Digital Demodulation for Bioimpedance Measurement Stability
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Solution Overview
Problem
Conventional methods for digital demodulation of thoracic electrical bioimpedance signals suffer from limitations such as temperature-dependent diode characteristics, loss of critical waveform details due to bandwidth constraints, and unstable data due to ripple in digitization, leading to inadequate amplitude resolution for detecting small changes in bioimpedance related to cardiac and respiratory functions.
Innovation Solution
The method employs digital demodulation through correlation techniques, separating base impedance from changes in bioimpedance using low and high pass filters, and differentiating the results to obtain the rate of change, enabling high-resolution measurement of bioimpedance changes over time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If diode rectifier circuit is used for demodulation, then the demodulated signal can be obtained, but the diode characteristics change with temperature causing measurement instability
Solution Approach 1:
The patent replaces the analog diode rectifier circuit with a digital signal processing approach. The measured impedance signal is digitized and processed using digital demodulation techniques including correlation with reference signals and digital filtering, eliminating temperature-dependent diode characteristics and improving measurement stability.
2Measurement precision
If high-pass or band-pass filter is applied to the voltage signal, then the signal can be conditioned, but critical waveform details are lost due to bandwidth constraints
Solution Approach 1:
The patent segments the signal processing into distinct digital stages: demodulation through correlation with in-phase and quadrature reference signals, followed by separate digital filtering operations. This allows selective extraction of impedance magnitude and phase information while preserving waveform details through appropriate digital filter design.
Solution Approach 2:
The patent transforms the impedance signal from the time domain to the frequency domain through digital correlation and filtering operations. By adjusting digital filter parameters such as cutoff frequencies and order, the system can selectively pass or attenuate frequency components to preserve critical waveform details while removing noise.
3Measurement precision
If the time constant of the smoothing low-pass filter is chosen high, then the demodulated signal can be smoothed, but the bandwidth of the desired demodulated signal is limited and critical waveform detail is lost
Solution Approach 1:
The patent employs dynamic digital filtering where the filter characteristics can be adjusted based on the specific measurement requirements. Digital filter parameters such as cutoff frequency and order can be changed in real-time to optimize the balance between smoothing and bandwidth preservation, unlike fixed analog RC filters.
4Extent of automation
If digitization of a signal with ripple is performed, then the signal can be converted to digital form, but unstable data is produced due to dependence on sampling position within the carrier period
Solution Approach 1:
The patent performs preliminary demodulation through digital correlation with reference signals before digitization and filtering. By extracting the envelope signal through correlation with in-phase and quadrature references and applying digital low-pass filtering, the ripple is removed prior to final digitization, ensuring stable data independent of sampling position.
Data Source
AI summary
Methods and apparatus for digital demodulation of signals obtained in the measurement of electrical bioimpedance or bioadmittance of an object. One example comprises: generating an excitation signal of known frequency content; applying the excitation signal to the object; sensing a response signal of the object; sampling and digitizing the response signal to acquire a digitized response signal representing the response signal with respect to frequency content, amplitude and phase; correlating, for each frequency fAC of the excitation signal applied, digitized samples of the response signal, with discrete values representing the excitation signal; calculating, using the correlated signals for each frequency fAC of the excitation signal applied, complex values for the bioimpedance Z(fAC); providing, over time, a set of digital bioimpedance waveforms Z(fAC,t)); separating the base bioimpedance Z0(fAC), from the waveforms; and separating the changes of bioimpedance ΔZ(fAC,t), from the waveforms.