Bioelectrical Impedance Spectrum Measurement System Based on Single Chip Microcomputer

0 Introduction
For a long time, the study of the electrical properties of biological tissues has been a hot topic in biomedical engineering. The electrical properties of biological tissues are an important aspect of people's understanding of life substances.
As an important electrical parameter, the electrical impedance of life substances occupies a very important position in the study of electrical properties. The physiological and pathological characteristics of biological tissues can be reflected by electrical impedance. Many studies have shown that the frequency dependence of the electrical properties of biological tissues is very strong. Therefore, the multi-frequency electrical impedance method is often used to study the electrical properties of biological tissues. At present, domestic and foreign scholars have predicted early diseases, monitored during treatment, and distinguished normal tissues from pathological tissues by studying the impedance characteristics of biological tissues. In these research reports, it was found that the impedance characteristics of relevant biological tissues were mainly measured using ready-made impedance analyzers. These instruments are not designed for biological tissues, so there are problems such as damage to the measurement object, non-real-time, and expensive. At the same time, the research on the spectrum measurement system of biological tissues is gradually unfolding, and there are a few related reports abroad. Therefore, based on these studies, an advanced, optimized, convenient and fast bioelectrical impedance measurement system is summarized and designed. The system can automatically scan from 1 Hz to 10 MHz in a short time, and record and display the amplitude and phase corresponding to each frequency point. At the same time, the system can be connected to a PC, which has strong scalability.

1 Measurement principle
Biological tissues have different impedance characteristics at different frequencies. The two-electrode method is used to measure the impedance characteristics of biological tissues. The circuit consists of an electrode system Zx composed of electrodes and tissues and a standard pure resistor Rs without inductance in series. Since the voltage generated by the signal source is only millivolts, the voltages detected at both ends of Zx and Rs are amplified by the same proportion to become UZ and UR. Assume that the output voltage of the signal source Ui=Umsin(ωt). At the same time, the relationship between the signal sources Ui, UZ and UR satisfies the parallelogram law. The currents I, UZ and UR passing through Zx and Rs can be expressed as:

By using equation (2) and equation (3), we can calculate equation (4):

Therefore, the amplitude-frequency and phase-frequency characteristics of biological tissues can be obtained through measurement and calculation.

2 Bioelectrical impedance spectrum measurement system
2.1 System overview
The principle of the bioimpedance spectrum measurement system is shown in Figure 1. The system mainly includes power module, experimental platform, signal detection, MCU, LCD display, keyboard, SD card storage, USB interface, etc.

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The experimental platform is an electrode system composed of a measurement object and electrodes, in which the electrodes are self-made coplanar electrodes, as shown in Figure 2. First, prepare the measurement object in a suitable measurement environment and start the measurement system. The single-chip microcomputer generates a sine signal from low frequency to high frequency by controlling the DDS chip, which acts on the electrode system. After the measurement system processes the signal, it converts the signal into A/D and stores the data in the single-chip microcomputer. The single-chip microcomputer analyzes and processes the data, displays the values ​​of |Zx| and θ(ω) at different frequencies through the LCD, and stores the data in the SD card. The PC can directly read the information in the SD card through the USB interface and analyze it.

2.2 Signal source part
The signal generator is an important part of the system. It is necessary to ensure that the output sine wave signal has low distortion, stable amplitude and adjustable amplitude, phase and frequency. The system adopts direct digital synthesizer (DDS) technology and selects AD9852 chip to generate voltage signal. AD9852 is a new direct digital frequency synthesizer produced by Analog Devices, USA. It has the characteristics of fast frequency conversion speed, high spectrum purity, wide operating temperature range and high integration. The working voltage is 3.3 V. There is a 4-20 times programmable clock multiplication circuit on the chip. The maximum clock of the system can reach 300 MHz, the output frequency can reach 120 MHz, and the frequency conversion speed is less than 1μs. There are 12-bit D/A converter, 48-bit programmable frequency register and 14-bit programmable phase register inside. It has 12-bit amplitude tuning function and can generate high-stability analog signals with programmable frequency, phase and amplitude control, which can fully meet the experimental requirements. The circuit principle block diagram of the signal source is shown in Figure 3.

The clock of the signal source uses an external 20 MHz active crystal oscillator, and the system clock is obtained by multiplying the frequency through the internal clock multiplier. Since the signal directly output by the DDS chip contains the interference component of the internal clock, in order to suppress the spurious, a low-pass filter should be added to the output end of AD9852.
The maximum output amplitude of AD9852 is determined by the resistor RSET at pins 5 and 6, RSET=39.93/IOUT. The output current range of AD9852 is 5~20 mA. When the output typical value is 10 mA, it can provide the best SFD (Spurious Free Dynamic range) performance, so RSET=3.9 kΩ is selected. The output voltage amplitude of the digital-to-analog converter is -0.5~+1 V, and the output signal power cannot directly meet the requirements, so the output signal must be amplified.

