SARA-G450 - System integration manual
UBX-18046432 - R08 Design-in Page 109 of 143
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2.7.1.2 Guidelines for microphone and loudspeaker connection circuit design
Figure 70 shows an application circuit for the analog audio interface, connecting a 2.2 kΩ electret
microphone and an 8 Ω or 4 Ω loudspeaker:
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.
Using an 8 Ω or 4 Ω loudspeaker, an external audio amplifier must be provided on the application
board to amplify the low power audio signal provided by the downlink path of the module, so that
the external audio amplifier will drive the 8 Ω or 4 Ω loudspeaker, since differential analog audio
output of the module is not able to directly drive loads with such a resistance rating.
As in the example circuit in Figure 70, follow the general guidelines for the design of an analog audio
circuit:
Provide an appropriate supply to the used electret microphone, providing a clean connection from
the MIC_BIAS supply output to the microphone. It is suggested to implement a bridge:
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 kΩ) and R3 must be equal to the microphone nominal intrinsic resistance
value (e.g. 2.2 kΩ).
Provide a series resistor at the MIC_BIAS supply output and then mount a good bypass capacitor
to provide additional supply noise filtering, as the R1 series resistor (2.2 kΩ) and the C1 bypass
capacitor (10 µF).
Do not place a bypass capacitor directly at the MIC_BIAS supply output, since an appropriate
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 an appropriate 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 70). This
provides a high-pass filter for the microphone DC bias with a corresponding cut-off frequency
according to the value of the resistors of the microphone supply circuit. Then connect the signal
lines to the microphone.
Provide suitable 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 70).
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 70).
Use a microphone designed for GSM applications, which typically has an internal built-in bypass
capacitor.
Provide a 47 nF series capacitor at both SPK_P and SPK_N analog downlink outputs for DC
blocking (C8 and C9 Murata GRM155R71C473K capacitors in Figure 70). Then connect the lines to
the differential input of an appropriate external audio amplifier, differential output which must be
connected to the 8 Ω or 4 Ω loudspeaker (see the Analog Devices SSM2305CPZ filter-less mono
2.8 W class-D audio amplifier in the circuit described in Figure 70).
Provide suitable parts on each line connected to the external loudspeaker as noise and EMI
improvements, to minimize RF coupling, according to the EMC requirements of the custom
application:
o Mount a 27 pF bypass capacitor (e.g. Murata GRM1555C1H270J) from each loudspeaker line
to solid ground plane (C6 and C7 capacitors in Figure 70).
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