Design concepts for the implementation of two basic functions for measurement of electrical impedance are presented: current injection and voltage measurement. At relatively high frequencies, the application of an alternating current through the body or a body segment results in electromagnetic stray fields that reduce the amount of current actually injected into the tissue under study. It is shown that electrical isolation and small dimensions of the isolated section are indispensable in order to substantially reduce these stray currents. The paper describes a new wideband current source configuration driven by direct digital sine wave synthesis (DDS) presenting very low stray currents due to a symmetrical layout. Two implementations of the actual current source circuit are presented: (1) a voltage-controlled system and (2) a current conveyor-based circuit.
A wideband input amplifier with transformer coupling is described. The current source, amplifier, and (in case of tomography) multiplexer are also situated on an electrically isolated front end. The presented concepts are applied in a new electrical impedance tomograph (EIT) presently under construction in our department.