Immittance, Admittance, and Impedance

Immittance is the general term eovering the duality of admittance and impedance. 

Driving point immittance is defined with excitation and response at the same port, the result is not influenced hy any contrihution from transmittance components. There are two possible ratios vi/ii (impedance) and ii/vi (admittance). 

Transmittance is defined with excitation and response at different ports. The transfer function H(w) of voltages at two different ports is  

Such a dimensionless function is used to emphasize signal transmission and not tissue characteristics. 

A two-port four-electrode black box may use port 1 for controlled current excitation and port 2 for zero current potential measurement. The ratio excitation current to measured potential is called admittance [siemens]. However, it is transfer admittance, implying that the transfer admittance is a transmission parameter and therefore strongly dependent on the distance between port 1 and 2. Therefore transfer admittance is not directly characterizing the tissue. 

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