TOBY-L4 series - System Integration Manual
UBX-16024839 - R04
Design-in
Page 106 of 143
Guidelines for connecting one external analog audio device output
Figure 62 and Table 46 describe application circuits connecting the uplink path of the analog audio interface of
TOBY-L4 series modules to analog audio output of generic external analog audio devices:
One differential analog audio output is connected to an analog audio uplink path of the module
One single-ended analog audio output is connected to an analog audio uplink path of the module
The same circuits can be implemented for the
MIC1_P
/
MIC1_N
and
MIC2_P
/
MIC2_N
differential analog
audio inputs of the module.
Guidelines for the connection to a differential analog audio output:
The
MIC1_P
/
MIC1_N
and/or
MIC2_P
/
MIC2_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 illustrated in the Figure 62 left side.
Guidelines for the connection to a single-ended analog audio output:
A suitable single-ended to differential circuit must be inserted from the single-ended output of the external
audio device to the
MIC1_P
/
MIC1_N
and/or
MIC2_P
/
MIC2_N
balanced input of the module, as
illustrated in the Figure 62 right side: 10 µF series capacitors are provided to decouple the bias present at the
module input, and a voltage divider is provided to adapt the signal level from the external device to the
module.
Additional guidelines for any connection:
Audio devices with differential analog I/O are preferable, as they are more immune to external disturbances.
The DC-block series capacitor acts as a high-pass filter for audio signals, with a cut-off frequency depending
on both the values of the capacitor and on the input impedance of the device. For example: for differential
input impedance of 600
, the two 10 µF capacitors will set the -3 dB cut-off frequency to 53 Hz, while for
a single-ended connection to a 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 the correct
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 the last one.
The signal levels can be adapted by setting the internal gains, but additional circuitry must be inserted if the
audio device output level is too high for
MIC1_P
/
MIC1_N
and/or
MIC2_P
/
MIC2_N
, as the voltage dividers
present in the circuits illustrated in Figure 62 right side to properly adapt the signal level.
TOBY-L4 series
GND
MICx_P
Analog OUT (-)
Analog OUT (+)
Audio Device
GND
MICx_N
C1
C2
TOBY-L4 series
GND
MICx_P
Audio Device
GND
MICx_N
Analog OUT
R2
R1
C3
C4
Figure 62: Application circuits to connect the module to audio devices with suitable differential or single-ended output
Reference
Description
Part Number – Manufacturer
C1, C2, C3, C4
10 µF Capacitor X5R 0603 5% 6.3 V
GRM188R60J106M – Murata
R1
0
Resistor 0402 5% 0.1 W
RC0402JR-070RL – Yageo Phycomp
R2
Not populated
Table 46: Example of components for connecting audio devices with suitable analog differential or single-ended output