恒流源在高精度数字多用表中的设计与实现毕业论文外文翻译.doc

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1、附录1 英文参考资料The constant current source in high precision digital design and implementation of multiple tables The constant current source in instrumentation, electronic equipment and high-tech science and technology is an important component of. In high precision digital multimeter, has the basic fun

2、ction of the resistance measurement, and the method of resistance measurement is usually through a resistance voltage (R-U) conversion circuit, which adopts the method of constant current source.Precision constant current source resistance measurement accuracy depends largely on the flow through the

3、 measured resistance, when the constant current source with high accuracy, stable voltage measured resistance, can improve the measurement accuracy of resistance. Therefore, the design of a good stability, high accuracy of the constant current source is very important.According to the actualneeds,in

4、 the device selection,considering the influence ofnoise,Wen Piao,design aconstant current sourcewith good stability.1 constant current source is introducedThe basicfunction of a constant current sourceis toeliminate or reducethe power supply voltage,load resistance and theinfluenceof environmental t

5、emperature on theoutput current.The constant current sourceaccording to the adjustmentof different ways,can be divided into directand indirectadjustment ofconstant current sourceadjustableconstant current source.Directadjusting typeconstant current sourcecircuit is simple,the mainpower supplyandcons

6、tant currentdevices,using thenonlinear characteristicof constant current devicesare used to directly adjustthe load current.For example,a constant current diode,but itscurrent valueis limited andstabilityis poor.The indirectadjustmentis a constant currentcircuit composed of electronic tubes, transis

7、tors and other electronic componentstogether constitutethe.For example,feedback OPamplifier based onconstant current source,the use of suchcircuitcan produceexcellentperformanceconstant current source.1.1basic structurefeedbackconstant current sourceFeedback constant current source through thenegati

8、ve feedback effect, to make comparison between the amplifier two input voltage are equal,so as to maintain aconstant output current. This circuit is connected in series with the load change on the adjustment ofthechange of resistanceto compensatethe output current,so it is also called the series adj

9、ustable constant current source. Itmainly consists of a reference voltage source, a comparison amplifier,adjust thesampling resistancepipe,and other parts.Specific work process:through the sampling resistor to the output current into a voltage feedback amplifier input, to compare, in comparison with

10、 the reference voltage, the voltage error amplifier amplified to control the resistance adjusting pipe to adjust the output current, maintain constant output current.1.2basiccircuit modelThere are many specific forms of feedback circuit of constant current source,according to the loadconnection mode

11、of constant current source,divided into loadgrounding typeand loadof floatingtype,because ofthe constant current sourceis designed for the application of digital multiresistance measurement function in the table, and the digital multimeter in measuring electric wirewhen the black pen and sheet resis

12、tance digital multimeter internal circuit board is connected, so the measuredload resistanceat one endis grounded,so thedesignof the constant current sourceshould begrounded loadtype.2 constant current source circuitdesignOn the basis of the design of high precision digital multi function meter meas

13、uring resistor, the resistance profile of different needs of different constant current source, and therefore need to design a 4 constan current output of the constant current source circuit.Combined with the design offeedback constant current source principle, by different samplingresistance value,

14、so as to achieve4differentconstant current sourceoutput.Inthe design of constant current source,a reference voltageselection is very important,itdirectly affects thestabilityof the constant current source.The reference voltagerequirements ofhigh precision,smalltemperature drift,need to choosethe ref

15、erence voltagechipwith low noise.Based on the above requirements, considering the choice of Linear Technology Corporation shunt voltage reference chip LM399, typical output voltage reference value 6.95V. It has the following characteristics: drift coefficient, 0.5ppm/C; dynamic resistance is small,

16、the maximum of 1 ohm;noise voltage effective value of maximum 20 V initial accuracy of2%.LM399 internal thermostatic heater, can improve the effect of temperature drift, stable performance for a long time. The constant current source circuit in the amplifier offset voltage,temperature drift,noisevol

17、tagewill affect theaccuracy of theconstant current source,sothe erroramplifiershould havehigh input impedance,low noiseand lowleakage current.Based on the above considerations,the designuses the ADI companyAD706,two operational amplifierswith internal.It is abipolarfield effect transistor(BiFET)of t

18、heinput stage,so it hashighinput impedance,lowinput bias current,low offset voltage, low offset voltage drift, low power consumption etc.Select the JFET to replace the transistor regulator, voltage controlled element isJFET.A transistor is acurrent controldevice, it will introduce the base currentto

19、 constant current source output, in a small constant current source output error,gatelevel JFETnocurrent flows,so theeffectofconstant current sourcethan thetransistoreffect of small,constant current sourcecan obtainsmall error.The whole circuitdesign canhave4differentconstant current sourceoutput cu

20、rrent,output currentsampling resistorswitchingconstant current sourceby choosing different.The circuitis divided into two parts,the firstpart is composed ofLM399,U201A,JFETand Q201. Theamplifierconstant currentreceiver;the second part is JFET Q202, Op U201B(R5,R6,sampling resistance,R7,R8)aconstant

21、current output current source circuit.In summary,in view of the high precisiondigital versatileresistance measurementfunctionin the table,the design oftheconstant current sourcewith high stability.Through the analysis ofa typicalconstant current sourcecircuit structure and working principle,some key

