外文翻译-- Study of Voltage Control Current Source in Electrical Impedance Tomography System

Study of Voltage Control Current Source in Electrical Impedance Tomography System Li Zhangyong,Xu Zhui,Ren Chaoshi,Wang Wei,Zhao Dechun,Zhang Huiquan Biomedical Engineering Research Center Chongqing University of Posts and Telecommunications Chongqing,China E-mail: AbstractA steady and efficient Voltage Control Current Source(VCCS)with broad bandwidth plays a very important role in the quality of final images for Electrical Impedance Tomography(EIT)system.Therefore,two models of current source are proposed in the paper:Advanced Howland VCCS and Double Operational Amplifiers VCCS.Both of the two models are simulated through a software named Multisim,and the results of simulation show that both models can keep an output impedance higher than 1M when the frequency changes from 10 kHz to 1 MHz and the load varies from 100 to 1000.The results also show that both models meet the requirements of VCCS in EIT system,they are hoped to provide usefull approaches for the design of practical VCCS of EIT system.Keywords-;Electrical Impedance Tomography;Howland circuit;double oprational amplifier current source circuit I.INTODUCTION EIT technology is based on the various impedance in different organizations of the human body,it can make full use of the physiological and pathological information which carried by human body impedance,a small and safety current is injected into human body by stimulating electrode,then voltage can be measured by measuring electrode,at last the voltage information is rebuilt into images that describe the electrical characteristics inside human body through a certain calculating algorithm1.EIT has been widely concerned because of its three advantages:no harmful to body,functional imaging and fitting for long-term medical care and it is also one of major topics of modern bio-informatics and biomedical engineering.The research of EIT began in the 70s of last century;the EIT technology has made considerable progress in just 30s.Some groups and organizations have worked on the research of the clinical applications in some fields like:detection of breast cancer,brain and abdominal bleeding custody,measurement of lung function and so on 234.It is time for the transition from theoretical research to clinical application.EIT system includes software and hardware components:software component focuses on algorithm and imaging;hardware consists of Digital oscillator circuit,Voltage Control Current Source(VCCS),Control and data acquisition system,Voltage measurement circuit and Electrode array.VCCS plays a great important role in the process of imaging and the quality of final images,so the design of VCCS in EIT system is especially important 5.An ideal VCCS in EIT system requires high precision and stability which means approximately infinite output impedance.In this paper,two models of current source are proposed:Advantaged Howland(AH)circuit and Double Operational Amplifier(DOA)current source circuit,both of the two models are analyzed and simulated,and the results of analysis and simulation are processed and compared to find a better method.II.MATERIAL AND METHODS A.Howland Circuit The V-I conversion can be achieved when Current Series Negative Feedback(CSNF)were added into amplifier circuit,Howland circuit includes both CSNF and Voltage Positive Feedback(VPF),So it is a more practical current source circuit as is shown in Fig.1.When load resistance RL decreases,on the one hand,iO will increase because of the inner resistance;on the other hand,uP will decrease,then uO will decrease and iO decrease too;When the value of resistances meet(1),the increased value causes by the reduction of LR approximately equal to the decreased value causes by VPF,then V-I conversion is finished and a steady current computed by(2)can be given.3214RRRR=,(1)4uiIOR.(2)This work was supported by National Natural Science foundation of China(No.60901045),Chongqing Science Technology foundation(No.CSTC2009AC5149)and Chongqing education commission foundation(No.KJ090505).978-1-4244-4713-8/10/$25.00 2010 IEEE Fig.1 Howland circuit B.VCCS Based on AH Circuit VCCS based on AH circuit is shown in Fig.2.A feedback resistor 4Ris added to Howland circuit which helps to reduce the influence of current source causes by the variation of load resistance,so the output impedance is increased furthermore 6.As is shown in Fig.2,when the value of resistances meet(3):45213RRRRR+=.(3)Then the output current iLis computed by(4):1L34i/VRR.(4)Equation(4)shows that the value of iLis related to signal voltage1V,periphery resistances3Rand4Rbut is not related to loadLR C.VCCS Based on DOA Current Source Circuit DOA current source circuit is shown in Fig.3.A negative feedback is added to the inverting input port of U1,the in-phase input port is connected with a voltage follower(U2)and 4Rwhich also compose a sum operational circuit.Then the output current can be calculated by(7):341123434uuPPRRVRRRR=+(5)2111u(1)uOPRR=+.(6)Let 1234RRRR=,Then:10iVR=L.(7)Equation(7)shows that the value of iLis related to signal voltage1V,periphery resistance 0Rbut is not related to loadLR.Fig.2 Advanced Howland circuit D.VCCS In EIT System The principle of EIT technology requires:the more steady the output current is,the closer the obtained impedance information approach to the truth;a wide frequency rage between 10 kHz and 1 MHz is required in order to obtain the information of imaginary part in impedance 78;the output current must be smaller than 5mA for the consideration of the security of human body.So far,several models of VCCS have been proposed:DOA VCCS,AH VCCS,Mirror Current(MC)VCCS and so on.The DOA model increases the output impedance of current source by adding a voltage follower with high input impedance to the feedback circuits;it is used more wildly but only applied to a bandwidth between 0 and 200 kHz which is lower than the requirement of EIT.The AH model needs only single operational amplifier and several resistor to obtain a high output impedance 9,so the selection of operational amplifier is of great importance.The MC model has a strict requirement on the symmetry of the current mirror.But thanks to the developments of modern electronic technology and integrated circuit manufacturing,some operational amplifiers integrate current mirror itself and the bandwidth became broader and broader.So,current feedback operational amplifier AD844 is used in the design.The AD844 has several features like:wide bandwidth(60MHz),fast settling(100 ns to 0.1%),slew rate(2000 V/s);quickly response to signal,low distortion,low noise,low temperature offset,excellent performance on AC/DC;current mirror is integrated inside10.So the DOA VCCS with the application of AD844 owns both the features of DOA VCCS and advantages of current mirror,and the problem of insufficient bandwidth can be overcame.Fig.3 DOA current source circuit The design of practical VCCS for EIT system is an important factor for the quality of final results.It is a problem that must be solved for the research of EIT.Therefore,the models mentioned above are hoped to provide references for the design of VCCS in EIT system.ACKNOWLEDGMENT This work was supported by National Natural Science foundation of China(No.60901045),Chongqing Science Technology foundation(No.CSTC2009AC5149)and Chongqing education commission foundation(No.KJ090505).REFERENCES 1 Dijksdra A M,Brown B H,Leathaerd A D.Review Clinical Application of Electrical Impedance Tomography.Journal of Medical Engineering&Technology,1973,Vol.17,pp.88-89.2JAN C.DE MUNCK,THEO J.C,FAES,ROB M.Heethaar,The