BGS3 Hardware Interface Description
3.11 Audio Interfaces
63
BGS3_HD_v01.000d
Page 55 of 109
2010-03-26
Confidential / Released
3.11.2.3
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 MICNx
resp. MICPx input terminals. The voltage divider between VMIC and AGND is necessary to
bias the input amplifier. MICPx is automatically self biased to the MICNx 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, MICPx 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. 47k
R
VMIC
= 470Ohm
C
k
= typ. 100nF
C
F
= typ. 22µF
V
MIC
= typ. 2.5V
V
bias
= typ. ½ V
MIC
= 1.25V
Figure 18:
Line input configuration with OpAmp
