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SARA-G3 and SARA-U2 series - System Integration Manual

UBX-13000995 - R08

Objective Specification

Design-in

Page 133 of 188

2.7.1.3

Guidelines for external analog audio device connection circuit design

The differential analog audio I/O can be used to connect the module to an external analog audio device. Audio
devices with a differential analog I/O are preferable, as they are more immune to external disturbances.
Figure 77 and Table 43 describe the application circuits, following the suggested circuit design-in.

Guidelines for the connection to a differential analog audio input:



The

SPK_P

SPK_N

balanced output of the module must be connected to the differential input of the

external audio device by means of series capacitors for DC blocking (e.g. 10 µF) to decouple the bias present
at the module output, as described in the left side of Figure 77

Guidelines for the connection to a single ended analog audio input:



A proper differential to single ended circuit must be inserted from the

SPK_P

SPK_N

balanced output of

the module to the single ended input of the external audio device, as described in the Figure 77 right side:
10 µF series capacitors are provided to decouple the bias present at the module output, and a voltage
divider is provided to properly adapt the signal level from module output to external audio device input

Guidelines for the connection to a differential analog audio output:



The

MIC_P

MIC_N

balanced input of the module must be connected to the differential output of the

external audio device by means of series capacitors for DC blocking (e.g. 10 µF) to decouple the bias present
at the module input, as described in the Figure 77 left side

Guidelines for the connection to a single ended analog audio output:



A proper single ended to differential circuit has to be inserted from the single ended output of the external
audio device to the

MIC_P

MIC_N

balanced input of the module, as described in the Figure 77 right side:

10 µF series capacitors are provided to decouple the bias present at the module input, and a voltage divider
is provided to properly adapt the signal level from the external audio device output to the module input

Additional guidelines for any connection:



The DC-block series capacitor acts as high-pass filter for audio signals, with cut-off frequency depending on
both the values of capacitor and on the input impedance of the device. For example: in case of differential
input impedance of 600



, the two 10 µF capacitors will set the -3 dB cut-off frequency to 53 Hz, while for

single ended connection to 600



external device, the cut-off frequency with just the single 10 µF capacitor

will be 103 Hz. In both cases the high-pass filter has a low enough cut-off for proper frequency response



Use a suitable power-on sequence to avoid audio bump due to charging of the capacitor: the final audio
stage should be always enabled as last one



The signal levels can be adapted by setting gain using AT commands (see

u-blox AT Commands Manual

[3],