The BIA device is based on the AD8302 Gain and Phase Detector (Fig. 3), which can be used to find the impedance and phase of an RC circuit and has already been shown to work in a BIA device.19 The Gain and Phase Detector takes two sinusoidal signals as inputs and outputs two dc potentials, Vphase and Vmag. Here Vphase is proportional to the phase shift Du between the inputs, and Vmag is proportional to the
logarithm of the ratio of the connected impedance Z to the reference resistance Rref. Thus, we have Vphase / Du and Vmag / log10ðZ=RrefÞ. (See Ref. 18 for the full conversion formulas.) There are multiple methods of electrode placement in BIA including hand-to-hand, foot-to-foot, and hand-to-foot.13 Our device uses hand-to-hand electrodes that are designed with conductive tape on plastic tubing (Fig. 2). Four electrodes are needed for BIA, two electrodes to carry current and two to measure the potential across the body. Tetrapolar voltage probes minimize the effect of the skin-electrode impedance Zcontact on the measurement of the body’s impedance Zbody (Fig. 4).20