Operation of Lorentz-Force MEMS Magnetometers With a Frequency Offset Between Driving Current and Mechanical Resonance

The paper discusses the operation of Lorentz-force-based microelectromechanical magnetometers at a driving-current frequency slightly lower than the device resonance frequency. Among the advantages with respect to operation at resonance, there are a higher achievable signal to noise ratio (thanks to the lower permitted pressure and damping coefficient, which have now no influence on the maximum sensing bandwidth) and the possibility of driving more magnetometers in series through a single current source, enabling the fabrication of low-power 3-axis magnetic field sensors. A partial drawback is represented by a loss in gain-factor. Experimental results obtained on a sample device confirm the trade-off between gain-factor decrease and bandwidth increase. Guidelines for an optimized design of Lorentz force magnetometers are given together with a comparison with other state-of-the-art technologies through the introduction of a figure of merit. In particular, it is shown how Lorentz force devices can reach better performance in terms of minimum detectable magnetic flux density per unit current consumption and bandwidth.