Research On Signal Acquisition Circuit Of Long-period Geomagnetic Field Source

Long-period geomagnetic detection is one of the important geophysical methods,which has important scientific significance in resource exploration,environmental protection,disaster prevention and mitigation.At the end of the last century,many developed countries have launched long-term magnetotelluric sounding deep exploration program,China launched in 2008,named "deep exploration special",to expand the access to resources.At present China's long-period geomagnetic equipment development is mainly composed of China University of Geosciences(Beijing)and Jilin University.The domestic use is mainly imported long-term magnetotelluric sounding system.Many long-term earth-earth systems strictly restrict the export to China.restricting the process of deep exploration plan in China."Eleventh Five-Year" national high-tech research program to start the deep mineral resources exploration,hoping to break through the deep resource exploration technology aspects of the barrier,this paper comes from "863" major instrument special-long-period distributed earth electromagnetic observation system(2014AA06A612).Long-period electromagnetic signal is weak,the signal period is extremely long,and need to solve the impact of flicker noise,so this paper designed based on the chopper principle device long-period geomagnetic field source signal acquisition circuit to achieve high resolution,high stability,low noise,Low drift acquisition circuit design.The acquisition circuit is divided into seven parts: input protection circuit,input matching circuit,filter circuit,programmable gain circuit,analog-to-digital converter circuit,digital-analog converter feedback compensation circuit and FPGA controller circuit.For each part of the circuit structure and principle had been In-depth analysised and simulated.The digital-mode feedback compensation circuit can effectively compensate the background field of the input signal by the digital-analog conversion circuit and the subtraction circuit,and the dynamic component of the input signal was amplified by the program-controlled gain control circuit in the loop.,increasing channel gain and signal to noise ratio,increasing the effective resolution of analog-to-digital converters.Using FPGA as a controller,the use of parallel processing to achieve multichannel signal high-precision synchronous acquisition and processing.Two sets of prototypes were developed by stacking.The prototype was tested according to the quasi-calibration standard,the noise test,the inter-channel crosstalk test and the channel consistency test were carried out,and the field experiment was carried out with LEMI-417 system.The results showed that the effective value of the noise of the electric field was 0.4?V,the maximum value of the noise of the magnetic field is 0.08 nT,the inter-channel crosstalk and the channel consistency of the prototype had reached the design requirements.The results of the field experiments showed that the data obtaineded by the LMT instrument developed by this paper were treated with the same software as the LEMI-417 system,and the resistivity results were highly consistent.