EES3 Hardware Interface Description
3.15 Audio Interfaces
71
EES3_HD_v01.100b
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2009-08-12
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3.15.2.2
Differential Microphone Input
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 be-
tween microphone and module.
While VMIC is switched off, the input voltage at any MIC line should not exceed ±0.25V relative
to AGND (see also
). In this case no bias voltage has to be supplied from the cus-
tomer circuit to the MIC line and any signal voltage should be smaller than Vpp = 0.5V.
VMIC can be used to generate the MICP line 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 line 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
):
•
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 set-
ting leads to a higher current consumption 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
Vbias = 1.0V … 1.6V, typ.
1.5V
Figure 25:
Differential microphone input
