Bioimpedance Measurement Circuit Stray Capacitance Extraction
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Solution Overview
Problem
Bioimpedance spectroscopy technologies face challenges in accurately measuring body composition and hydration status due to errors from stray capacitance, instrumentation variations, and lack of noise analysis, leading to unreliable results.
Innovation Solution
The solution involves measuring stray capacitance, positioning preamplifiers close to sensing electrodes, using input logic circuits to isolate parts of the measuring circuit, employing mirrored Howland current sources for precise current control, and applying sine wave fitting algorithms to reduce noise and improve measurement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Difficulty of detecting and measuring
If bioimpedance spectroscopy is used to measure body composition and hydration status, then measurement capability is provided, but measurement precision deteriorates due to stray capacitance and instrumentation variations
Solution Approach 1:
The patent extracts and separately measures the stray capacitance component from the total measured capacitance. By isolating the stray capacitance measurement (Cstray) from the body capacitance measurement, the system can subtract this error source from the total measurement, thereby improving the precision of body composition measurements while maintaining the ability to measure body hydration status.
Solution Approach 2:
The patent changes the measurement parameters by performing measurements at multiple frequencies (at least two different frequencies) and analyzing the frequency-dependent behavior of the impedance. This allows the system to separate and identify the stray capacitance parameter from the body tissue parameters, improving measurement accuracy without losing the capability to assess body composition across different frequency ranges.
2Ease of operation
If preamplifiers are positioned away from sensing electrodes for ease of connection, then ease of operation improves, but measurement precision deteriorates due to increased noise and stray capacitance
Solution Approach 1:
The patent introduces a buffer amplifier or shielded cable as an intermediary between the sensing electrodes and the preamplifier. This intermediary component allows the preamplifier to be positioned away from the sensing electrodes for ease of connection while maintaining low noise performance, as the buffer or shielded cable isolates the high-impedance electrode inputs from the effects of cable capacitance and interference.
3Device complexity
If standard measurement circuits are used without isolation circuits, then device complexity is reduced, but reliability deteriorates due to noise and interference
Solution Approach 1:
The patent applies preliminary action by incorporating isolation circuits and noise filtering stages at the beginning of the signal path, close to the sensing electrodes. This preliminary noise reduction and isolation prevents interference from propagating through the rest of the measurement system, ensuring reliable measurements without requiring complex noise reduction techniques throughout the entire device.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the accuracy and reliability of body composition and hydration status measurements by correcting for stray capacitance and noise, providing individualized references and real-time metabolic modeling.
Implementation Method 1
measuring the resistance and reactance, that is, the impedance, of the human body directly at a multitude of frequencies
Implementation Method 2
measuring the stray capacitance of the measuring circuit
Implementation Method 3
The Cole model of body impedance as interpreted by Cornish... a current at low frequency flows through the extracellular water mass while at higher frequencies it flows through both the extracellular and intracellular water mass
Data Source
AI summary
One embodiment of an apparatus for analysis of body composition and hydration status by detecting resistance of the human subject at zero and infinite frequency including a method for measuring indirectly extracellular water mass, intracellular water mass, lean body mass, and body fat mass; daily changes of extracellular water mass, intracellular water mass, lean body mass, and body fat mass; and acute changes of extracellular water mass and intracellular water mass; and for individualized calibration of these indirect measurements.In addition, a method for fitting mathematical models to serial measurements of indirectly measured lean body mass and fat mass and for dynamic indirect individualized measurement using minimum variance estimation and prediction of daily changes of the body composition defined as change of glycogen store, change of fat store and change of protein store; daily utilized macronutrient energy intake defined as utilized carbohydrate, fat, and protein caloric intake; daily macronutrient oxidation rate defined as rate of carbohydrate oxidation, fat oxidation, and protein oxidation; daily resting metabolic rate; daily unknown forms of energy losses or gains; daily rate of endogenous lipolysis; daily nitrogen excretion; daily gluconeogenesis from protein; daily determination of extracellular water mass; daily determination of intracellular water mass; and acute change of extracellular water mass and intracellular water mass.