Contents I. Introduction
Several papers reported in the literature deal with the measurement and evaluation of impedance for biomedical applications [1]–[4]. In these papers, two- and four-electrode tissue impedance measurement systems are explained. There are a lot of applications in which the impedance of the tissue can give some insights into the state of the organ or the state of some illness (e.g., ischemia [5], hypoxia [6], and tumours [7]). In all these cases, a clear impedance measurement (information on both phase and magnitude versus frequency) is necessary. These impedance measurement systems are basically based on the injection of a sinusoidal current (with a variable frequency) through the electrodes and the measurement of the resulting voltage. The output signal is an amplitude-modulated voltage, and to obtain the resistance, signal processing is needed. Usually, an analog demodulator is implemented using a mixer, a low-pass filter, a sampler, and a final analog-to-digital converter (ADC). Other solutions imply the use of synchronous sampling to demodulate bioelectric impedance signals [3]. Opposite to these frequency-domain techniques, time-domain measurement techniques have also been reported as a new method to measure bioimpedance. The use of single stimulation square wave and measurement of the resulting current intensity at three given times [8] or a direct analysis of the electrode voltage drop are some examples [9]. The main drawback of both solutions is the need for fast ADCs and powerful data processors. Some of the measurement systems above [9] have been also applied to implantable medical devices.
