The Design Of The Low Frequency And High Precision LCR Meter

With the rapid development of the electronics industry,a variety of electronic equipments have constantly upgrading towards more high-precision,high response speed and low-cost direction under the pressure of market competition.Therefore,it has a very important practical value to accurately measure the specific parameters of the impedance of electronic components,and LCR measuring instrument is the current mainstream equipment in measuring resistance,capacitance and the parameters of the inductance impedance.This paper designs a LCR measuring instrument with high precision and simple circuit structure.The volt-ampere measuring principle based on free-axis,which has the advantages of simple method and high precision compared with other principles.The main body of the measuring system is composed of power supply,microprocessor control unit,sine wave signal generator,bridge circuit,controllable gain amplifier circuit,switch type phase sensitive detector and A / D conversion module.Among them,the microprocessor use 32-bit micro-controller STM32F103C8T6,which supports a variety of peripherals,working frequency up to 72 MHz,meeting the design requirements and is cost-effective.Sinusoidal signal generation circuit generate sinusoidal signal which has accuracy frequency,adjustable phase and square wave signal which has 90 degrees difference between two phase allowing by the post-level phase-sensitive phase detection circuit,with the method of the PWM output function combined with multi-stage low-pass filter analog DAC,and then applying a way of direct digital frequency synthesis.This method greatly simplifies the circuit structure compared to the conventional application of the DDS chip or the sinusoidal generator designed by the FPGA.At the same time,it is easier to ensure that the phase difference between the two orthogonal square wave signals is 90 degrees.The bridge circuit is the main part of the system,which can strictly guarantee that the component under test has the same current as the known impedance flows during the measurement.The part of the circuit used for vector voltage separation is using differential three op amp structure,which has high common mode rejection to improve the measurement accuracy.Taking into account the accuracy requirements of the measuring instrument,this paper uses successive approximation differential A / D converter chip AD7915 whose resolution up to 16 bits as a A / D converter circuit,the chip can collect bipolar analog voltage signal,the maximum acquisition range is ?5V.Phase-sensitive detection part of the circuit use the switch-type phase sensitive detection circuit which has a simple structure,at the same time this article also optimize and improve the output of traditional switch phase-sensitive detector,through introduce reference signal into the input of the phase-sensitive detector,eliminate the main interference peak of output signal,greatly inhibited the output signal ripple,effectively enhance the system response speed and measurement accuracy.The LCR measurement circuit designed in this paper is simple in structure,portable in volume and supports parallel and parallel equivalent measurements.It can be used in routine measurement,especially when its error can be reduced to below 1% after debugging.