Table 20: Analog Audio Signals
2.14.2 Reference Schematics
Depending on the module, the headphone output signals can have a DC offset. A standard
solution is adding series capacitors to the headphone signal lines. If the headphone output signals
are used only as line output signals, 1µF series capacitors are sufficient. If the signals are used for
driving headphones, larger capacitors (47µF and more) are recommended. Some Colibri modules
provide a virtual headphone ground that can be used instead of the series capacitors to reduce the
BOM cost. Please note this solution only works if the attached device is isolated from the module
ground. For example, this works perfectly for headphones but does not work for an audio amplifier
that uses the same ground as the module.
The line-in and microphone signals do not require serial capacitors since they are already placed
on the module. Some microphones (e.g., the widely used electret microphones) require phantom
power. The reference schematic below shows a suitable solution for common electret microphone
capsules. Please note that some microphones require phantom power on the middle ring of the
3.5mm jack, while others need to be powered over the tip of the 3.5mm jack which is also used for
the audio signals. The Colibri modules feature a special analog ground for the microphone. This is
a switched ground. Using this ground instead of the regular analog ground allows switching off the
phantom power when the microphone is not used.
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