The Design of Digital Multi-channel Analyzer Based on FPGA

Transcription

1 Available online at ScienceDirect Energy Procedia 9 (0 ) 48 4 Abstract Asian Nuclear Prospects 0 (ANUP0) The Design of Digital Multi-channel Analyzer Based on FPGA ZENG Weihua * China University of Geosciences, BeiJing, China A Digital Multi-Channel Analyzer (DMCA) based on FPGA(Field Programmable Gate Array) is introduced. The nuclear signals are processed by EPC40Q40. It uses digital signals acquired by ADC to do calculus,seek the peaks,discriminate the threshold,and then form the energy spectrum. DMCA communicates with energy spectrum processing terminal(for example: PC) by high-speed RS485 bus interface. The software on PC can control DMCA and display energy spectrum. And the energy spectrum of 7 Cs is given,which is obtained from NaI( Tl) detector and processed by DMCA. 0xx The Authors. Published by Elsevier Ltd. 0 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection Selection and and/or peer-review peer-review under responsibility under responsibility of Institute of of Nuclear Institute and of New Nuclear Energy and Technology, New Energy Tsinghua Technology, University Tsinghua University Keywords: Digital Multi-Channel Analyzer ; FPGA ; Digital Integration ; RS485. Introduction Digital Multi-Channel Analyzer (DMCA) is a key component of nuclear radiation measurement and analysis instruments. It is capable of realizing rapid A/D conversion of random and fast analog nuclear signals and thus obtaining digital nuclear signals. DMCA will obtain the nuclear spectrum after digital integration, peak-seeking and threshold discrimination on the digital nuclear signal. Compared with * Corresponding author. Tel.: ; fax: address: zwh@cugb.edu.cn The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of Institute of Nuclear and New Energy Technology, Tsinghua University doi:0.06/j.egypro.0.07.

2 Zeng Weihua / Energy Procedia 9 (0 ) analog multi-channel analyzer, DMCA has the following advantages: rapid analysis speed, high pulse through rate, more abundant pulse information, using programmable device to implement a flexible data processing algorithm, which make DMCA can be widely used in nuclear spectral analysis, nuclear radiation measurement, industrial CT, medical CT and many other fields.. Principles of DMCA Fully-digital technology aims to take sample of the original waveform of the nuclear signals as early as possible and preserve the original information of nuclear cases as much as possible, and then extract from the sampling signals by digital processing in accordance with the purpose of signal analysis []. By obtaining the waveform, time and energy information simultaneously of nuclear pulse signals, it can realize functions that analog technology is unable to achieve: both of its analysis accuracy and flexibility are better than the analog analysis. Therefore, DMCA is able to effectively improve the performance of the spectrometer, and further promote the application of nuclear spectrometer as well as the level of measurement and analysis [,]. Lots of integrated chips and hardware& software programs may be used to develop and manufacture DMCA [4]. A design and implementation of digital multi-channel analyzer base on single FPGA main chip will be introduced.. Overall Design of DMCA System As shown in figure, the digital multi-channel analyzer is composed of the A/D,FPGA(Field Programmable Gate Array),SRAM(Static RAM), communication interface, power supply unit etc.. The analog signal of nuclear pulse is transferred to the high-speed ADC for analog-to-digital conversion after front-end processing, and ADC will transfer the converted digital signal into FIFO(First-in First-Out) within FPGA under the control of the FPGA internal logic. After filtering preprocessing, pole-zero cancellation, digital integration and peak detection processing of the digital nuclear pulse signal within FPGA, the peak value of nuclear pulse signal will be obtained. Nuclear spectrum data formed by peak signals of nuclear pulse will be stored at the SRAM outside of FPGA, and then be transferred to personal computer by high-speed RS485 interface for subsequent nuclear spectral analysis, or be connected with other nuclear spectral processing terminals via 6-bit parallel bus interface. CLK+ CLK- SRAM V IN+ AD ADC EPC40Q40C8N FPGA RS485 bus interface V IN- 6-bit bus interface V8 V V V5 V Power Supply Fig.. Block diagram of DMCA system

