Bio-impedance Excitation System: A Comparison of Voltage Source and Current Source Designs

The key component in any bio-impedance measurement system is the excitation subsystem. Bio-impedance measurement can be performed by applying either current or voltage through the electrodes and then by measuring the resulting voltages or current respectively. A current source based excitation system can be useful for lower frequencies (i.e. up to 1 MHz). For a mammography system, many useful characteristics of the breast tissues lie above 1 MHz. The performance will degrade if a current source is used as an excitation system due to the higher output impedance and high precision requirement for an EIM system. Therefore a wideband excitation source covering higher frequencies, above 1 MHz, with an acceptable level of output impedance is required. This paper reports on a performance comparison of a traditional Enhanced Howland based current source with a proposed voltage controlled voltage source (VCVS). Results are compared to establish their relationship to performance parameters: bandwidth, output impedance, SNR, and phase difference over a wide bandwidth (i.e. up to 10 MHz). The objective of this research is to show which design is the most appropriate for constructing a wideband excitation source specifically for EIM or for any other EIT related biomedical application which requires a wideband system.