Research On The Key Technology Of The Front-end Circuit Of Bio-Electronic System

Body composition are maintained at a certain equilibrium state.When the human body physiology or psychology changes,the body composition will change.So the health state can be real-time detected by analyszing the components of the human body.Clinical experiments show that human body composition can be analyzed by measuring human impedance.An impedance measurement system is designed,it can be used to get the impedance of human body.The system uses the DDS to produce sine signal that the frequency can be controlled by the users.The Delta-sigma modulator is used to convert the parallel digital code produced by DDS to serial output.Then analog low pass filter is used to filter the high frequency noise of the modulator output.Finally,the voltage signal is converted to the current signal by the Howland current source.The current signal generated by Howland current source is injected into the tissue through a pair of electrode.The voltage difference at the electrode is amplified by the INA amplifier.And then the analog signal after the amplification is converted to the serial digital signal through the Delta-sigma modulator.Finally,the digital filter is used to reduce the rate of the modulator to output the digital code and filter the high frequency noise signal from the modulator output.The computer interface is written by Lab VIEW,and the working mode of the system is modified by controlling FT245,and the data collected by the system is stored.Compare with the previous impedance measurement system,the proposed system in this paper has three advantages.It can reduce the scale and power consumption of the system by using the digital Delta-sigma modulator and a low pass filter to convert the DDS output digital signal to analog output.And the SNR of the filter can reach the lowest level of 52 d B.secondly,a PMOS is cascaded behind the Howland current source is cascaded,that can increase the output impedance of the constant current source.And the output impedance of the constant current source can reach 1M Omega.Thirdly,the CIC decimation filter with interlaced structure is used to reduce the pass band attenuation.Compared with the filter,the filter with interleaving structure can make the stop band attenuation increase by 2-10 d B.In the system test process,the function and performance of each independent module are verified by using the known resistance network,and then the performance of the whole circuit system is analyzed.The minimum signal to noise ratio of the whole system is 46 d B,and the maximum signal-to-noise ratio can reach 62 d B.