An Automatic Loop Gain Enhancement Technique in Magnetoimpedance-Based Magnetometer

Abstract

A low-power, low-noise, and high-bandwidth mag-1 netometer that utilizes the magnetoimpedance (MI) element as a 2 sensor head is presented. The MI element has a high sensitivity, 3 and it can be implemented in the mm-scale through the MEMS 4 process. The analog front-end (AFE) circuit of the magnetometer 5 includes a digital calibration scheme that automatically enhances 6 the loop gain of the system, resulting in high bandwidth and 7 low-noise characteristics. The AFE circuit is designed based on 8 a switched-capacitor (SC) approach, and its dedicated switching 9 scheme can suppress the folded noise of an amplifier. A single-10 coil magnetic negative feedback architecture with correlated 11 double sampling (CDS) enables to achieve a high dynamic range 12 (DR) and stable passband gain in addition to simplifying the 13 structure of the MI element. The AFE chip of the magnetometer 14 is implemented in a 0.18-µm CMOS process, and it achieves an 15 8-pT/ √ Hz noise floor within a 31-kHz bandwidth and the DR of 16 96 dB, where the power consumption is 1.97 mW. 17 Index Terms-Analog front-end (AFE), biomagnetic, digital 18 calibration, Internet of Things (IoT), magnetic feedback, magne-19 toimpedance (MI) element, magnetometer. 20 I. INTRODUCTION 21 A BIOMAGNETIC sensing technique such as magneto-22 myography (MMG) or magnetoencephalography (MEG) 23 is one solution for capturing biological information with a min-24 imum invasive approach. Implantable MMG has the potential 25 to acquire fast neuronal magnetic activity, which corresponds 26 to the action potential of neurons close to skeletal muscle 27 with high spatiotemporal resolution [1], [2], [3], as opposed 28 to an approach with an optically pumped magnetometer that 29 achieves low noise but a relatively large size because of 30 the optical system [4]. Magnetometers for such applications 31 require of low noise less than 100 pT/ √ Hz, high bandwidth 32 over 10 kHz, low power, and small size because they are 33 implanted. Furthermore, a wide input range over 100 μT is 34 desired because there is a need to accept the geomagnetic field 35 and artifact without saturating the signal. A magnetic negative 36

A Benchmark of Integrated Magnetometers and Magnetic Gradiometers

“…Academic groups have been revisiting fluxgate [24] and GMI [28] in highly integrated devices, paving the road towards a full integration with the magnetic core and coil onchip. The MEMS device [39] has also substantial potential as its calculated thermal noise floor (110 fT/cm −1 √ Hz) is several decades lower than its current performance.…”

“…We also incorporated giant magneto-impedance (GMI) devices [26], [27] for their superior high-speed capability than fluxgate. We also added a recent GMI development [28], partially integrated with the mm-scale magnetoimpedance element offchip. One 3D MEMS magnetometer [29] was also added to acknowledge its single-chip (or single-package) integration potential.…”

“…We coded a Noise_Model class in Python decoding the specification string, and performing the calculation of the RMS noise in the same 500-Hz bandwidth for all sensors. As an example, the specific case of [28] is treated in the Appendixes. All devices are treated in the Supplementary Notes.…”

“…xMR and fluxgate devices can resolve even smaller fields with noise approaching 10 nT at the expense of the range due to the saturation effect in the soft magnetic layers. As mitigation, the fluxgate devices [23], [24] and the GMI device [28] employ feedback via another coil wrapped around the same magnetic core, thereby improving the NFR, and are optimum in the range [10 nT, 1 mT].…”