Delta-sigma digital magnetometer utilizing bistable spin-dependent-tunneling magnetic sensors

Abstract

A digital magnetometer, utilizing a micron-sized spin-dependent-tunneling (SDT) sensor as a field-dependent bistable difference node within an oversampling first-order delta-sigma (Δ−Σ) analog-to-digital converter, was designed and prototyped. The Δ−Σ magnetometer can be fabricated as a single-chip device. It is intended for integration directly with digital components, facilitating development of low-cost magnetic-field sensing systems for commercial and military navigation, security, and linear-magnetic-field sensing applications. The Δ−Σ magnetometer operates by pulsing the SDT element along its hard axis (HA) in order to set the magnetization of the free layer into an unstable equilibrium orientation along the HA. In the absence of an easy-axis (EA) magnetic field, thermal fluctuations result in an equal probability for the magnetization to relax into one of the two stable orientations along the EA. Application of an EA magnetic field increases the probability for the magnetization to relax into the EA orientation parallel to the applied field. Thus, repeatedly pulsing the SDT sensor along the HA and monitoring the magnetoresistance results in a digital bitstream, where the number of 1s within a fixed length of the bitstream provides a measure of the applied field. The SDT sensor bitstream is thus integrated in order to produce an EA feedback signal. The raw bitstream from the SDT sensor provides the digital output of the magnetometer. The length of the bitstream summing window is the oversampling ratio, which determines the minimum possible digital resolution. A prototype Δ−Σ magnetometer was constructed, and several different bit shapes were tested in order to optimize performance. The digital output of the device was found to provide a count that is linearly proportional to the applied field over a range determined by the EA feedback driver current limit and loop gain.