Design And Implementation Of MRS Signal Full-wave Acquisition System

Water is one of the most valuable natural resources for the survival of mankind. To solve the water crisis, high-tech investigation for groundwater has become an urgent task. Twenty years on, the only direct detection for groundwater is the ground geophysical methods MRS. It uses the proton magnetic moment which displays under the earth magnetic field. Along the earth magnetic field vertical direction, we add an alternating magnetic field in to stimulate the underground hydrogen proton; the proton magnetic moment is able to deviate the earth magnetic field direction to form the macroscopic magnetic moment. After the excitation field stops, the macroscopic magnetic moment can restore to and produces the weak signal along the earth magnetic field direction. With the signal, we can gather the information about the water content, the depth and the porosity of the underground water. As the signal is so weak (only nV), the method is easily disturbed by the noise. And the frequency is (1.278~2.556 kHz). So a high performance acquisition system is required. This paper is to design an acquisition system that fits MRS signal. MRS signal acquisition system is the key unit of the MRS instrument. Its performance decides the whole instrument's exploration ability.Group of MRS Instruments (Jilin University) has already developed the MRS underground water investigation instrument JLMRS-I for shallow groundwater investigations (down to approximately 150 m depth). The existing acquisition system detects the envelope of MRS signal. This system uses 51 Single-chip to control 16-bit A/D to sample MRS signal. But it has the shortage in storage capacity, acquisition speed, accuracy and the amount of information collected. In view of the particularity of MRS signal and the inadequacies of current system, this paper proposes a new program: ARM7+CPLD+ AD7760+DDS. MRS signal full-wave acquisition system is given out based on the program. It is consisted by signal conditioning circuit, A/D circuit, CPLD circuit, ARM circuit, DDS circuit, communication circuit and PC software.Signal conditioning circuit uses low noise, high speed op amp (OP37). It transfers single-ended signal to differential signals, ensuring the integrity and stability of MRS signal.A/D circuit selects the high-precision, high sampling rate, 24-bit parallel A/D (AD7760). Its reference voltage is 2.5V which is provided by low noise, high accuracy ADR431. For AD7760's digital circuitry and FIR Filter is 2.5 V Power Supply, so its digital pins are connected to the bank of CPLD which is 2.5V power supply.CPLD has four banks. Each bank can be powered by different voltage to work as voltage-level translator. Also it has the functions: AD7760's reading timing control, data latches, address decoding and so on.ARM circuit completes the minimum system design. It is the core control part of the entire acquisition system. ARM circuit supports emulation and debugging via a JTAG serial port.This paper has completed the design of peripheral circuits for AD9850. The frequency could be programmed according to the Lamor frequency and the noise of the local by writing cotrol words to AD9850.Communication circuit provides the RS-485 interface circuit. After all the data stored in 64k static RAM, the data then could be sent to the PC to process via the communication circuit.PC software is designed based on MATLAB GUI. After receiving data from serial port, the PC software draws a graph, and also shows the sampling frequency, sampling time and other information. The software is simple and easy to operate.Through the experiments and improvement, MRS signal full-wave acquisition system's performance is improving continuously. The results showed that the system can work stablely and reliablely.