TOBY-L4 series - System Integration Manual
UBX-16024839 - R04
Design-in
Page 104 of 143
Guidelines for connecting one external microphone
Figure 61 and Table 45 show an application circuit connecting the uplink path of the analog audio interface of
TOBY-L4 series modules to one electret microphone:
One external 2.2 k
electret microphone is connected to an analog audio uplink path of the module
The same circuit can be implemented for the
MIC1_P
/
MIC1_N
and
MIC2_P
/
MIC2_N
differential analog audio
inputs of the module.
As in the example circuit in Figure 61 and Table 45, the following general guidelines are recommended for the
design of an analog audio circuit connecting an external electret microphone:
Provide a correct supply to the used electret microphone, as for example providing a clean connection from
the
MIC_BIAS
supply output to the microphone. It is suggested to implement a bridge structure:
o
The electret microphone, with its nominal intrinsic resistance value, represents one resistor of the bridge.
o
To achieve good supply noise rejection, the ratio of the two resistances in one leg (R2 / R3) should be
equal to the ratio of the two resistances in the other leg (R4 / MIC), i.e. R2 must be equal to R4 (e.g.
2.2 k
) and R3 must be equal to the microphone nominal intrinsic resistance value (e.g. 2.2 k
).
Using the
MIC_BIAS
supply output of the module, provide a suitable series resistor at the
MIC_BIAS
supply
output and then mount a suitable large bypass capacitor to provide additional supply noise filtering. See the
R1 series resistor (2.2 k
) and the C1 bypass capacitor (10 µF).
Do not place a bypass capacitor directly at the
MIC_BIAS
supply output, since a suitable internal bypass
capacitor is already provided to guarantee stable operation of the internal regulator.
Connect the reference of the microphone circuit to the
MIC_GND
pin of the module as a sense line.
Provide a suitable series capacitor at both
MIC1_P
/
MIC1_N
and/or
MIC2_P
/
MIC2_N
analog uplink inputs
for DC blocking (as the C2 and C3 Murata GRM155R71C104K 100 nF capacitors in Figure 61). This provides
a high-pass filter for the microphone DC bias with the correct cut-off frequency according to the value of
the resistors of the microphone supply circuit. Then connect the signal lines to the microphone.
Provide the correct parts on each line connected to the external microphone as noise and EMI
improvements, to minimize RF coupling and TDMA noise, according to the custom application requirements.
o
Mount an 82 nH series inductor with a Self Resonance Frequency ~1 GHz (e.g. the Murata
LQG15HS82NJ02) on each microphone line (L1 and L2 inductors in Figure 61).
o
Mount a 27 pF bypass capacitor (e.g. Murata GRM1555C1H270J) from each microphone line to the
solid ground plane (C4 and C5 capacitors in Figure 61).
Use a microphone designed for GSM applications, which typically has internal built-in bypass capacitor(s)
with Self-Resonant Frequency in the cellular frequency range(s).
Provide additional ESD protection (e.g. Bourns CG0402MLE-18G varistor) if the analog audio lines will be
externally accessible on the application device, according to the EMC/ESD requirements of the custom
application. Mount the protection close to an accessible point of the line (D1-D2 in Figure 61).
