LISA-C2 series and FW75-C200 - System Integration Manual
UBX-13000620 - R21
Early Production Information
System description
Page 40 of 103
1.11.1.1
Uplink path (differential analog audio input)
The pins related to the differential analog audio input are:
•
MIC_P / MIC_N
: Differential analog audio signal inputs (positive/negative). These two pins are provided with
internal series 100 nF capacitors for DC blocking that connect the module pads to the differential input of a
Low Noise Amplifier. The LNA output is internally connected to the digital processing system by an
integrated sigma-delta analog-to-digital converter
The analog audio input is selected when the parameter <main_uplink> in AT+USPM command is set to “Analog
microphone”: the uplink analog path profiles use the same physical input but have different sets of audio
parameters (for more details, refer to u-blox C2 series AT Commands Manual [3], AT+USPM, AT+UMGC,
AT+UUBF, AT+UHFP commands).
There is no microphone supply pin available on the module: an external low noise LDO voltage regulator should
be added to provide a proper supply for a microphone.
1.11.1.2
Downlink path (differential analog audio output)
The pins related to the differential analog audio output are:
•
SPK_N / SPK_P
: Differential analog audio signal output (positive/negative). These two pins are internally
directly connected to the differential output of a low power audio amplifier, for which the input is internally
connected to the digital processing system by to an integrated digital-to-analog converter.
•
The Analog audio output is selected when the parameter <main_downlink> in AT+USPM is set to Analog
earpiece.
•
The differential analog audio output can be directly connected to a headset earpiece or handset earpiece but
is not able to drive an 8
Ω
speaker.
Warning: excessive sound pressure from headphones can cause hearing loss.
Name
Description
Remarks
MIC_P
Differential analog audio input (Positive)
Internal DC blocking capacitor
MIC_N
Differential analog audio input (Negative)
Internal DC blocking capacitor
SPK_P
Differential analog audio output (Positive)
No DC blocking capacitor required
SPK_N
Differential analog audio output (Negative)
No DC blocking capacitor required
Table 20: Signals relating to analog audio functions
The audio pins ESD rating is 1 kV (contact discharge). A higher protection level could be required if the lines
are externally accessible on the application board. A higher protection level can be achieved by mounting
an ESD protection (e.g. EPCOS CA05P4S14THSG varistor array) on the lines connected to these pins.
All corresponding differential audio lines must be routed in pairs, be embedded in GND (have the ground
lines as close as possible to the audio lines), and maintain distance from noisy lines such as VCC and from
components such as switching regulators.
If the audio pins are not used, they can be left unconnected on the application board.
1.11.1.3
Hands-free profile
The hands-free profile is used by default.
Hands-free functionality is implemented using appropriate digital signal processing algorithms for voice-band
handling (echo canceller and automatic gain control), managed via software (refer to u-blox C2 series AT
commands manual [3], AT+UHFP command).
Figure 22 shows an example of an application circuit connecting a 2.2 k
Ω
electret microphone and an 8
Ω
speaker to the LISA-C200 module, with an external low noise LDO voltage regulator to provide a proper supply
for the microphone and with an external audio amplifier to amplify the low power audio signal provided by the
module differential output.
