1. Scheme design
1. Theoretical analysis of digital multimeter
The core component of the digital multimeter is the A / D converter, which converts the input analog quantity into a digital quantity, and turns on or off each group of analog switches in the A / D converter according to the specified timing to ensure the normal A / D conversion get on. The single-chip computer reads the digital quantity converted by A / D and performs the corresponding conversion, and then displays it on the liquid crystal module. When measuring DC voltage, an automatic range conversion circuit is used to read the digital signal of MC14433 through a single-chip microcomputer to control the analog switch, thereby changing the size of the feedback resistor to achieve different choices of gears; measuring AC voltage, using AC-DC conversion circuit to convert AC into DC, and then use the method of measuring DC voltage to measure AC. Among them, when the input voltage is less than 200mV, it must first pass through the amplifier circuit composed of NE5532; measure the resistance, use the operational amplifier to use the inverse proportional operation method and the method of constant current source to convert it into voltage, and then pass the A / D conversion circuit and Single-chip computer for calculation.
2. Theoretical basis during the selection of the IT company
(1) MSP430F149 single-chip microcomputer: MSP430 series single-chip microcomputer is a 16-bit ultra-low power mixed signal processor (Mixed Signal Processor) of Texas Instruments (TI) which was introduced to the market in 1996. It integrates many analog circuit peripherals (such as ADC, DAC, analog comparator, etc.) and common digital modules (such as SCI, SPI, I2C, watchdog, PWM, CAP, timer / counter) inside the chip. Because it has the advantages of strong processing power, fast calculation speed, high integration, rich external devices, ultra-low power consumption, and has a high cost performance, it has been widely used in many fields. MSP430 MCU can use assembly language or C language for programming. It supports JTAG debugging, and its hardware emulator is a simple parallel port adapter, and is suitable for all MSP430 microcontrollers. Its ultra-low power consumption can be as low as 0.1uA in sleep mode, and there are seven low-power modes to choose from. Multiple I / O support interrupt modes to meet the system's need for external interrupts. Its software is the Embedded Workbench integrated development environment provided by IAR. This software has a friendly man-machine interface and can well support C language development. Because this system belongs to the instrument design, it involves the requirements of automatic control and low power consumption. MSP430 MCU has rich I / O ports and low power consumption, so it is the best choice for this system design.
(2) TL431 voltage stabilization device: a constant current of 10mA is required when measuring a small resistance. The system design uses a TL431 voltage stabilization device to generate a stable 2.5V voltage as a reference to design a precision constant current source. TL431 is a three-terminal adjustable shunt reference source with good thermal stability. Its output voltage can be arbitrarily set to any value in the range from Vref (2.5V) to 36V with two resistors. The typical dynamic impedance of the device is 0.2, the voltage fluctuation difference is plus or minus 0.4%, and the noise output is low. Compared with other products, the TL431 is cheap and inexpensive, so TL431 is used as the voltage reference device for system design.
(3) OP07 operational amplifier: OP07 is a commonly used low-noise high-precision operational amplifier, with very low input offset voltage, very low temperature drift, very low input noise voltage amplitude, high common mode rejection ratio (-126dB) And good stability. The input offset voltage is 10uV, the input offset voltage temperature drift is 0.2uV / ℃, the power supply voltage range is wide, and the input impedance is high. It can be widely used in integrating circuits, precision absolute value circuits, comparators and precise amplification circuits of weak signals.
3. Demonstration of digital multimeter scheme
1) Automatic range conversion module
Use software to achieve. By reading the digital signal of MC14433 through the single-chip microcomputer to control the analog switch, the size of the feedback resistor can be changed to realize different choices of gears, which can easily meet the requirements of the measurement range. This scheme is simpler in circuit and easier to implement.
2) A / D conversion module
Using the internal A / D conversion module of the single chip microcomputer makes the peripheral circuit simple and the design convenient.
3) AC-DC conversion module
Scheme 1: Bridge rectifier circuit, which can obtain DC voltage by using the unidirectional conductivity of the diode. However, because the design circuit is complicated and time is limited, this scheme is not adopted.
Solution 2: Use the AD627 true effective value conversion chip to convert the AC voltage signal to a DC voltage signal with an amplitude equal to the AC effective value. When measuring the DC voltage, this solution has a simple circuit, fast response, low distortion, and work. Stable and reliable.
4) Resistance measurement module
Option 1: Use the principle of resistance voltage division, connect the measured resistance in parallel with the known resistance, and obtain the known resistance through the voltage ratio. But this measurement error is very large and inaccurate.
Option 2: The measurement method of large resistance is different from that of small resistance. The ratio method is adopted for large resistance, the circuit is simple and easy to operate; the constant resistance plus small signal amplification method is adopted for the measurement of small resistance. A current of 10mA flows through the resistor to produce a voltage drop of 0 ~ 20mV and then magnify 10 times, that is, 0 ~ 200mV measurement, which solves the problem of large heat flowing through a small resistance and large heat. The error of this scheme mainly depends on the accuracy of the constant current source. As long as the constant current source is constant, the measurement can reach a very high accuracy.
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