AirPrime Module Datasheet

Hardware Design Guidelines
Rev 1 Apr.17 23 41110693
No two audio amplifiers are the same so it is important to ensure that you follow
the design recommendations provided by the supplier of the amplifier.
When connecting to external devices such as laptops in a “line In” configuration,
there is a possibility that the sleeve of the audio cable can carry noise derived
from the laptop. Also external devices connected through the audio cable sleeve
can have different ground potentials which causes currents to flow through the
sleeve ground. In this case it is important to isolate the sleeve ground from the
audio amplifier to avoid the noise coupling to the amplifier and degrading the
signal source. In this case add a filter circuit or ferrite bead connecting the sleeve
ground to the ground on the application board as shown in Figure 4-7 below.
Figure 4-8: Isolating the Sleeve Ground from the Amplifier
Some amplifiers do not support “capless” operation and require AC coupling on
the input and output of the amplifier. Series capacitance can introduce clicks and
pops during turn off and turn on as voltage transients occur across the capacitors
causing audible spikes on the audio output.
Choice of capacitors used in these configurations is important; avoid using
ceramic capacitors as these tend to have high voltage coefficients. Use low
voltage coefficients capacitors such as tantalum or electrolytic capacitors to
reduce low frequency distortion effects.
In differential mode the tolerance of the source and feedback resistors which set
the gain of an amplifier is important. 1% resistors give a 40dB CMRR whereas
0.1% resistors give a 60dB CMRR. Use 0.1% resistor arrays for even better
performance.
The BC127/BC127-HD/BC127-EXT can be used to drive amplifiers in a single
ended format. Simply terminate the +ve differential outputs directly to the audio
ground. An example of a single ended connection is shown below.