Magnetic Beads Detecting System Design Based On Spin Valve Giant Magneto-Resistance Sensor

Biosensors has a very wide range of applications in medical research, clinical diagnosis and ecological agriculture, et al. In recent years, the biosensor has been in a hot research topic in the field of sensing technology. Since the giant magneto-resistance (GMR) effect was found, Giant magnetoresistance biosensor has opened up a new research direction in the field of biosensors. Giant magnetoresi stance biosensor is mainly composed of GMR sensor and signal detection circuit, the test object is biological samples containing immune magnetic beads markers. GMR sensor has several advantages:high sensitivity, wide detection range, resistance to bad environment and low cost, et al, and the GMR sensor is more suitable for the integration requirement by using existing semiconductor microelectronics preparation technology. At present, the giant magnetoresi stance materials have multilayer film structure, tunnel junction structure and spin valve structure and so on, and the GMR sensor made by them can be used in the detection of biological sample.The general steps of biological detection by GMR biosensor are:First, fixed biological probe on the surface of the sensor for specific detect; Second, the target molecular in the biological sample combined with immune magnetic beads by the specific binding between biological molecules; Third, the target creature components in the sample solution will be captured by the biological probe fixed on the surface of the sensor when biological sample fluid flows through the sensor’s surface; Finally, the immune magnetic beads will generate additional magnetic field under the action of external magnetic field, and GMR sensor has a output signal under the action of additional magnetic field, so as to realize the detection of biological sample. The direct detection objects of the GMR sensor is immune magnetic beads and its detection level will directly determine the quality of biological detection.Besides the GMR sensor, another factor influencing the detection performance is signal detection circuit. Because of the GMR biological sensor’s output signal is very weak, the weak signal detection technology is usually required in order to improve the detection sensitivity. Lock-in amplifier is often used in the experiment. The lock-in amplifier is based on relevant principle to test the signal, it can maximize compress the bandwidth and limit noise and thus pick up the weak signal buried in strong noise, and it has high stability and flexibility. But the commercial lock-in amplifier has several shortcomings:bulky, expensive and not suitable for site test, etc. Therefore, the design of weak signal detection circuit according to the characteristics of the GMR biological sensor’s output signal has practical application value. The goal of this issue is to design a magnetic bead detection system, the main work is as follows:①According to the working principle and characteristics of the spin valve GMR sensor, determine its work mode and its output signal detection method, the overall design scheme of magnetic beads detecting system was put forward.②Design and manufacture the helmholtz coil according to actual needs of the detection system.③According to the principle of correlation detection, we made the modeling and the simulation to the correlation detection circuit which belongs to the weak signal detection circuit by using Matlab/simulink. Completed the design of DDS circuit and A/D conversion circuit control module based on the FPGA chip on the Quartus Ⅱ11.0software platform. Furthermore, completed hardware design of the weak signal detection circuit and draw the corresponding PCB diagram.④The weak signal detection circuit test experiment was carried out, test results show that the design of signal detection circuit can achieve the expected goal.⑤Build magnetic beads testing system. First, did the characteristics test of the spin valve GMR sensor and determine the linear working area; Then, the magnetic beads detecting experiment was carried out. Magnetic beads detecting experiment results show that the system can realize the qualitative detection of magnetic beads.