Unlike accelerometer, pressure, and gyroscope MEMS sensors, which use mechanical sensing elements unaffected by electromagnetic components on the PCB, magnetometers are sensitive to the magnetic fields generated by other circuit components.
The most common magnetic field sources are:
permanent ferromagnets (as found in speakers or buzzers)
induced fields within any ferromagnetic material lacking a permanent field (such as sheet steel)
fields generated by current flows (ranging from strong currents in power supply tracks to smaller currents within coils).
The post is here and a link to the NXP app note (PDF) is here.
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