22、factors which have impact onthe accuracy of the constant current source,puts forward the design schemeof the constant current source,the scheme adopts thefeedback circuitcomposed ofoperational amplifierwith low noiseto the design,thehardware principle diagramand give concrete.The experimental result

23、s show that: the design of theconstant current source canmeet the highprecision digitalmultimeterresistance measurement function application.The circuit has smallvolume andhigh reliability,flexibledesign,has a good value.附录2 译文恒流源在高精度数字多用表中的设计与实现恒流源在仪器仪表、 电子设备及高新科学技术中属于一个重要的部件。在高精度的数字多用表中，具有基本的电阻测量功

24、能，而电阻测量的方法通常是经过电阻电压(R-U)转换电路，即采用恒流源的方法。电阻的测量精度很大程度上取决于流过该被测电阻的恒流源电流的精度，当恒流源电流精度高时，测得电阻的电压稳定，能够提高电阻的测量精度。 因此，设计一个稳定性好、 准确度高的恒流源是十分关键。本文根据实际的需要，在器件选型上，考虑到噪声、温漂的影响，设计出稳定性好的恒流源。1 恒流源介绍恒流源的基本作用是消除或消弱电源电压、负载电阻和环境温度变化对输出电流的影响。恒流源按照调整方式的不同，可以分为直接调整型恒流源和间接调整型恒流源。直接调整型恒流源电路简单，主要有供电电源和恒流器件构成，利用了恒流器件的非线性特性对负载电流

25、进行直接的调整。例如恒流二极管，但其电流值有限且稳定度也较差。 间接调整型是由电子管、晶体管和其他电子元件一起构成的恒流电路。例如基于 OP 放大器的反馈型恒流源，利用此类电路可以制作出性能优良的恒流源。1.1 反馈型恒流源基本结构反馈型恒流源是通过负反馈的作用，使加到比较放大器的两个输入端电压相等，从而保持输出电流恒定。 这种电路是依靠与负载串联的调整管的内阻变化来补偿输出电流的变化，故又称为串联调整型恒流源。它主要由基准电压源、比较放大器、调整管、采样电阻等部分构成。具体的工作过程：通过采样电阻把输出电流转变成电压，反馈给比较放大器输入端，在与基准电压相比较，放大器把误差电压放大后去控制调

26、整管的内阻对输出电流进行调整，维持输出电流恒定。1.2 基本电路模型反馈型恒流源的具体电路形式有很多，根据恒流源的负载连接方式不同，分为负载接地型和负载浮制型，由于所设计的恒流源是针对数字多用表中的电阻测量功能的应用，而数字多用表在测量电阻时其中黑表笔端与数字多用表内部电路板的地线相连，因而所测负载电阻有一端是接地的，故所设计的恒流源应是负载接地型的。2 恒流源电路设计依据所设计的高精度数字多用表的电阻测量功能，不同的电阻档需要不同的恒流源电流，因而需设计一个有4种恒定电流输出的恒流源电路。设计结合反馈型恒流源基本工作原理，通过选择不同的采样电阻值，从而达到4路不同的恒流源电流输出。在恒流源的

27、设计中，基准电压的选择十分重要，它直接影响着恒流源的稳定性能。基准电压要求精度高，温漂影响小，需要选择低噪声的基准电压芯片。基于以上的要求，综合考虑后选择了凌特公司的并联电压参考芯片 LM399，输出基准电压的典型值为6.95V。 它具有以下特点：温漂系数小， 0.5ppm/；动态电阻小，最大 1；噪声电压有效值最大 20V；初始精度2%。LM399 内部自带恒温加热器，能够改善温度漂移带来的影响，长期的稳定性能好。其恒流源电路中的运放的失调电压、 温漂、 噪声电压等会影响恒流源电流的精度，因此误差比较放大器应该具有高输入阻抗、 低噪声和超低漏电流。基于以上考虑，设计采用 ADI 公司的 AD

28、706, 内部含有两个运算放大器。它是具有双极性场效应晶体管（BiFET）的输入级，因此具有高的输入阻抗、 低输入偏置电流，低失调电压，低失调电压温漂，低功耗等特点。调整管选择 JFET 替代了晶体管， JFET 是压控型元件。晶体管是电流控制型器件，它的基极电流会引入到恒流源电流输出端，在小电流恒流源输出时误差影响大，JFET 的栅级没有电流流入，故对恒流源电流的影响远比晶体管的影响小，可以获得误差极小的恒流源。所设计的整体电路可以有4路不同的恒流源电流输出，通过选择不同的采样电阻来切换恒流源的输出电流。电路分两部分，第一部分由 LM399、 运放 U201A、 JFET Q201 等构成恒流接收器；第二部分有运放U201B、 JFET Q202、 采样电阻（R5,R6,R7,R8）等构成恒流源电流输出电路。综上所述，针对高精度数字多用表中的电阻测量功能，设计了一种高稳定度的恒流源。通过分析典型的恒流源的电路结构和工作原理，研究了影响恒流源精度的一些关键因素，提出了恒流源的设计方案，该方案采用由低噪声的运算放大器构成的反馈型电路来设计，给出了具体的硬件原理图。实验结果表明：设计的恒流源能够满足高精度的数字多用表中电阻测量功能应用。该电路具有较小的体积和高可靠性，设计方式灵活，具有很好的使用价值。