Magnetic Field Sensing Device Using MTJ Bridge and Offset Cancellation
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Solution Overview
Problem
Conventional magnetic sensors face challenges in achieving sensitivity, minimizing offsets, and expanding dynamic range for low field measurements, particularly in mobile applications where cost, circuit area, and power consumption are concerns, and they are affected by temperature changes and mechanical stress.
Innovation Solution
A magnetic field sensing device utilizing a Wheatstone bridge configuration with four magnetic tunnel junction elements and a current source to adjust sensitivity by supplying temporally spaced currents, sampling the bridge output, and determining the magnetic field by subtracting the differences in sampled values to eliminate offsets and noise, while using an unpinned synthetic antiferromagnetic reference layer for enhanced sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If AMR based devices are used for low field sensing, then sensitivity and CMOS compatibility are achieved, but sensor size becomes large (square millimeters) and power consumption increases
Solution Approach 1:
The patent changes the fundamental sensing mechanism from AMR to MTJ, exploiting quantum tunneling effects and magnetic anisotropy parameters to achieve high sensitivity in a compact form factor. The MTJ structure with synthetic antiferromagnetic reference layer enables sensitivity comparable to AMR but with dramatically reduced area.
Solution Approach 2:
The patent employs composite material structures including synthetic antiferromagnetic layers combining CoFeB and CoFe layers, oxidized MgO tunnel barriers, and Ta seed layers to achieve both compact size and high sensitivity. These composite structures enable the MTJ to outperform conventional AMR devices.
2Measurement precision
If MTJ or GMR elements are used in Wheatstone bridge structure, then sensitivity increases and temperature dependence is eliminated, but manufacturing variations cause residual offset and device complexity increases
Solution Approach 1:
The patent segments the sensing function into four distinct MTJ elements arranged in a Wheatstone bridge, where each element can be independently optimized. This segmentation allows differential measurement that cancels common-mode errors while maintaining high sensitivity.
Solution Approach 2:
The patent creates four identical copies of the MTJ sensing element in a bridge configuration, where three elements serve as references and one as the active sensor. This copying approach enables offset cancellation through differential measurement, eliminating the need for complex compensation circuits.
3Measurement precision
If sense layer thickness and shape are optimized for sensitivity, then measurement precision improves, but useful range and linear range are limited
Solution Approach 1:
The patent introduces a current line that dynamically adjusts the magnetic field environment of the sense layer during measurement. By applying compensation currents, the system can shift the operating point of the MTJ elements, enabling the sensor to maintain linearity and sensitivity across a broader dynamic range.
Solution Approach 2:
The patent changes the operational parameters of the MTJ sensor by applying external currents to the current line, which modifies the effective magnetic anisotropy and switching fields. This parameter adjustment allows the sensor to adapt to different measurement ranges while maintaining high sensitivity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides high sensitivity, minimizes offsets, and expands the dynamic range of magnetic field measurements, reducing power consumption and maintaining accuracy in low field applications by effectively subtracting out DC and low-frequency noise, thus improving the signal-to-noise ratio and accommodating larger field ranges without loss of resolution.
Implementation Method 1
magnetic tunnel junction (MTJ) sensors and giant magnetoresistance (GMR) sensors
Implementation Method 2
A current source supplies a variable current to the at least one current line to adjust the sensitivity of the sensor
Implementation Method 3
MTJ, GMR, and AMR sensors have been employed in a Wheatstone bridge structure to increase sensitivity
Data Source
AI summary
A magnetic field sensing device for determining the strength of a magnetic field, includes four magnetic tunnel junction elements or element arrays (100) configured as a bridge (200). A current source is coupled to a current line (116) disposed near each of the four magnetic tunnel junction elements (100) for selectively supplying temporally spaced first and second currents. Sampling circuitry (412, 414) coupled to the current source samples the bridge output during the first and second currents and determines the value of the magnetic field from the difference of the first and second values. A method for sensing the magnetic field includes supplying a first current to the current line (116), supplying a second current the current line (116), sampling the value at the output for each of the first and second currents, determining the difference between the sampled values during each of the first and second currents, and determining a measured magnetic field based on the determined difference.