Holding zero switching scheme for multi-channel impedance measurement

This paper presents a design, implementation and performance of a novel data collection scheme for multi-channel impedance measurement, which is intended for developing high performance electrical tomography systems, even without parallel processing strategy. The hardware of electrical tomography employs electrical switching parts instead of mechanically moving parts to implement object scanning, which features electrical tomography faster acquisition rate and lower cost compared to radioactive tomography. However, frequent electrical switching operations in measurement inevitably result in charge/discharge processes in capacitors and other energy storage components. The processes will produce residual charges in the capacitors, which will act as additional voltage source in the future excitations and measurements. This effect becomes inacceptable when increasing the excitation frequency. Therefore, the authors developed the holding zero switching (HZS) scheme to handle this effect. An FPGA based measurement system is designed to implement this scheme. Simulation and experimental results show the strategy could effectively eliminate the impact of residual charges to the measurements. In this way, a possible electrical tomography system with a single A/D could reach the data acquisition rate of above 1000 frames per second without sacrificing the system signal to noise ratio.