Key result
Sine-wave AC-bridge amplifiers reduce total voltage noise ~52% versus standard DC-bridge amplifiers.
Effect estimate: improvement of 4.8 dB (square) or 6 dB (sine)
Absolute Event Rate: 25% vs 52%
An ac-bridge amplifier significantly reduces noise and improves signal-to-noise ratio for non-invasive ballistocardiogram measurements on an electronic weighing scale compared to standard dc-bridge amplifiers.
May enhance subtle cardiac signal detection; leaves open clinical validation of ac-bridge amplifiers.
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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Inan et al. (2010) studied this question. ac-bridge amplifier (square or sine wave excitation) vs. standard BCG amplifier with dc-bridge excitation was evaluated on Total input-referred voltage noise (rms) integrated over 0.3-10 Hz (improvement of 4.8 dB (square) or 6 dB (sine)). An ac-bridge amplifier using a sine-wave source reduced total input-referred voltage noise to 25 nV compared to 52 nV for a standard dc-bridge amplifier, an improvement of 6 dB.
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