AirPrime Module Datasheet

Hardware Design Guidelines
Rev 1 Apr.17 21 41110693
The left channel is used for mono operation, leave the right channel unconnected.
For dual mono operation the right channel forms the auxiliary channel; connect as
per stereo example schematic.
By default mic bias will go high for a hands-free and for a music connection. The
behavior of Mic Bias can be configured. Please refer to the Melody UART manual
for configuration options.
Mic bias is 2.6V and can source up to 2.8mA of current. Melody fixes it at a
certain voltage. Please refer to the manual. The microphone input goes into a
buffer stage so its input resistance is high.
Connecting Speakers
The analogue output is not AC coupled. The analogue output circuit comprises a
DAC, a buffer with gainsetting, a low-pass filter, and a class AB output stage
amplifier. The output is available as a differential signal between SPKR_LN and
SPKR_LP for the left channel, and between SPKR_RN and SPKR_RP for the
right channel. The amplifiers expect a load impedance of 16 to 32 Ohms. Peak
voltage output depends on the gain setting though one should expect nothing
more than a peak to peak voltage of 1.5V with a DC offset of around 1.2V at
maximum gain.
External Amplifier Support
On the BC127/BC127-HD/BC127-EXT (non MFI builds), PIO3 goes high when an
audio link is active. That PIO can be used for turning ON and OFF an external
amplifier.
The BC127/BC127-HD/BC127-EXT is designed to provide direct speaker drive
however in the case where higher audio power is required, the BC127/BC127-
HD/BC127-EXT can drive an external amplifier.
The BC127/BC127-HD/BC127-EXT speaker outputs are DC coupled to the
Bluetooth IC internal to the module. The speaker outputs are referenced to 1.8V.
Sierra Wireless recommends connecting the BC127/BC127-HD/BC127-EXT to
an external amplifier in a differential configuration for enhanced noise immunity. In
this configuration common mode noise is rejected by the amplifier configuration.
In many applications supply noise can significantly degrade audio performance so
supplying filtering and layout grounding is essential to gain best performance in
audio applications. This is especially important in USB powered applications or
where a USB source is used to charge a battery powered applications, USB
supplies vary significantly with noise levels and supply stability. Good grounding
and isolation of noise ground currents is essential for superior audio applications.
In addition to steady state noise levels from USB supplies, the plugging and
unplugging of USB chargers can cause large supply transients that ripple through
the supply chain to cause clicking and popping in the audio domain. Sierra
Wireless recommends following good noise immunity pcb design practices,
ground isolation, short residual current return paths and the use of ferrite bead
and large decoupling capacitors on USB supply connections.