Related Experiment Video
Updated: Jun 12, 2026

Muscle Velocity Recovery Cycles to Examine Muscle Membrane Properties
Published on: February 19, 2020
A low-noise ac-bridge amplifier for ballistocardiogram measurement on an electronic weighing scale
1Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA. omeri@stanford.edu
Abstract:
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.
Related Concept Videos
Instrumentation Amplifier
To overcome this challenge, an ECG machine utilizes an instrumentation amplifier. This specialized amplifier is...
Design Example: Strain Gauge Bridge or Wheatstone Bridge
Wheatstone Bridge
Thus, for accurate resistance measurements, a...
Equipments Used To Measure Blood Pressure
This invasive approach involves cannulating a peripheral artery. During each cardiac contraction, pressure generates mechanical motion within the catheter, transmitted through rigid, fluid-filled tubing to a transducer. This transducer converts mechanical motion into electrical signals displayed as waveforms on a monitor. An automatic flushing system prevents blood backflow. Due to the potential risk of unexpected arterial blood loss, this method is primarily used in intensive...
Measurement of Blood Pressure
Bridge rectifier
Operationally, the bridge rectifier allows current flow through two of its diodes during each...
