Specifications

XT65/XT75 Hardware Interface Description
3.14 Audio Interfaces
s
XT65_XT75_HD_v01.001 Page 61 of 133 2007-1-8
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R
A
has to be chosen so that the DC voltage across the microphone falls into the bias voltage range of 1.0V to
1.6V and the microphone feeding current meets its specification.
The MICNx input is automatically self biased to the MICPx DC level. It is AC coupled via C
K
to a resistive divider
which is used to optimize supply noise cancellation by the differential microphone amplifier in the module.
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
).
This circuit is well suited if the distance between microphone and module is kept short. Due to good grounding
the microphone can be easily ESD protected as its housing usually connects to the negative terminal.
3.14.2.2 Differential Microphone Input
Figure 24 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 7.1). 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 24: Differential microphone input