  We present a method developed to actively compensate common-mode magnetic
disturbances on a multi-sensor device devoted to differential measurements. The
system uses a field-programmable-gated-array card, and operates in conjunction
with a high sensitivity magnetometer: compensating the common-mode of magnetic
disturbances results in a relevant reduction of the difference-mode noise. The
digital nature of the compensation system allows for using a numerical approach
aimed at automatically adapting the feedback loop filter response. A common
mode disturbance attenuation exceeding 50 dB is achieved, resulting in a final
improvement of the differential noise floor by a factor of 10 over the whole
spectral interval of interest.
