A Mixed-Signal Control System for Lorentz-Force Resonant MEMS Magnetometers

This paper presents a mixed-signal closed-loop control system for Lorentz force resonant MEMS magnetometers. The control system contributes to 1) the automatic phase control of the loop, that allows start-up and keeps self-sustained oscillation at the MEMS resonance frequency, and 2) output offset reduction due to electrostatic driving by selectively disabling it. The proposed solution proof-of-concept has been tested with a Lorentz force-based MEMS magnetometer. The readout electronic circuitry has been implemented on a printed circuit board with off-the-shelf components. Digital control has been implemented in an FPGA coded with VHDL. When biased with 1 V and a driving current of $300 \mu ~A_{\textrm {rms}}$ , the device shows 9.75 pA/ $\mu \text{T}$ sensitivity and total sensor white noise of 550 nT/ $\sqrt {\textrm {Hz}}$ . Offset when electrostatic driving is disabled is 793 $\mu \text{T}$ , which means a 40.1% reduction compared when electrostatic driving is enabled. Moreover, removing electrostatic driving does not worsen bias instability, which is lower than 125 nT in both driving cases.