A typical form of noise to audio signal in a Bluetooth design is caused by Bluetooth system (RF) transmitting data in 625 us time slots. 625 us time slot corresponds to 1,6 kHz noise and five slot packet in A2DP profile corresponds to 320 Hz noise. The amount of noise coupled to audio signal depends strongly on the design. Since the noise is common mode noise for the audio, the most effective way to avoid the problem is to use fully differential signals as far as possible.
The following figure shows an example schematic for a stereo headset using one microphone and without an external audio PA. The headphones can be connected directly to WT32 without any external components. Fully differential signals give perfect common mode rejection and excellent audio quality.
The following figure shows an example how to connect WT32 to stereo jack connector. The PA converts fully differential signals to single ended. R6 and R7 help to reduce pops and clicks when plugging the headphones, and R8 + C14 form a zobel network. The purpose of the zobel network is to tune out the inductance of the speaker coil and thus making the effective load to become resistive. The values chosen depend on the speaker coil selected by the customer and thus some optimization may be required. The component values can be approximated by calculating
where Rs is the series resistance of the speaker and
where fd is the frequency where the impedance of the speaker doubles.
General rules for PCB layout:
Route the analogue audio lines away from noise sources, e.g. antenna, RF, clock, high-speed data, etc.
Pay attention to audio input lines and/or microphone because those are very sensitivity to noise
Minimize size of current loops
If differential mode is use, the audio traces should be routed as differential pairs and they should run parallel all the way to the pins of the module. For example,
Note: you can also refer to the WT32 Evaluation Kit schematic and PCB layout at