Miniaturizated Respiratory Monitoring: Synchronous Demodulation & Bi-Directional Ultrasonic Flow Sensing

Abstract

The issues surrounding optimization of respiratory monitoring are of size, power, and SNR (Signal to Noise Ratio). Miniaturization and optimization of components within a respiratory monitoring system is valuable for integration into existing non-invasive ventilators, including, but not limited to the FOT (Forced Oscillation Technique) source, sensors, signal processing and generation. This document contains a low power analog Lock-in Amplifier (LIA) in CMOS 0.18um to measure respiratory impedance at low frequencies (4Hz < f < 100), and a speed of sound invariant, ultrasonic based flow meter for reduction of series impedance when sensing flow. This LIA relaxes requirements on signal input power by maintaining a precise frequency selection in the modulation and demodulation of FOT, improving the effective bandpass quality factor when averaged over long time periods, and significantly reducing the power requirement of impedance calculations. This LIA, when integrated with current ventilation devices, would allow for long term exploration of respiratory health through a diverse band of FOT frequencies and open more avenues for research with large sets of data to improve respiratory diagnostics. The ultrasonic based flow meter, leverages TDC (Time to Digital Conversion) to measure the time of flight of ultrasonic waves within the breathing path, instead of using a conventional differential pressure measurement across a known impedance. This ultrasonic sensor uses a symmetric style of measurement that cancels the effect of changes in speed of sound, isolating flow from temperature and humidity variations during breathing. This ultrasonic sensor is a more compact, bi-directional (positive and negative flow) and inexpensive form of capturing flow that is capable of highly linear operation, high resolution and sampling rate, while not adding series impedance to the measurement. Together, these components comprise a large portion of a functional respiratory monitoring platform.