Specifications

AC65/AC75 Hardware Interface Description
3.15 Audio Interfaces
s
AC65_AC75_HD_v01.002 Page 60 of 117 2006-10-30
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3.15.2.2 Differential Microphone Input
Figure 25 shows a differential solution for connecting an electret microphone.
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 consump-
tion in SLEEP modes.
• micVccCtl=0. VMIC is permanently switched off.
R
A
= typ. 1k
R
VMIC
= 470Ohm
C
K
= typ. 100nF
C
F
= typ. 22µF
V
MIC
= typ. 2.5V
V
bias
= 1.0V … 1.6V, typ. 1.5V
Figure 25: Differential microphone input