Design of broadband DC amplifier based on MSP430F449 single chip microcomputer

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Design of broadband DC amplifier based on MSP430F449 single chip microcomputer [Copy link]

According to the design requirements, the maximum voltage gain ≥ 60dB is achieved within a wide signal bandwidth (0-10MHz), and the gain can be adjusted continuously or preset in 5dB steps. This is the biggest difficulty and one of the key points of the design. Another difficulty is that the maximum output voltage sine wave effective value Vo of the post-stage power amplifier module on a 100Q load is ≥ 10V. Since the low end of the bandwidth is 0Hz, that is, a DC signal, the zero drift of the amplifier circuit is also a difficult problem to solve. In addition, in the design of the entire amplifier, its cost needs to be considered. 1. Data processing and control core selection plan 1: Use AT89S52+FPGA to realize signal gain control, data processing and human-machine interface control functions. Since this system does not involve a large amount of data storage and complex processing, the resources of FPGA cannot be fully utilized and the cost is high. Plan 2: Use a single-chip microcomputer to realize unified control and data processing of the entire system. The single-chip microcomputer MSP430F449 is a 16-bit ultra-low power microprocessor with rich on-chip peripherals and strong computing power. It supports online programming, is very convenient to use, and has a high cost performance. Therefore, the second scheme is adopted. 2. Signal gain control and power amplification scheme design Scheme 1: Use triodes to form a multi-stage amplifier circuit to achieve a gain of ≥60dB, and use discrete components to build the post-stage power amplifier. This scheme has low cost, but it is difficult to pair transistors, the circuit design is complex, the step adjustment of gain is difficult to achieve, the working point debugging is cumbersome, and the circuit stability is poor, which is easy to produce self-excitation. Scheme 2: Use integrated chips, such as using low-noise, precision-controlled variable gain amplifier AD603 as the gain control core device, and using a high-voltage output broadband op amp to complete the power output. AD3603 has high temperature stability, and its gain (dB) is linearly related to the control voltage (V). Using D/A output control voltage can achieve precise digital control. But the cost is high. The circuit has high integration, simple design, and short design cycle. In summary, Scheme 2 is adopted. 2. Overall scheme design and system block diagram The overall system block diagram is shown in the figure below. Overall scheme description: The input signal of this system passes through the pre-amplifier circuit, the post-stage program-controlled amplifier and the final power amplifier, achieving a maximum voltage gain of 90dB. The post-stage power amplifier uses a broadband operational amplifier with high voltage output to improve the effective value of the output voltage. The single-chip microcomputer MSP430F449 is used as the data processing and control core. The control voltage of AD603 is adjusted by the D/A converter, the gain is adjusted by program control, and the post-stage program-controlled amplifier circuit channel is switched by the relay to achieve the control function of the amplifier gain ×1, ×10, ×100... The passband selection is achieved by switching the two-way elliptical filter by the relay. Manually adjust the continuously adjustable potentiometer to continuously change the control voltage of AD603 and achieve the continuous gain adjustment function. Zero drift problem. The zero drift and offset of the amplifier are mainly generated by the input end of AD603. Whenever the gain of AD603 is adjusted to a different level, the output DC deviation voltage is also different. Before each test, the system short-circuits the input of AD603, samples the output voltage of the zeroing amplifier with the internal ADC of MSP430F449, and uses the microcontroller and digital algorithm to control the D/A converter to output the corresponding adjustment voltage, and controls the output of the zeroing amplifier to zero. This not only suppresses the DC zero drift, but also realizes the automatic zeroing calibration function.