Signal to noise ratio in bioelectrical impedance measurements using synchronous sampling

The authors have analyzed and measured the signal-to-noise ratio (SNR) for three coherent amplitude demodulation methods applied to detecting physiological events from electrical impedance measurements. They compare the theoretical performance of a synchronous demodulator based on an analog multiplier (AMD), a synchronous detector (SD) based on a switched-gain amplifier and a synchronous sampling method (SS) implemented using the floating capacitor technique. The authors have considered the influence of noise outside the passband for the detected signal (NOISE), i.e. physiological noise, harmonics from the injected signal (HARM), power line interference (INT) and common mode voltage (CMV). Overall, when measuring transthoracic impedance, SS using the floating capacitor technique yields up to twice as much noise as compared to SD using the AD 630, but if additional filtering is used then the same SNR results