A FPGA-Based Multi-Frequency Current Source for Biological EIT System

Multi-frequency electrical impedance tomography has been evolving from the frequency-sweep approach to the multi-frequency simultaneous measurement technique which can drastically reduce measuring time and will be increasingly attractive for time-varying biological applications. In order to improve the output impedance of current source and obtain more impedance information at different frequencies simultaneously, a FPGA-based multi-frequency current source is designed. New features of the proposed multi-frequency current source include feedback loop to improve the output impedance of current source, amplitude control circuit to adjust amplitude of current source and the initial phases which are considered for reducing crest factor (CF) of multisine excitation signals to obtain a further SNR improvement, compared to random phases which are used in previous design. The output performance of this current source is evaluated by simulation and actual measurement. The output impedance of current source with feedback loop is greater than the one without feedback loop. The stability of current source with feedback loop outperforms the current source without feedback loop for resistive and capacitive loads in simulation. The simulation and measurement results show that output impedance of current source significantly improves by adding feedback loop, and the multi-frequency current source can perform stably and provide high output impedance and multi-frequency simultaneously excitation signals, which is suitable for multi-frequency EIT system.