Application Of Digital Lock-in Amplifier In Magnetic Beads Detection Based On FPGA

Weak signal detection is a very important part in the field of signal detection, it has a very wide range of applications, such as Radar, geological exploration, acoustic spectrum detection, environmental monitoring gas detection, biological samplesand detection and so on. There are many methods to detect weak signals, and the lock-in amplifier is one of the most common methods for weak signal detection. The commonly used lock-in amplifier is universal, larger, and expensive. In order to meet the requirement of the instrument, the research of all kinds of special lock-in amplifier in recent years has been paid much attention by people. This paper to quantitative detection of the concentration of magnetic beads as the background, based on FPGA as means for the realization, using the relevant principle designs a special digital lock-in amplifier which was used for to detect the magnetic beadsDue to the discovery of the giant magnetoresistance effect, the high sensitivity of the GMR sensor came into being, GMR sensor,detection circuit and magnetic beads used for the detection of biological samples are called GMR biosensor. Its works roughly: biological samples and immunomagnetic beads combined constitute of the biological probe as one part and another part of the biological probe fixed on the surface of the GMR sensor. Under the action of the external excitation field, immunomagnetic beads will generate additional magnetic field also, the superposition of the additional magnetic field and the original magnetic field will make the GMR sensor output signal changes, to establish the correspondence between the output signal and the amount of the magnetic beads can complete the detection of biological samples. However, due to the GMR sensor output signal is very small and often are buried by the noise, detection circuit if can't detect the weak change of output effectly, the detrction of biological target is also impossible. Therefore, whether detection circuit can detect the output of GMR biosensor is important, its performance will directly decide and restrict the overall performance of the GMR biosensor. Therefore, the application of weak signal detection technology in the detection circuit is essential.Lock in amplifier technology is one of the weak signal detection technologies, and is widely used, can able to detect the signal 10^-5 weaker compared to noise. The effect quite in a center frequency can be adjusted, and the bandwidth is less than the width of 0.004 HZ bandpass filter. Compared with the weak signal detection method of narrow band filtering and sampling integral, the lock-in amplifier has higher stability and more flexibility. However, the commercial lock-in amplifier need considering the universal and multifunctional, often a huge equipment and expensive, difficult to achieve today biological detection which asks for fast, cheap, scene detection and easy operation. Therefore, a lock-in amplifier which only used for magnetic detection system is meaningful. This paper is based on the magnetic detection platform, using special lock-in amplifier as the main line, main work is as follows:?1? Through the platform of DSP Builder, Designing the digital lock-in amplifier based on FPGA used to magnetic beads detection, in particular by DDS truncation algorithm and improved parallel distributed algorithm we completed the design of the reference signal module and low pass filter module respectively; according to the actual needs of the application to design magnetic signal pre processing circuit, complete amplifying, filtering and A / D conversion related functions?2? building digital lock-in amplifier and magnetic detection platform together to a complete magnetic measurement system, determine the GMR sensor working mode and characteristic curve, and dilution and mixture ratio of magnetic beads solution, to be ready for final bead assay.?3? Test is done in the bead test system, given magnetic concentration and output voltage curve of the. The experimental results show that: beads at 15.5 Gaussian DC bias magnetic field, 0.5 Gaussian and AC excitation magnetic field environment, detection circuit can detect the weak output changes of magnetic beads solution with concentration magnitude at 10^-5mg/mL, system sensitive degree: 2.05 * 105 mv.mL/mg.