3 40 Zeng Weihua / Energy Procedia 9 ( 0 ) Hardware Circuit Design 4.. Design of preprocessing front-end and data acquisition circuit The signal that output from the detector will be transferred to the analog front-end and data acquisition circuit after preliminary amplification processing. Figure shows the analog front-end and data acquisition circuit schematic diagram. Single-ended analog signal is converted into differential signal with better anti-jamming capacity through ADA497, and then transferred to A/D chip--ad9649 for analog-to-digital conversion under the control of FPGA internal logic. The FPGA used here is EPC40Q40C8N provided by Altera Company, which, by adopting SOPC(System On a Programmable Chip) technology, integrates high-density logic element (FPGA), static storage (SRAM) and embedded processor (ARM) in programmable logic devices, realizing a perfect combination of speed and programmability. FPGA performs the following functions in the system: data acquisition of nuclear spectrum, digital filter processing, data buffering (realizing FIFO through FPGA), digital integration, peak-seeking and serial data communication with personal computer. Data acquisition and digital signal processing achieved by a single FPGA chip have brought into the fullest play the parallel processing capabilities of the FPGA chip and reduced the dead time of the multi-channel analyzer [5,6], while simplify the hardware circuit design of multi-channel analyzer, reducing its power consumption. V8 V E 0uF/6V J BNC R8 50 SIG_P SIG_N C 0uF C 0uF R 76.8 SIG_C_P SIG_C_N R R5 00 R7 00 R 00 AD_IN_P AD_IN_N R 00 U E ADA497-YCPZ-R AD_OUT_E_N R FB- AMP_PD 0uF/6V R4 PD IN+ + OUT- AD_OUT_N 00 VCM 0 IN- - AD_OUT_P OUT+ R6 4 9 VCM FB+ VOCM E5 00 AD_OUT_E_P R0 0uF/6V V C4 E 0.uF0uF/6V VS+ 8 VS+ 7 VS+ 6 VS+ 5 ADC_VIN_N C0 0pF ADC_VIN_P ADC_CLK_P ADC_CLK_N CLK+ CLK- SPI_nCS 4 CSB SPI_CLK 5 SCLK/DFS SPI_DIO 6 SDIO/PDWN ADC_VIN_P VIN+ ADC_VIN_N 0 VIN- VCM 7 VCM U AD9649BCPZ-65 Int Vref VREF DRVDD A C7 D7 5 D6 4 D5 D4 D 0 D 9 D 8 D0 7 RBIAS 8 SENSE 6 0.uF AD_OR MODE/OR AD_DCO DCO AD_D D 0 AD_D D 9 AD_D D 8 AD_D0 D0 7 AD_D9 D9 6 AD_D8 D8 AD_D0 AD_D AD_D AD_D AD_D4 AD_D5 AD_D6 AD_D7 R0 0K(%) Fig.. Scheme of analog front-end and data acquisition 4.. A/D converter A/D converter is a key component of the digital multi-channel analyzer, which can directly affect performance of DMCA. In this design, AD is used as its AD converter, which has the following characteristics: Conversion bit: 4-bit Sampling rate: -65Msps Low power consumption: 75mW at operating mode; 4mW at suspend mode Input mode: differential mode Output mode: 4-bit parallel bus Differential nonlinearity: ±0.LSB Integral nonlinearity: ±0.5LSB

4 Zeng Weihua / Energy Procedia 9 (0 ) With high resolution, fast conversion rate, user-friendly control, low power consumption and other advantages, AD is particularly suitable for rapid data acquisition, image processing, communications and other fields. 4.. FPGA EPC40Q40, the digital signal processor in the system, is responsible for A/D conversion control, digital signal processing, RS485 communication control and other function in the system. Integrated resources within the EPC40Q40 chip include: 9,600 logic elements, 4Kbit RAM, 6 multipliers and 4 phase-locked loops. In FPGA, the A/D conversion control module runs in parallel with the digital signal processing module of nuclear pulse, connected with each other by FIFO, which is realized by RAM within the FPGA. Compare to the digital multi-channel composed of single DSP system, digital multichannel composed of FPGA chip with parallel processing capabilities can effectively reduce the dead time of the multi-channel analysis. DMCA composed of a single FPGA chip is able to simplify the PCB layout design and thus effectively reduce the power consumption of DMCA RS485 interface Spectroscopy data of DMCA is transferred to the spectral processing system of personal computer via bus interface. In this design of DMCA, a high-speed half-duplex RS485 differential bus is used as the communication interface for the digital multi-channel. As a high-speed RS485 transceiver for power supply within a wide voltage range, SN65HVD08D can provide a communication speed of 0-0Mbps with power supply voltage of.v or 5V. Figure shows the block diagram of RS485 interface circuit. Build a USART communication module in FPGA so that FPGA is able to record the spectroscopy data in SRAM into this USART module, which will be converted into RS485 data and then transferred to personal computer. Compared to USB interfaces, high-speed RS485 interface circuit is simple, as only one RS485 converter with a high-speed USB interface is needed to be connected with the personal computer while without designing the complex USB driver on PC, further simplifying the design of the spectral data processing software. 485_RXD 485_nRE 485_DE 485_TXD 4 U RO VCC /RE B DE A DI SN65HVD08D 5V R48 4.7K(OPEN) R5 4.7K(OPEN) 5V J5 4 RS485 5V Fig.. Scheme of high-speed RS485 bus interface

