Abstract
Tunnel magnetoresistance (TMR) is a kind of magnetic sensor with the advantages of low cost and high sensitivity. For ultra-weak and low-frequency magnetic field measurement, the TMR sensor is affected by the 1/f noise. This paper proposes an AC modulation method with impedance compensation to improve the performance. The DC and AC characteristics of the sensors were measured and are presented here. It was found that both the equivalent resistance and capacitor of the sensors are affected by the external magnetic field. The TMR sensors are connected as a push–pull bridge circuit to measure the magnetic field. To reduce the common-mode noise, two similar bridge circuits form a magnetic gradiometer. Experimental results show that the sensor’s sensitivity in the low-frequency range is obviously improved by the modulation and impedance compensation. The signal-to-noise ratio of the sensor at 1 Hz was increased about 25.3 dB by the AC modulation, impedance compensation, and gradiometer measurement setup. In addition, the sensitivity of the sensor was improved from 165.2 to 222.1 mV/V/mT. Ultra-weak magnetic signals, namely magnetocardiography signals of two human bodies, were measured by the sensor in an unshielded environment. It was seen that the R peak of MCG can be clearly visualized from the recorded signal.
Highlights
Academic Editor: Arcady ZhukovMeasurement of the ultra-weak low-frequency magnetic field with a highly sensitive magnetic sensor is an essential technology in many applications, such as biomagnetism measurement, detection of submarine objects, and geological surveys
This paper proposed an AC modulation method with impedance compensation to improve the sensitivity of Tunnel magnetoresistance (TMR) sensors for ultra-weak magnetic field measurement
It was found that with AC excitation, both the resistance and capacitance of the TMR sensor were affected by the external magnetic field
Summary
Measurement of the ultra-weak low-frequency magnetic field with a highly sensitive magnetic sensor is an essential technology in many applications, such as biomagnetism measurement, detection of submarine objects, and geological surveys. Magnetocardiography (MCG) is a technique to measure the weak magnetic fields of the human heart, and is used for heart health monitoring and disease diagnosis [1,2]. Because the MCG signal is very weak, a very highly sensitive magnetic sensor is required to take a measurement. Tometer is in the range of a few fT/ Hz, which is capable of measuring the magnetic field of the human heart and brain (magnetoencephalogram) [4]. SQUIDs require complex cryogenics and consume liquid helium, which is a scarce natural resource, making it bulky in size and expensive to build and use. High demand exists for the development of alternative sensors
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