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. 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. 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. 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.