Flat Bandwidth Expansion for Spin-Valve GMR Magnetic Field Sensors

The solid-state magnetic field sensors developed in recent years are transducers that convert magnetic fields under test into digital or analog voltage outputs. [1] The system consists of the front-end sensing element and the rear-end signal processing unit. The key performances of magnetic field sensor are demonstrated by its sensitivity, noise, linear range, and frequency bandwidth. The ultimate frequency response and noise behavior of the sensing system are determined by the driving method for the front-end sensing element. For quasi-static geomagnetic applications, flipping or modulation techniques is used to minimized the hysteresis for high accuracy measurement of DC field [2], but the available flat bandwidth is limited to less than one tenth of modulation frequency [3]. In contrast, a flat bandwidth over 30 kHz is observed for the DC-field-biased GMR sensor with a magnetic field feedback [4], which is ideal for high frequency applications, e.g. eddy-current sensing. However, the output under a DC-biased field is hysteretic and hence it is not suitable for DC measurement. For industry and high-end consumer applications, a magnetometer with a large bandwidth is necessary for mixed AC and DC measurement. A straight forward implementation of high bandwidth DC/AC magnetometer is to combine the DC sensor with and an AC induction coil [5], [6]. However, the hybrid magnetometer of this kind has a feature size much larger than the solid-state sensor in it. In addition, the output levels for the two element sensors could be very different, inducing more complexity in signal processing. In this work, we explore the flat bandwidth expansion technique using two spinvalve GMR magnetic field sensors driven by different schemes, i.e. field modulation and DC-field-bias. Fig.1 shows the block diagram for the analog signal processing of the system. The low-frequency GMR sensor (LF sensor) is driven by AC magnetic modulation field to achieve high linearity and low hysteresis. A low-pass filter is used to extract the DC output induced by the nonlinear voltage-field relation of spin valve. In this way, the complexity of driving circuit with the inclusion of synchronous detection is avoided. The high-frequency GMR sensor (HF sensor) driven by a DC magnetic field bias has higher hysteresis in the low frequency range, but it exhibits high sensitivity and good linearity for high frequency field measurement. The LF sensor is suitable for detection of DC and low frequency magnetic field, while the HF sensor is capable of AC measurement at higher frequencies. The combined sensor output of the system has a wide bandwidth and is suitable for both DC and AC measurement. To combine the output of the two sensors, the output of HF sensor is high-pass (6 dB/oct) filtered at 100 Hz, and the output of LF sensor is low-pass (6 dB/oct) filtered at 100 Hz. The amplifier gains of the two outputs are fine tuned to achieve a uniform sensitivity, and the two outputs are combined with a summing amplifier. The observed outputs of LF and HF sensors as well as the combined signals are shown in Fig. 2. It was found that the front-end sensitivities are 9.8 V/T for LF sensor and 251 V/T for HF sensor. The total sensitivity of the combined GMR sensing system is 2312 V/T with the total amplifier gains of 500 for the LF sensor and 20 for the HF sensor, supply voltage is 0.69 V for LF sensor and 2.61 V for HF sensor. The linearity error of the combined output signal is 2.2% within the ± 4μT range. The field noise spectral density is 65 nT$/ \surd $ Hz at 1 Hz. The output of summing amplifier is well consistent with theoretical values predicted by numeral calculations from the circuit model. The flat bandwidth of the combined output is expanded to the range from DC to above 10 kHz. The maximum deviation of normalized sensitivity is 1.7% at 100 Hz and the 3-dB flat bandwidth is 40 kHz. The phase lag of the combined output changes slowly to from 0° to - 15° with an increasing frequency from 1 Hz to 1 kHz. The phase lag increases to -130° at 10 kHz and becomes random at frequencies above 100 kHz. Further improvement in the uniformity of frequency response is possible by optimizing the circuit parameters of low-pass and high-pass filters to fine tune their characteristic frequencies and gains. The observed frequency response of our system surpasses the existing GMR sensing systems reported by Tyler et al. [7], for which the 5% flat bandwidth is 530 Hz, and 10% flat bandwidth is 1.515 kHz. Our system exhibited a 5% flat bandwidth of 1.7 kHz and a 10% flat bandwidth of 8.3 kHz, while the response to quasi-static sweeping field at frequencies below 0.1 Hz shows negligible hysteresis. The broad bandwidth of our system makes it suitable for detecting the DC and AC magnetic fields in nondestructive evaluation [4]. It is also suitable for monitoring the environmental field in transient motion tracking with the software gyroscope [8] consisting of magnetic sensor and accelerometer. This work is supported by the Ministry of Science and Technology of Taiwan under Grant No. MOST MOST 105-2221-E-151-038.Fig. 1.Schematics of the signal processing unitFig. 2.Normalized sensitivity of LF sensor, HF sensor, and combined output of two sensors. Brown: LF sensor, Purple: HF sensor, Blue: Output of summing amplifier.