Designing Exciting Circuit For Electrical Impedance Tomography

Electrical Impedance Tomography (EIT) is a new functional imaging technology. EIT bases on the physical principles that the human body's different organizations have different electrical impedance. EIT apply weak current at the surface of human body, and measurement voltage. Then according to certain algorithms, EIT reconstruct the impedance distribution image within the body. Compare with the existing conventional medical imaging techniques, EIT's advantages include non-invasive, non-radioactive, portable, affordable, long-term observation for bed, etc., Recently, EIT is attracting the attention of many researchers at home and abroad.The main content of this paper is based on the existing theory to develop and implement high-bandwidth high-output impedance excitation circuit for electrical impedance tomography system.The paper first studies on the theories of sine wave generator, including the Wilson Circuit and DDS technology. Use commercial DDS chip for high-performance EIT system sine wave generator. The paper uses dual 1:2 analog switch TS3USB221 and 1:16 analog switch CD74HCT4067-Q1 to build analog switch array to meet the gating function of 64 electrode EIT system. Then studies the measurement methods of voltage-controlled current source'performance. Study the method to eliminate errors with the frequency increases. Compared the performance of existing voltage-controlled current source based on independent components by circuit simulation and actual circuit testing. Study on the characteristics of the enhanced Howland circuit. Improve Howland circuit to achieve high output impedance voltage controlled current source. Achieved an enhanced Howland circuit based on AD8610, the output current amplitude error is less than 0.5%, when frequency is less than 200 KHz, the output impedance is greater than 80KΩ. This circuit'output impedance and amplitude stability performance is better than other voltage-controlled current sources. The coupling capacitance by parasitic capacitance effects of decreased the output impedance performance of enhanced Howland circuit when the frequency is greater than 200 KHz. To overcome the effects of capacitor, propose a further improvement of the VCCS by using generalized impedance converter circuit. Based on PSPICE simulation results, this improvement could get output impedance of 1MΩat 1MHz. Also maintain a good linearity, could meet the needs of medical EIT system.Finally, this paper summarizes the research work carried out and recommendations for future research are given.