Circuit Design Of Multi-frequency Data Acquisition System Of Electrical Impedance Tomography

Electrical Impedance Tomography(EIT) is a new medical imaging technology developed in nearly forty years. It takes biological electrical impedance distribution or change as imaging target, injecting the safety exciting current into human body and measuring the induction voltage through electrodes attached on body surface, then using the algorithm to reconstruct the human internal impedance distribution, thus for disease detection and clinical care. Compare with the existing conventional medical imaging techniques, EIT’s advantages include non-invasive, fast, convenient, portable, safe and affordable. It is suitable for the patients to carry on the long-term, continuous clinical care, and has important potential value and broad application prospect.This paper mainly designed a hardware circuit of multi-frequency EIT data acquisition system, which controlled by Field Programmable Gate Array(FPGA) and can work under 1KHz~1MHz excitation frequency in 16 electrodes system. The simulation and hardware circuit test showed that the designed data acquisition circuit has good performance.The designed EIT data acquisition system hardware circuit includes the front-end signal conditioning module, analog multiplexer module, digital phase sensitive demodulation module, current and voltage acquisition module. In the front-end signal conditioning module, selecting the AD8065 as voltage follower, using AD8250 and AD8253 as two levels of programmable gain amplifier(PGA), so as to realize the signal buffer and flexible dynamic range adjustment. In order to improve the common mode rejection and signal transmission quality, put the electrode, buffer, amplifier circuit as close as possible to the human body surface. In the analog multiplexer module, establishing circuit model of multiplexer, doing some simulation and actual circuit test. Through comparison, choosing ADG1206 and ADG1406 as incentive electrode multiplexer and measuring electrode multiplexer. In digital phase sensitive demodulation module, using Matlab to simulate the demodulation process, studying the influence of sampling points in each cycle to the demodulation precision, giving reference to the choice of ADC. Using LTC1420 as analog to digital converter and 74AC16373 as data latch to realize the signal conversion, data storage and transmission.Through the software simulation and actual circuit testing, it is proved that the front-end signal conditioning module can work in 1KHz~1MHz frequency, and has good performance of signal buffering, filtering, differential amplifying, the dynamic range adjustment ratio can reach 1000 times and above. The selected analog multiplexers’ distribution parameters have little influence to the signal in the high frequency period, and can realize the basic no loss of current and voltage transmission. Digital phase sensitive demodulation can get more accurate signal amplitude and phase information, but needs front-end ADC with at least 12-bit accuracy, more than eight times of signal frequency sampling rate. LTC1420 is a 10 MSPS, 12-bit sampling A/D converter, which used to acquire current and voltage signal.This hardware circuit designed in this thesis can be used in broadband, multi-frequency EIT data acquisition system. The system may, according to the number of electrodes for flexible configuration, as long as change signal front-end conditioning module number and analog multiplexers’ channel. It has very good experiment and development value.