introduction Bioimpedance technology is a non-invasive technique that does not cause any harm or side effects to the human body during measurement. It is one of the directions of human medical development [1]. The mechanism of the bioimpedance analyzer [2] is that the biological tissue has different conductive effects on the external current field. When a low-frequency current with a fixed frequency is applied to the surface of the human body, more than 70% of the muscle tissue containing water is a good conductor, while the water contains water. Fewer fat tissue is approximately an insulator, so the measured impedance value can be used to calculate the body composition and electrical imaging of electrical impedance [3]. There are a variety of bioimpedance analysis instruments on the market, but they are expensive, cumbersome to use, complicated in design, and difficult to popularize in family health care. Therefore, this design proposes a new idea and develops a high-performance small bio-impedance analyzer that can measure the impedance modulus and phase information of each segment of the human body at multiple frequencies. The instrument has the advantages of high integration, low cost, convenient use, safety and reliability. 1. System hardware structure and principle 1.1 System principle The highly integrated impedance analysis chip AD5933 is a high precision impedance conversion system with an on-chip frequency generator and a 12-bit, 1MHz sampling rate ADC (analog-to-digital converter). The on-chip frequency generator can emit an excitation signal at any specified frequency, and the gain is controlled by the preamplifier to reach the target impedance. The returned signal is sampled by the ADC and sent to the on-chip DSP engine. The DSP engine performs an FFT operation and calculates each The real and imaginary parts of the impedance at the output frequency. This system adopts the design method of the upper and lower position machine [4]. The whole system is controlled by the upper computer PC sending the corresponding commands through the USB. The scanning application starts the scanning frequency starting point, the end point and the step size, and sends it through the USB interface. After the lower computer resolves the corresponding protocol, it converts to the control command of AD5933, configures AD5933 to complete the corresponding operation, and the lower computer uses the LPC2148 chip of ARM7 core as the main controller, and controls the AD5933 chip through I2C bus. The chip passes the analog front end of the four-electrode method and uses the on-chip DSP to calculate the characteristic impedance of the human body. The main controller retrieves it through the I2C interface, and the data is transmitted to the upper computer PC for display and analysis via the USB interface, and finally the impedance to the human body is completed. Sweep measurement of modulus and phase. At the same time, the system uses safety isolation measures between the human body and the PC to ensure the safety of the tested human body. In this design, in order to facilitate the calibration measurement, the parameters obtained by the E2PROM memory calibration of the I2C bus interface are used. In addition, the design uses the PCA9535 chip to convert the I2C signal into an I/O signal as a control bit to control the front-end electrode required for the measurement. It is possible to simultaneously measure the impedance information of multiple sections of the human body [5]. 1.2 system hardware structure The overall system hardware framework is shown in Figure 1: Figure 1 hardware frame diagram 1.2.1 USB communication: This design uses the LPC2148 on-chip USB2.0 controller to complete USB communication. The controller supports 16 logical endpoints. It uses control endpoint 0 and bulk endpoint 2 in the design, and uses full-speed mode for USB communication. 1.2.2 Isolation: In order to ensure that the measured organism is separated from the PC voltage, isolation technology is used, including power isolation and isolation of the I2C signal. The I2C signal is isolated using the ADUM2251, which provides a bidirectional channel and a unidirectional channel to isolate the I2C interface. The chip has strong isolation and protection performance, rated isolation voltage of 5000V, and can last for 1 minute. The power isolation uses DC/DC power isolation module. The module has built-in EMI filter unit, input overvoltage, overcurrent and output short circuit, overload protection circuit. 1.2.3 Measuring front electrode: A four-electrode method [6] is employed, in which a pair of excitation electrodes are used to provide an excitation current, and a pair of electrodes are used to measure the voltage drop across the body. The excitation electrode adopts a pressure-flow conversion design to convert the excitation voltage signal from the AD5933 into an AC constant-current signal, and the multi-frequency constant current is lifted to 2.5V through the human body [7], and the measured electrode is measured in the human body. The generated voltage is passed through a differential amplifier with high input impedance and high common-mode rejection ratio, and sent to the back end for AD conversion processing and impedance characteristic calculation to ensure measurement accuracy. Compared with the relatively simple two-electrode method, the simulation front end of the whole measuring electrode is shown in Fig. 2. Figure 2 Four-electrode simulation front end Extraction Kit Magnetics Bead Method
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