SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26
Design-in
Page 149 of 217
2.7
Audio interface
2.7.1
Analog audio interface
SARA-G300, SARA-G310 and SARA-U2 modules do not provide analog audio interface.
2.7.1.1
Guidelines for microphone and speaker connection circuit design (headset / handset modes)
SARA-G340 and SARA-G350 modules provide one analog audio input path and one analog audio output path:
the same paths are used for both headset and handset modes, so that basically the same application circuit can
be implemented for both headset and handset modes.
Figure 83 shows an application circuit for the analog audio interface in headset and handset modes, connecting
a 2.2 k
electret microphone and a 16
receiver / speaker:
External microphone can be connected to the uplink path of the module, since the module provides supply
and reference as well as differential signal input for the external microphone.
A 16
receiver / speaker can be directly connected to the balanced output of the module, since the
differential analog audio output of the module is able to directly drive loads with resistance rating greater
than 14
.
As in the example circuit in Figure 83, follow the general guidelines for the design of an analog audio circuit for
both headset and handset modes:
Provide proper supply to the used electret microphone, providing a proper 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 resistance in one leg (R2/R3) should be
equal to the ratio of the two resistance 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
).
Provide a proper series resistor at the
MIC_BIAS
supply output and then mount a proper 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 proper 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 proper series capacitor at both
MIC_P
and
MIC_N
analog uplink inputs for DC blocking (as the C2
and C3 100 nF Murata GRM155R71C104K capacitors in Figure 83). This provides a high-pass filter for the
microphone DC bias with proper cut-off frequency according to the value of the resistors of the microphone
supply circuit. Then connect the signal lines to the microphone.
Provide proper 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 83).
o
Mount a 27 pF bypass capacitor (e.g. Murata GRM1555C1H270J) from each microphone line to solid
ground plane (C4 and C5 capacitors in Figure 83).
Use a microphone designed for GSM applications, which typically has an internal built-in bypass capacitor.
Connect the
SPK_P
and
SPK_N
analog downlink outputs directly to the receiver / speaker (which resistance
rating must be greater than 14
).