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2.3 Measurement part
The measurement circuit is shown in Figure 4.

From theoretical analysis, we know that the selection of chip AD8302 can obtain the amplitude ratio and phase difference of the two signals output from the electrode. AD8302 is the first monolithic integrated circuit produced by Analog Devices in the United States specifically for amplitude and phase measurement. It can simultaneously measure the amplitude ratio and phase difference between two input signals in the frequency range from low frequency to 2.7 GHz. It contains two precision matched broadband logarithmic detectors, a phase detector, an output amplifier group, a bias unit and an output reference voltage buffer. The device integrates two precision matched logarithmic detectors on a chip, so the error source and related temperature drift can be reduced to a minimum.
The amplitude and phase measurement principle of AD8302 is mainly based on the logarithmic compression function of the logarithmic amplifier. Its general mathematical expression is: VOUT=VSLPlog(VIN/Vz) where VIN is the input voltage, Vz is the intercept, and VSLP is the slope. AD8302 uses the above logarithmic compression principle to measure the amplitude and phase of the two input channel signals through two precisely matched wideband logarithmic detectors. The equations are as follows:

In the formula: VINA and VINB are the input signal amplitudes of channels A and B respectively; φ(VINA) and φ(VINB) are the input signal phases of channels A and B respectively; VCP is the reference voltage VCP=900 mV; RFISP=30 mV/dB; RFIφ=10 mV/(°).
Therefore, the two voltage signals are outputted by AD8302 with gain and phase voltage values; and then converted by the 10-bit ADC built into the C8051F340 microcontroller, the specific parameters of gain and phase are obtained, and the impedance to be measured can be directly obtained after calculation.
2.4 Other parts
In the design of this system, in addition to the signal source and detection parts, keyboard, display, data storage and other parts are also indispensable. The keyboard is mainly used to set the signal source parameters, including the start and end frequencies, frequency steps, start and reset of the signal. The keyboard adopts a 3×2 matrix structure.
The display part uses the EDM12864B graphic dot matrix LCD display module, which is mainly used to display the specific setting information and output status of the signal parameters, such as the frequency and amplitude of the current output signal, the spectrum characteristic parameter value (amplitude and phase) of the measured object, etc.
The external data storage uses an SD card. This mode is convenient and fast. When the measurement is completed, the measurement data is stored in the SD card; then the SD card is removed and inserted into the card reader, and the data file can be directly read and processed on the computer. In addition, a USB interface is also designed on the system. When the USB is connected to the computer, the data file in the SD card can be directly read through the control of the program.

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3 System software
The system software design completes the initialization of the MCU and each external chip, including the MCU clock, A/D initialization, AD9852 initialization, AD8302 initialization, as well as the keyboard, LCD display, USB, SD card initialization, etc. After the initialization is completed, write the control word to the DDS chip AD9852 to control and output the signal; write the control word to the amplitude phase test chip AD8032 to control the output mode of measuring the output amplitude difference and frequency ratio. In addition, complete the detection of key status, real-time display, data storage, USB query, etc. The system software flow is shown in Figure 5.

The driving and control of AD9852 is the key and difficulty of system software setting. To initialize AD9852, first of all, the working mode should be set. This design adopts the monotonic mode (Single-Tone), which requires the output of sinusoidal signals of different frequencies from 1 Hz to 10 MHz. First, the value of the frequency control word of each required frequency point is calculated. The value of the frequency control word is determined by the following equation: FTW = output frequency × 248 / system clock frequency; then, the corresponding frequency table value is entered in the memory, and the corresponding frequency value is obtained through programming and sent to the frequency control word of DDS to control the output frequency of DDS. After a round of detection is completed, the next frequency value is sent. In addition, the timing design of the program should be specially considered when designing the program. The
detection part is mainly an A/D conversion process. ADC is a module that comes with the microcontroller, and the program uses an interrupt method to read the A/D conversion data. When the conversion is completed, the amplitude ratio and phase difference are read, and the impedance of the measured tissue is directly obtained through calculation.

4 Conclusion
The biological tissue impedance spectrum tester designed here can quickly, accurately and automatically complete the measurement of biological tissue electrical impedance (frequency characteristics) in the range of 1 Hz to 10 MHz. The instrument has a complete, simple and low-cost structure. The use of DDS chip ensures the quality of the signal, and the AD8302 chip greatly simplifies the amplitude and phase measurement circuits and improves the measurement accuracy. The system can well complete the measurement of biological tissue electrical impedance spectrum characteristics, greatly improve and expand related measurements, and is of great significance to related research.