Design of a high signal to ratio electrical impedance plethysmograph

The authors describe a high-sensitivity impedence plethysmograph for determining the blood pressure of pigeons. The method described can be used for the study of human atherosclerosis. An instrument which increases the signal-to-noise ratio, therefore increasing the sensitivity, was designed. The signal obtained from the subject being tested consists of an amplitude modulated waveform with an index of modulation on the order of 0.1%. This instrument reduces the carrier amplitude in the amplitude modulated (AM) waveform and amplifies the signal before demodulation. The AM signal is then demodulated and any DC offset present is balanced out. The result is an increase in the index of modulation and a higher signal-to-noise ratio at the output