Research on the calibration device of integrated operational amplifier parameter tester

       Integrated operational amplifiers (hereinafter referred to as integrated op amps) are widely used in many military and civilian electronic systems due to their small size, light weight, low power consumption, and high reliability. They are one of the key components of the electronic systems of intelligent weapons and equipment. In recent years, with the rapid development of microelectronics technology, integrated op amps have become increasingly perfect in both technical performance and reliability, and are widely used in my country's military systems. The quality of integrated op amps is related to the safety of specific projects and even the country.

　　With the widespread application of integrated operational amplifier parameter testers (hereinafter referred to as op amp testers) in the defense, military and civilian fields, its quality issues are particularly important. Traditional op amp tester calibration solutions can no longer meet the requirements of defense and military industries, and the calibration of op amp testers faces severe challenges. Therefore, how to standardize and improve the test accuracy of op amp testers and ensure the accuracy of military op amp devices is a key issue that should be solved at present.

　　At present, there are mainly the following calibration schemes for operational amplifier testers (or analog device test systems) at home and abroad: calibration board method, standard sample method and standard parameter simulation method.

　　Comparing the above three solutions, we can see that the first two methods only calibrate the PMU unit, current source, voltage source, etc. used inside the instrument, and do not involve the closed-loop test circuit part of the instrument itself. They have great limitations and it is difficult to ensure the parameter test accuracy of the integrated op amp device of the op amp tester. The standard parameter simulation method is directly aimed at the test fixture, and its calibration method has certain feasibility, but further research is needed in terms of calibration accuracy, versatility, and test automation. Therefore, by improving the standard parameter simulation method, calibrating the op amp tester, and developing an integrated op amp parameter tester calibration device, the parameter accuracy and calibration range can meet the needs of most domestic op amp testers; in terms of versatility, it can calibrate instruments using the "closed-loop test principle".

　　System performance requirements

　　The main task of this project is to study the calibration methods of operational amplifier testers at home and abroad, improve the more practical standard parameter simulation method, use parameter standards with higher indicators to calibrate operational amplifier testers, realize the automatic calibration of operational amplifier testers, and automatically generate calibration original records and calibration certificates.

　　Hardware Design of Calibration Device

　　The calibration scheme covers most of the parameters given by the operational amplifier tester on the market, including 10 parameters such as input offset voltage, input offset current, input bias current, etc. By studying the "closed-loop test principle" of integrated operational amplifier parameters, it can be known that some parameter calibrations require the use of a "closed-loop test loop", while some can be calibrated by directly connecting to the corresponding standard instrument for measurement. For several parameters that use a "closed-loop test loop", they are mainly calibrated through a compensation power supply device and an analog power supply device.

　　1 Calibration circuit design

　　The input offset voltage VIO is defined as the DC compensation voltage added to the two input terminals when the output voltage is zero (or a specified value). The integrated operational amplifier can be simulated as an ideal integrated operational amplifier with a voltage at the input terminal. By adjusting the compensation power supply device to give the input a voltage Vcompensation that is equal to and opposite to the VIO voltage, the input can be equivalent to V=VIO+Vcompensation=0, and the tested integrated operational amplifier and the interface circuit are equivalent to an ideal operational amplifier with zero input offset voltage. Then, adjust the analog power supply device to give the analog standard operational amplifier input offset voltage parameter value. By comparing the digital multimeter reading with the test value of the calibrated operational amplifier tester, the error value is calculated to complete the VIO parameter calibration.

　　2. Single chip microcomputer control circuit design

　　The single-chip microcomputer uses AT89S51, which is a low-power, high-performance CMOS 8-bit single-chip microcomputer with a 4kB ISP (In-system programmable) FLASH ROM that can be repeatedly erased and written 1000 times. It is manufactured using ATMEL's high-density, non-volatile storage technology, is compatible with the standard MCS-51 instruction system and 80C51 pin structure, and integrates a general-purpose 8-bit central processing unit and ISP FLASH storage unit.

　　In this design, a single-chip microcomputer is used to control the signal relay to realize the circuit test state conversion. The signal relay is HRS2H-S-DC5V from HKE, which can quickly complete the test state conversion and only requires a 5V power supply from the single-chip microcomputer, which is convenient for parameter calibration. In addition, the relay jump is driven by the PNP transistor S8550.

