Specifications

AC65/AC75 Hardware Interface Description
3.15 Audio Interfaces
s
AC65_AC75_HD_v01.002 Page 61 of 117 2006-10-30
Confidential / Released
3.15.2.3 Line Input Configuration with OpAmp
Figure 26 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 attached 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 using 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. 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 26: Line input configuration with OpAmp