A low-noise ac-bridge amplifier for ballistocardiogram measurement on an electronic weighing scale.
Ballistocardiography is a non-invasive technique for evaluating cardiovascular health. This note presents an ac-bridge amplifier for low-noise ballistocardiogram (BCG) recording from a modified weighing scale. The strain gauges in a commercial scale were excited by an ac source-square or sine wave-and the differential output voltage resulting from the BCG was amplified and demodulated synchronously with the excitation waveform. A standard BCG amplifier, with a simple dc-bridge excitation, was also built and the performance was compared to both the square- and sine-wave excited ac-bridge amplifiers. The total input-referred voltage noise (rms) integrated over the relevant BCG bandwidth of 0.3-10 Hz was found to be 30 nV (square wave source) or 25 nV (sine-wave source) for the ac-bridge amplifier and 52 nV for the standard amplifier: an improvement of 4.8 dB or 6 dB, respectively. These correspond to input-referred force noise (rms) values of 5 mN, 4 mN and 8.3 mN. The improvement in SNR was also observed in recorded waveforms from a seated subject whose BCG signal was measured with both dc- and ac-bridge circuits.
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Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA. omeri@stanford.edu
Kovacs
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D000669:Amplifiers, Electronic; D016477:Artifacts; D001450:Ballistocardiography / Q000295:instrumentation*; D001837:Body Weights and Measures / Q000295:instrumentation*; D004581:Electronics / Q000295:instrumentation*; D006801:Humans