　　3 LCD display circuit design

　　The intelligent color LCD display VK56B is a product of the Beijing branch of Shanghai Radio and Television Group. It has the characteristics of small size, low power consumption, no auxiliary radiation, long life, ultra-thin, vibration-proof and explosion-proof. The LCD uses an industrial-grade CPU and is equipped with a secondary character library. It can receive control command data through the serial port or the three-state data bus parallel port, and process the received commands and data by itself to display various curves, graphics and Chinese and Western fonts that the user wants to display in real time. The interface circuit of AT89S51 and the intelligent LCD VK56B is shown in Figure 3. The single-chip microcomputer and LCD adopt parallel communication design. The LCD itself has a three-state data bus parallel port (the parallel port is CMOS level) and can communicate with the host. It has 12 external lines connected to the single-chip microcomputer, namely D0~D7, WRCS, BUSY, INT and GND. Among them, WRCS is the logical OR of the chip select signal and the write signal, and the rising edge is valid; the BUSY signal is high (CMOS level) to indicate busy; INT is the interrupt application signal, and the low level is valid.

　　Software Design for Calibration Device of Integrated Operational Amplifier Parameter Tester

　　The software part includes the upper computer software and the lower computer software design. The upper computer software completes the communication between the PC and the single-chip microcomputer and the calibration data processing; the lower computer software is the single-chip microcomputer source program. This design uses Keil C to complete the conversion of test status, serial communication with the upper computer, and real-time display of test parameters.

　　1 Host computer software design

　　The upper computer software is mainly divided into three parts: the parameter setting part mainly completes the information entry of the calibrated operational amplifier tester; the calibration part completes the calibration of various parameters; the data processing part completes the automatic report of the calibration certificate and original records. The "parameter setting" part mainly completes the data entry of the calibrated operational amplifier tester; the "calibration" part mainly completes the calibration process of 10 parameters such as input offset voltage and input offset current through the cooperation of the lower computer; "generate calibration certificate", "generate original record", "preview calibration certificate", "preview original record" mainly realize the automatic processing of calibration data.

　　The lower computer software is mainly written by Keil C, and the dynamic display of calibration parameters and the conversion of test status are completed by the lower computer software. It consists of two parts, one is the ST7920 LCD driver, and the other is the microcontroller serial port communication program. Here is a brief introduction to the writing of the VK56B LCD driver. Among them, TW is the pulse width of the WRCS signal, TSU is the data setup time, and TH is the data hold time. The specific requirements of these parameters are: TW is not less than 16ns, TSU is not less than 12ns, T is greater than 0ns, TH is not less than 5ns, and TI is not less than 2μs.

　　The source code for the bus port communication subroutine is shown below.

　　PSEND:

　　JB PBUSY, PSEND; detect bus port busy signal

　　PUSH DPH

　　PUSH DPL

　　MOV DPTR, #8000H; Assume that the address assigned to the display by the user is 8000H

　　MOVX@DPTR,A

　　CLR P1.0; the low level pulse width of P1.0 is not less than 2μs

　　NOP

　　NOP

　　NOP

　　NOP

　　NOP

　　SETB P1.0

　　POP DPL

　　POP DPH

　　RET

　　Some issues that need to be paid attention to during the development of calibration equipment

　　● The devices of the interface circuit are powered by a high-resolution, high-stability, low-ripple power supply, and the bias power supply of the device of the interface circuit is powered by a battery.

　　● The standard resistors in the calibration interface circuit unit use standard resistors with small temperature coefficients and accuracy better than 0.02%, and are then screened through power-on aging.

　　● The key to making the auxiliary circuit and compensation network of the calibration interface circuit unit is not to introduce any influence that will cause noise, self-oscillation, etc. to the instrument being calibrated. In the production of the circuit board, pay attention to the wiring, component sorting, good grounding, and electromagnetic shielding of the box.

　　● In order to ensure the standard uncertainty of standard parameters, different models of devices that meet the requirements will be purchased from abroad and strictly screened as standard samples for verification. The standard samples will be used to compare and verify with imported and domestic measurement (instrument) systems with good performance and stability.

　　● The auxiliary sample tubes used for testing must meet the specifications in the table (select auxiliary samples with allowable values ​​of parameters such as input offset voltage, input offset current, input bias current, etc. in Table 3 to calibrate the operational amplifier tester under test), otherwise the measurement results will be inaccurate.

　　Analysis of sources of uncertainty in calibration equipment

　　The sources of uncertainty in the voltage, current and other parameters of the integrated operational amplifier parameter tester calibration device mainly include inaccurate parameter measurements of digital multimeters, digital oscilloscopes, and digital nanovoltmeters, inaccurate parameters given by analog calibration devices and compensation calibration devices, and the repeatability of these parameter measurements.