Research Of A Portable Bioimpedance Measuring System

As a non-invasive,safe and easy-to-use detection method,bioimpedance measurement technology has been widely applied in the field of physiological information extraction and medical diagnosis.However,problems like narrow working frequency range,large volume,and high cost still exist in related devices.There is still room for improvement in the measuring methods and the design of systems.In order to meet the needs of impedance measurement in biomedical engineering,a miniaturized,high-precision and automatic impedance spectrum measuring system was developed based on AD8302.The amplitude ratio and phase difference between the voltage signals on two impedances were measured in the frequency range of 1 kHz to 1 MHz.The value of the impedance to be measured were indirectly calculated by the reference impedance.And the impedance spectrum can be obtained by testing in multiple frequencies.The system used the STM32F103 microcontroller as the control centre,usin the DDS and the voltage-controlled current source to generate a sinusoidal excitation current,which was input into the biological tissue through four electrodes of the interface circuit.Two analog multiplexers ADG1608 were used to adaptively suit the sampling resistors or phase-shifting capacitors to improve the accuracy of AD8302.By an analog switch ADG1634,the calibration impedance could be gated for performing least-squares calibration,which could effectively reduce system errors caused by chip manufacturing,testing parameters and other interference in circuit.Two instrumental amplifiers AD8421 were used to collect the response signals which were measured by the signal detector AD8302,while the results were calculated by the microcontroller.Users could operate and get the results through a touch screen.The U-disk and SD card output interfaces were also provided.This paper mainly focused on the system scheme and measurement interface circuit.The hardware and software design of the embedded system were also completed.Tests showed that enhancing the accuracy by least squares calibration and adaptive reference impedance matching is effective and feasible.The miniaturized and low-cost measuring system can operate in a wide frequency range and obtain accurate results with practical values.