A Novel Fully Differential Current Source for EIT/EIS Systems

J. Frounchi, M. H. Zarifi, and F. Dehkhoda

Microelectronic and Microsensor Research Laboratory, Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran, Email: jfrounchi@tabrizu.ac.ir

ABSTRACT

A novel fully differential current source for Electrical Impedance Spectroscopy (EIS) and Electrical Impedance Tomography (EIT) systems is presented. It achieves very high output impedance over a wide frequency range using four high-impedance current sources in an H-bridge connection. The load, a sample of human tissue, is placed between the left and right arms. To decrease harmonic distortion, all current sources have an equal dc level, so the transistors in the four current sources are always on. A common mode feedback circuit is employed at the differential outputs which can deal appropriately with any unbalanced differential current levels resulted from the mismatching in the circuit elements. We have implemented this circuit by the discrete elements and the small signal simulation results confirms that an output impedance of 135 MΩ can be expected at frequencies as high as 1 kHz. The output impedance at 1 MHz is above 153 kΩ. The large signal simulation also proves that this circuit can give a stable output current as high as 5 MHz with less than 1% total harmonic distortion.

Keywords Electrical impedance tomography, Electrical impedance spectroscopy, high frequency differential current source, output impedance.