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AC75 Hardware Interface Description
Strictly confidential / Preliminary
3.15.2.2
Differential Microphone Input
Figure 25 shows a differential solution for connecting an electret microphone.
R
A
C
F
V
Bias
R
A
Figure 25: Differential microphone input
The advantage of this circuit is that it can be used if the application involves longer lines
between microphone and module.
While VMIC is switched off, the input voltage at any MIC pin should not exceed ±0.25V
relative to AGND (see also section 5.1). In this case no bias voltage has to be supplied from
the customer circuit to the MIC pin and any signal voltage should be smaller than Vpp = 0.5V.
VMIC can be used to generate the MICP-pin bias voltage as shown below. In this case the
bias voltage is only applied if VMIC is switched on.
Only if VMIC is switched on, can the voltage applied to any MIC pin be in the range of 2.4V
to 0V. If these limits are exceeded undervoltage shutdown may be caused.
Consider that the maximum full scale input voltage is Vpp = 1.6V.
The behavior of VMIC can be controlled with the parameter micVccCtl of the AT command
AT^SNFM (see [1]):
•
micVccCtl=2 (default). VMIC is controlled automatically by the module. VMIC is always
switched on while the internal audio circuits of the module are active (e.g., during a call).
VMIC can be used as indicator for active audio in the module.
•
micVccCtl=1. VMIC is switched on continuously. This setting can be used to supply the
microphone in order to use the signal in other customer circuits as well. However, this
setting leads to a higher current consumption in SLEEP modes.
•
micVccCtl=0. VMIC is permanently switched off.
AC75_HD_V00.202
VMIC
R
VMIC
MICPx
GSM module
MICNx
C
K
AGND
Page 66 of 120
s
R
= typ. 1k
A
R
= 470Ohm
VMIC
C
= typ. 100nF
K
C
= typ. 22µF
F
V
= typ. 2.5V
MIC
V
= 1.0V ... 1.6V, typ. 1.5V
bias
27.04.2006
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