5 4 Zeng Weihua / Energy Procedia 9 ( 0 ) Software Design 5.. Digital integration and peak-detection Detector output signal will be transferred to high-speed A/D conversion circuit after the pretreatments such as preamplifier and so on to obtain charge integration signal by digital integration. A/D converter is liable for continuous high-speed sampling while FPGA will perform the functions such as digital integration of nuclear signal, peak detection and threshold discrimination. Set integral width as T, integrated output signal as U o (t), input signal as Ui (t), integrate Ui (t) as: Where (t) t U o ( t) Ui( ) d t T Ui (t) Uo ( t) [ Ui( ) Ui( T )] d Ui meets the following conditions: if -T< t < 0, then 0. Formula () can be converted into: U o (t) will have an extremum when the first derivative of U o (t) is 0, i.e. when ( t) dt 0 Ui ( ) Ui( T ) 0(0 t), U o (t) is an extremum. When duo ( t) d( t) turns to negative from positive, Ui( ) Ui( T ) 0(0 t) will turn to negative from positive correspondingly and U o (t) will be the maximum value, and conversely the minimum value. Therefore, each nuclear pulse peak can be found in the continuous input signals. To obtain Ui ( n T ), discrete signals obtained from A/D conversion will be shifted at T time intervals by FPGA. In the process when Ui ( n) Ui( n T ) gradually turns to negative from positive, Ui ( n) Ui( n T ) 0 will be the peak value of Uo (n). Therefore, the peak can be recorded into the corresponding address (channel) in SRAM. To improve the efficiency of peak-seeking, it is necessary to judge the threshold of discrete pulse signals prior to peak-detection. As peak-detection of signals with amplitude lower than the preset threshold is omitted, discrete nuclear pulse signals involved in peak-detection module are significantly reduced. 5.. Software design for personal computer duo, Software of personal computer is developed under Borland C++ integrated development environment, achieving lots of functions including RS485 serial communication, system parameter settings, spectrum display and spectroscopy data management, etc. 6. Performance Parameters This DMCA is capable of taking sample of nuclear pulses and obtaining energy spectrum. Figure 4 shows the energy spectra of 7 Cs rays on 75*75mm NaI(Tl) detector measured by this DMCA. The basic performance parameters of this DMCA are as follows: Number of channels: 04 (5-89 channels may be set) Energy resolution: 8.6 Integral nonlinearity: 0.8% Differential nonlinearity:.4% Pulses through rate: >00k CPS

6 Zeng Weihua / Energy Procedia 9 (0 ) Fig.4. A -ray spectrum of 7 Cs collected by DMCA 7. Conclusions A kind of DMCA is designed based on FPGA used to complete data acquisition, digital integration of nuclear signals, peak-detection, RS485 high-speed data transmission. spectrum display program on personal computer was developed. 7 Cs nuclear signals detected by NaI (Tl) detector are processed by DMCA to obtain 04-channel energy spectrum with energy resolution approximate to 8 while logic resources utilization within FPGA is less than 0. The basic functions of DMCA have been realized and further research will focus on realizing other functions on this hardware platform, such as digital filtering, waveform shaping of nuclear pulses, baseline elimination, pile-up rejection and other digital processing algorithms of nuclear pulses. By future work, DMCA will have more perfect functions and better performance. Acknowledgements Research for this paper was partially supported by the fundamental research funds for the central universities. References [] Wang Lei, Tuo Xianguo, Cheng Yi, etc. Design of DMCA based on DSP[J]. Nuclear Electronics & Detection Technology, 009, 9(4): [] Xiao Wuyun, Wei Yixiang, Ai Xianyun, etc. Research on Digital Multi-Channel Pulse Height Analysis Techniques[J]. Nuclear Technology, 005, 8(0): [] Ge Liangquan, Zeng Guoqiang, Lai Wancheng, etc, Development of Measurement System of Aviation Digital Gamma Energy Spectrum[J]. Progress Report on China Nuclear Science & Technology, 009, (Vol: Uranium Geology): 9-. [4] Ao Qi, Wei Yixiang, Wen Xiangyang, Design of DMCA based on DSP[J]. Nuclear Technology, 007, 0(6):5-54. [5] M. Grey, J. Goldsten, R. Maurer, D. Roth, C. Zeitlin. Data acquisition for the Combined Ion and Neutron Spectrometer (CINS). Nuclear Instruments and Methods in Physics Research B,009,67():9-4. [6] Ju Hahn Lee, Hyo Soon Jung, Hwa Youn Cho, Young Kwan Kwon, and Chun Sik Lee. A Novel Digital Pulse-Shape Analysis for High-Resolution Position-Sensitive Gamma-Ray Spectroscopy. NUCLEAR SCIENCE, 00,57(5):6-67.