Study On Digital Readout Method And Circuit For Weak Signal

Cancer is a persistent disease threatening human health and life.At present,heavy ion radiotherapy is recognized as one of the most advanced radiotherapy methods in the world.The experiments related to heavy ion therapy for tumors have been carried out in Institute of Modern Physics(IMP),Chinese Academy of Sciences(CAS)since 2006,and started to industrialize heavy ion radiotherapy devices in 2012.The beam position distribution and profile uniformity monitoring system at radiation terminal,which is constructed based on the multi-strip ionization chamber,is an important part of the heavy ion radiotherapy system,and the multi-strip ionization chamber outputs multichannel weak current signals.Based on the requirements of processing the multichannel weak current signals,the digital readout methods and circuits with the features of high integration and multichannel for processing the weak current signals will be studied in this dissertation,the goal is to find a novel digital methods and circuits which is most suitable for processing the weak signals from the beam position distribution and profile uniformity monitoring detector(multi-strip ionization chamber)of the heavy ion radiotherapy facility.There are three methods usually for digitizing the weak signals: The first method is real-time conversion of current to voltage followed by an analog-digital conversion(ADC),which can analyze the specific details of the signal,but it has a large amount of data,high cost and complex system.The second method is current to frequency conversion and followed by a counter,which is a processing method for a time period,which converts current into pulse output.A single pulse represents a fixed amount of charge.This method has a low cost and high technical difficulty.The third method is gated integrator combined with an ADC,this method is well coincided with the periodicity of heavy ion beam of radiation,relatively the complexity of the backend system and cost are reduced,it is a suitable digital readout method for the multi-channel weak current signals from a beam position and profile monitoring system in a heavy ion radiotherapy device.In order to implement the digital readout of multi-channel weak current signals from the multi-strip ionization chamber,a principle verification system is designed firstly.The front-end integral circuit board(FEICB)is designed with discrete devices,which is mainly composed of gated integrator circuit,multiplexer circuit,voltage attenuation and differential drive circuit,optocoupler isolation circuit,external trigger fan-out circuit,CPLD hardware circuit and firmware,power supply filter circuit.FEICB carries out front-end processing for weak current signals of 128 channels,and realizes digital readout by combining a DAQ system built with commercial CRIO components.And then a high integration digitizer system,or called it as an integrated system,is developed by adopting a newest current to digital convertor ASIC(Application Specific Integrated Circuit)chip with 128 input channels,the weak current signal can be directly converted into digital signals through this chip,then the data is transmitted to the FPGA through the serial peripheral interface,the FPGA uploads the data to PC for display and storage via gigabit Ethernet interface or USB interface.The integrated system mainly includes a current to digital conversion circuit with 128 input channels,FPGA hardware circuit and firmware,USB transmission module circuit,gigabit Ethernet transmission module,power module circuit,etc.After completing the design of the weak signal digitization system,the performance of the principle verification system and the integrated system were tested.The input current range of the verification principle system is-180 nA~180 nA(Means over a period of time),and the nonlinear error is less than 0.14%.Through field test,the principle verification system can well realize digital readout of multi-channel weak current signals from the multi-strip ionization chamber detector.The integrated system was tested in the laboratory environment,and the input current is in a range of-50 nA~550 nA(Means over a period of time).The integral time can be set in a range of 55 ?s~1000 ?s,and the dynamic range of charge measured is 32 pC~500 pC.The principle verification system realizes the digital readout of the bipolar weak current signal,at present,it has been widely used in heavy ion radiotherapy device,which solves the urgent need of work,but there are some shortages such as large dead time,large power consumption,large PCB board area,high cost and heat.The integrated system has the features of high integration,low power consumption,small dimensions,and low cost.Also,the integrated system has a capacity of implementing digital readout fast and continuously,which can reduce the dead time of beam position and profile monitoring system.