I'm using an ADAU1761 DSP to process some microphone signals and then send a value that I read with DSP Readback to a microcontroller, which then sends this value to an Android App on a smartphone via Bluetooth (or USB connexion).
Sadly, as soon as I connect the Bluetooth dongle or a USB cable (USB cable between my device using DSPs and the phone), the level that I read at the Readback is very noisy and level goes up. I monitor the Readback value in SigmaStudio while connecting and disconnecting the Bluetooth dongle.
I tried using a USB isolator circuit ( http://hifimediy.com/usb-isolator ) which uses an ADUM4160 chip to isolate USB GND. It helped reducing the noise especially for my connexion through USB cable, but didn't help much for the Bluetooth dongle.
I have tried 2 different Bluetooth dongles in the past and the 1st I used was inducing a lot of noise and the 2nd I use at the moment is less noisy but still not enough. Maybe there is a better Bluetooth dongle that wouldn't induce too much noise (sufficiently isolated...) ???
Is there a way to power the Bluetooth dongle (connected in USB port of my device) externally in a way that wouldn't induce interference in my Mic amps path? Like cutting the traces of power on the Bluetooth dongle and plugging them to a DC source or Battery pack? Would the USB communication from microcontroller to the Bluetooth dongle still work? Could it also be that the USB communication itself is inducing the noise in my Mic Amp path?