A Comparison of Different Methods for Measuring the Output Impedance of Current Sources Used in Bio-Impedance Measurement

Electrical impedance tomography, a non-invasive technique for reconstructing the spatial distribution of electrical properties, has been widely adopted in biomedical research. One of the core units of an EIT system is the current source, whose precision significantly affects the biological impedance demodulation. The precision of the impedance measurement is strongly dependent on the accuracy of the current source, which can be affected by the current source's output impedance. Hence, maintaining a high output impedance of a current source over the interesting bandwidth is a primary challenge of current source design. Even for the same current source, different measurement methods may yield varying output impedance values. This paper compares the output impedance measurement methods under various settings proposed in the literature for biomedical measurement: measurement with two similar known resistors, measurement with two significantly different known resistors, measurement of the resistive component when the capacitive component is ignored, and measurement with two unknown resistors. Through simulation verification under different auxiliary test resistor settings and across a wide frequency range, for those who only care about the resistance component that is not affected by capacitance, the output impedance measurement method that excludes the influence of capacitance is the most suitable one for the resistive component measurement of the output impedance in bio-impedance measurement. Based on in-depth qualitative and quantitative analyses, these methods can also be extended to current source measurements in various applications, guiding both selection and practical measurements.