The Research Of Digital Active Resistance Box Based On Thevenin Structure

Resistors are basic components used widely in the electronics industry.The requirements on accuracy and resistance of resistors vary according to different designs,and adjustable resistance within a certain range is required in some specific designs.Although conventional resistance box,mechanical potentiometer and digital potentiometer available on the market can realize adjustment of resistance to some degree,there are certain disadvantages.The adjustment of traditional resistance box needs manual operation,and there are mechanical contacts inside the box,which can achieve automatic regulation,meanwhile,reduce the accuracy due to mechanical wear and other factors;Resistance value of mechanical potentiometer can be continuously adjusted,while the resistance value in the access circuit cannot be obtained conveniently and quickly.Besides,its accuracy will be affected because of the prolonged use time;Digital potentiometer can realize digital control,but the problems such as low resistance resolution,small adjustable range,and large temperature coefficient of resistance cannot be ignored.Therefore,it is imperative for the electronics industry to find a resistor featuring a large adjustment range,high precision and digital adjustment.This digitally adjustable resistor is expected to simplify the complex manual operation of the conventional resistor box and get rid of the inconvenience caused by the resistance measurement of a mechanical potentiometer in circuit debugging.This research designs a digital active resistance box based on Thevenin structure.Inside the resistance box is a structure of a sampling resistor in series with a controlled voltage source.It can be concluded form the Ohm's law that,with constant port voltage,accurate control of port current can be realized indirectly through adjusting the controlled voltage source,which can also achieve the command of the equivalent resistance of electronic model.First of all,some linear relation should be established between port voltage and current to ensure stable resistance outputs generated by resistance box.Therefore,the instrumentation amplifier AD620 is furnished to convert the current in sampling reference resistor into the reference voltage of subsequent voltage dividing circuit,so that the linear relation can be realized.The process involves using two MAX5354(10-bit DAC digital-to-analog converter)and X9111(1024-position digital potentiometer)to constitute a two-stage voltage dividing circuit and reach the other end of the sampling reference resistor through co-directional amplifier.In this way,the input voltage U flows through the sampling reference resistor R,and is connected to the other end of the port through a controlled voltage source composed of active components like operational amplifier and DAC,thereby forming a single port network of Thevenin structure.Consequently,the equivalent resistance of the port can be adjusted by controlling the relation between voltage and current.The equivalent resistance value of resistance box output is the product of an adjustable coefficient X and the resistance of sampling resistor R.In order to guarantee a high precision and a large adjustable range of resistor box,low-temperature drift coefficient with an accuracy of 0.1% is selected,with its resistance being 10?,1K? and 100K?.At the same time,a double-switch structure is used in the research to achieve the switching of sampling resistor,so as to reduce the influence exerted by the on-resistance of switch upon the device accuracy.This research uses STM32 single-chip microcomputer to realize the digital control of bleeder circuit and sampling resistor switching circuit,thereby controlling the output resistance of resistance box automatically.The digital active resistance box characterized by Thevenin structure can be applied to both voltage source load and current source load.The output of resistance box,with overall accuracy better than 1% and simple structure,is adjustable from 10? to 10M?.It can communicate with PC through a serial port,which is convenient for further expansion.