Bio-impedance Measurement Unit with Baseline Cancellation Circuit
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Solution Overview
Problem
Existing bio-impedance measurement units face challenges in achieving high accuracy, particularly in distinguishing the small time-varying bio-impedance signals from the larger static signals, which can be obscured by noise in the reference current, leading to reduced signal-to-noise ratio (SNR) and requiring larger dynamic ranges and more complex setups.
Innovation Solution
A bio-impedance measurement unit comprising a current generator circuit, a readout circuit with an Instrumentation Amplifier (IA) and a baseline cancellation current circuit, which amplifies a reference current to form a second current that cancels out the baseline noise, allowing for the separation and measurement of the time-varying bio-impedance signal, thereby improving accuracy and reducing noise interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional bio-impedance measurement unit is used, then the measurement can be performed, but the noise in the reference current obscures the small time-varying bio-impedance signal, reducing measurement precision
Solution Approach 1:
The invention extracts and separates the time-varying bio-impedance signal from the dominant time-invariant bio-impedance component. By using a baseline cancellation current circuit that generates a copy of the reference current to subtract from the measurement current, the system isolates the small time-varying signal (e.g., heartbeat-induced impedance changes) from the large static impedance, thereby eliminating noise obscuration and improving measurement precision
Solution Approach 2:
The invention changes the parameter representation by converting the voltage measurement into a current domain through transconductance amplification. The baseline cancellation current circuit adjusts the DC component of the measurement current dynamically, allowing the system to operate with optimized current parameters that enhance the visibility of small AC variations superimposed on the large DC bio-impedance signal
2Measurement precision
If baseline noise cancellation is implemented, then measurement precision is improved, but device complexity increases due to additional circuits
Solution Approach 1:
The invention merges the baseline cancellation functionality directly into the existing transconductance amplifier circuit. The baseline cancellation current circuit is integrated with the transconductance stage, allowing the DC component cancellation to occur within the same operational framework as the AC signal amplification, thereby reducing overall device complexity while maintaining high measurement precision
Solution Approach 2:
The transconductance amplifier is designed to perform multiple functions simultaneously: it amplifies the time-varying bio-impedance signal while the integrated baseline cancellation current circuit concurrently removes the DC offset. This multi-functionality eliminates the need for separate dedicated cancellation circuits, reducing device complexity while achieving high measurement precision
3Measurement precision
If a 4-electrode setup is used to improve accuracy, then measurement precision increases, but device complexity and cost increase
Solution Approach 1:
The invention extracts and compensates for the electrode interface impedance effects through baseline cancellation. By measuring the voltage with the same electrode pair used for current injection and then canceling the baseline (DC) component that includes electrode impedance contributions, the system achieves accuracy comparable to 4-electrode methods while maintaining the simplicity of a 2-electrode configuration
Solution Approach 2:
The invention changes the measurement approach by focusing on the AC variations rather than attempting to directly measure the total impedance including electrode effects. The baseline cancellation removes the DC component containing electrode impedance, allowing the system to extract pure tissue impedance information from the AC signal, achieving high precision without requiring additional electrodes
Data Source
AI summary
A measurement unit for measuring a bio-impedance of a body, the measurement unit comprising a current generator circuit, a readout circuit, and a baseline cancellation current circuit,wherein the current generator circuit is configured to amplify a reference current to form a measurement current to be driven through a body to generate a measurement voltage representing the bio-impedance;wherein the readout circuit comprises a Instrumentation amplifier (IA) which has a transconductance stage and a transimpedance stage, wherein the IA is configured to:produce a first current in the transconductance stage, the first current being proportional to the measurement voltage,receive a second current from the baseline cancellation current circuit,produce an output voltage in the transimpedance stage, the output voltage being proportional to a difference between the first current and the second current and representative of the measured bio-impedance;wherein the baseline cancellation current circuit is configured to amplify the reference current by a factor to form the second current and deliver it to the IA,wherein the factor is such that that the absolute value of the difference between the first and the second current is below a threshold such that a baseline of the first current is cancelled by the second current.