Low-noise tunneling-magnetoresistance vector magnetometers with flux chopping technique

A concept for a low-noise three-axis magnetometer consisting of tunneling magnetoresistance (TMR) sensors and a flux chopper was designed, implemented, and characterized in this work. The TMR sensors used in this study were the model of TMR2102D from Multidimension Technology Inc. Three TMR sensors were aligned orthogonally on a printed circuit board (PCB) and mounted inside a cylindrical flux chopper. The cylindrical flux chopper including a soft magnetic shielding tube and enameled copper wires was 16 mm in length and 8 mm in diameter. The shielding tube was made of a cobalt-based soft magnetic ribbon, Metglas-2714A, from Metglas Inc. The flux chopper modulated the external magnetic flux density using the fluxgate effect, which made the sensors respond to the quasi-static field at the chopping frequency. The demodulated output showed a reduction in low-frequency noise to the level of 0.17 nT/√Hz@1 Hz. To demonstrate the technical feasibility for electronic compass application, the demodulated output of the vector magnetometer prototype was recorded by rotating the device in Earth's magnetic fields about a fixed axis. The Cartesian components Bx, By, and Bz of the Earth's fields at various azimuth angles were retrieved by performing a calibration algorithm to correct the non-orthogonality caused by the misalignment of TMR sensors and the chopper. The calibrated outputs were linear and orthogonal to each other with an angle error less than 1° and nonlinearity of 0.7%, indicating that the chopping technique is useful to realize an extremely low-noise three-dimensional magnetometer for the geomagnetic application.