A 3-D micromechanical multi-loop magnetometer driven off-resonance by an on-chip resonator

This paper presents the principle and complete characterization of a single-chip unit formed by microelectromechanical system magnetometers to sense the 3-D magnetic field vector and a Tang resonator. The three sensors, nominally with the same resonance frequency, are operated 200-Hz off-resonance through an ac current whose reference frequency is provided by the resonator embedded in an oscillating circuit. The sensors gain is increased by adopting a current recirculation strategy using metal strips directly deposited on the structural polysilicon. At a driving value of 100 μArms flowing in series through the three devices, the magnetometers show a sub-185 nT/Hz Hz resolution with a selectable bandwidth up to 50 Hz. Over a ±5-mT full-scale range, the sensitivity curves show linearity errors lower than 0.2%, with high cross-axis rejection and immunity to external accelerations. Under temperature changes, the stability of the 200-Hz difference between the magnetometers and the resonator frequency is within 55 ppm/K. Offset is trimmed down to the microtesla range, with an overall measured Allan stability of about 100 nT at 20-s observation time. [2016-0030]