CCC Bridge Circuit Design With Programme Control

The cryogenic current comparator can provide the most accurate current ratio device. And the uncertainty of its ratio can be less than 10-12. The cryogenic current comparator plays a very important role in precision electrical measurement. The cryogenic current comparator is mainly used for small current measurement and comparison of quantum hall resistance value. The paper is mainly aimed at the feedforward circuit design for cryogenic current comparator resistance bridge. The feedforward circuit with the main controller DSP is mainly composed of the two-outputting programmable current source and the resistance bridge circuit, and configured by the computer. The two-outputting current source not only can keep the high stability and the low noise, but also can output arbitrary ratio current with the high ratio stability.This paper introduces the reaserch background, the principle of cryogenic current comparator resistance bridge and the present situation of research at home and abroad. The feedforward design of the cryogenic current comparator resistance bridge consists of Lab VIEW control program, digital control circuit and analog current source circuit. And the paper analyzes the structure and function of the system by the design of hardware and software.The design of system hardware circuit mainly consists of digital control circuit and analog current source circuit. In the digital control circuit, DSP and FPGA working together are used to make the system to have the ability of high speed digital signal processing and strong logic.control. DSP which is the main controller use TMS320F2812 of the TI Company. DSP is used to execute commands and configure and drive modules which include SCI, AD, LCD and FPGA. FPGA which is the secondary controller uses EP2C8Q208C8 of the Altera Company. FPGA is used to convert control command into the corresponding control signal which is send to the analog current source circuit after completion of the conversion by optical fiber.In the analog current source circuit,the system mainly uses DA to output the adjustable ratio current. And this way can improve the control accuracy of the system. The system also designs lots of relays to switch the circuit state. By switched state, the system can make the current source more stable and the configuration more reasonable. And this way also improves the ability of automatic control of the system. In order to reduce interference of the current source caused by the digital control circuit and improve the system stability, the system uses the design of the optical isolation which is a way for the data communication and electrically isolation between digital control circuit and analog current source circuit by optical fiber. The design of the optical isolation improves the anti-interference ability of the system and ensure the stability.In the software design of the system, the paper introduced the Lab VIEW, CCS and QUARTUS II software and software development process. Based on the Lab VIEW, CCS and QUARTUS II software, the paper designs the Lab VIEW control program, the DSP program and the FPGA program. These programs are used to configure and adjust the current source. The paper analysis the program function, internal structure and connect.And these function are simulated.After the experiment, the relative stability of current source is less than 6×10-6 and the relative repeatability is less than 6×10-6. Measured in different ratio current, the error between the current ratio and its nominal value current ratio is less than 4×10-5, the relative stability is less than 4×10-6. By configuring different ratio current, the system can measure the different value of resistance.The design attains the design requirements.