75
EGS5_HD_v02.004
Page 64 of 123
2012-02-09
Confidential / Released
3.15.2.3
Line Input Configuration with OpAmp
shows an example of how to connect an opamp into the microphone circuit.
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 at-
tached as a simple first order RC-network (R
VMIC
and C
F
). If a high input level and a lower 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 us-
ing AT^SNFM=,1. In this case the current consumption in SLEEP modes is higher.
If desired, MICNx via C
K
can also be connected to the inverse output of the AC source instead
of connecting it to the reference potential for differential line input.
R
A
= typ. 47 k
R
VMIC
= 470
C
k
= typ. 100 nF
C
F
= typ. 22 µF
V
MIC
= typ. 2.5 V
V
bias
= typ. ½ V
MIC
= 1.25 V
Figure 26:
Line input configuration with OpAmp
