Electrical impedance tomography (EIT) utilizes electrodes placed on the surface of a body to determine the complex conductivity distribution within the body. When using a voltage source in EIT, it is necessary to know both the applied voltage and the resulting current with high precision, which cannot be achieved with current systems. This invention is directed to a voltage source for use in applied voltage EIT that includes the capability to measure both the applied voltage and the applied current.The invention provides a calibration circuit and a calibration algorithm which enables all voltage sources in an EIT system to be calibrated to a common standard.The calibration minimizes the impact of stray shunt impedance, passive component variability, and active component non-ideality.