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Journal of Applied Physics / Volume 99 / Issue 8 / PROCEEDINGS OF THE 50TH ANNUAL CONFERENCE ON MAGNETISM AND MAGNETIC MATERIALS / Magnetic Sensors (Not for Recording)

J. Appl. Phys. 99, 08B320 (2006); doi:10.1063/1.2171942 (3 pages)

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

J. Deak, A. Jander, E. Lange, S. Mundon, D. Brownell, and L. Tran

NVE Corp., 11409 Valley View Road, Eden Prairie, Minnesota 55344

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(Published online 3 May 2006)

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.

© 2006 American Institute of Physics

Article Outline

  1. INTRODUCTION
  2. MAGNETOMETER DESIGN AND OPERATION
  3. CONCLUSIONS

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KEYWORDS and PACS

Keywords

analogue-digital conversion, magnetometers, magnetic sensors, tunnelling magnetoresistance, microsensors, delta-sigma modulation

PACS

  • 07.55.Ge

    Magnetometers for magnetic field measurements

  • 07.07.Df

    Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

  • 07.10.Cm

    Micromechanical devices and systems

  • 84.30.-r

    Electronic circuits

ARTICLE DATA

Permalink
http://link.aip.org/link/doi/10.1063/1.2171942

PUBLICATION DATA

ISSN:

0021-8979 (print)  
1089-7550 (online)

Publisher:

$abstract.society.value is a Member of CrossRef American Institute of Physics

For access to fully linked references, you need to log in.
    L. Jiang, E. R. Nowak, P. E. Scott, J. Johnson, J. M. Slaughter, J. J. Sun, and R. W. Dave, "Low frequency magnetic and resistance noise in magnetic tunnel junctions," Phys. Rev. B 69, 054407 (2004)
    N. A. Stutzke, S. E. Russek, D. P. Pappas, and M. Tondra "Low-frequency noise measurements on commercial magnetoresistive magnetic field sensors," J. Appl. Phys..97, 10Q107 (2005)JAPIAU00009700001010Q107000001

    J. L. Garcia-Palacios and F. J. Lazaro, "Langevin-dynamics study of the dynamical properties of small magnetic particles," Phys. Rev. B 58, 14937 (1998).


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