Bio-Impedance Transducer Using Current Conversion for Noise Rejection
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Solution Overview
Problem
Existing bio-impedance measurement systems face challenges in efficiently measuring body impedance while minimizing noise interference from external electrical and magnetic sources, particularly common mode noise from AC power systems, which affects the accuracy of vital sign monitoring.
Innovation Solution
A bio-impedance transducer system that converts voltage signals to current signals using a high common mode rejection input stage, coupled with a continuous time sigma delta analog-to-digital converter (CTΣΔ ADC) to eliminate the need for anti-aliasing filters and improve noise rejection, and incorporates a current-based I-Q mixer for precise impedance measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional voltage measurement methods are used for bio-impedance measurement, then the measurement system is simple, but noise interference from external electrical and magnetic sources degrades measurement accuracy
Solution Approach 1:
The patent replaces conventional voltage-based measurement with a current-based measurement system. By converting the voltage signal to current and measuring current instead, the system achieves high common mode rejection ratio (CMRR) because current measurements are inherently less susceptible to voltage noise and electromagnetic interference from AC power systems.
Solution Approach 2:
The patent changes the measurement parameter from voltage to current. The input stage converts the voltage bio-impedance signal to a current signal, and this current signal is then processed through gain stages and down-conversion. This parameter change fundamentally improves noise rejection while maintaining measurement capability.
2Object-affected harmful factors
If high common mode rejection is achieved through voltage to current conversion, then noise rejection improves, but device complexity increases
Solution Approach 1:
The patent integrates multiple functions into a unified current-based measurement architecture. The same current measurement approach simultaneously achieves high common mode rejection, enables continuous-time sigma-delta ADC operation, and provides robust bio-impedance measurement without requiring separate voltage measurement and filtering stages.
Solution Approach 2:
The patent introduces current as an intermediary between the voltage bio-impedance signal and the digital measurement system. The voltage signal is converted to current at the input stage, and this current signal serves as the intermediary that is processed through analog stages and converted to digital form, simplifying the overall signal path while improving performance.
3Device complexity
If continuous time sigma delta ADC is used to eliminate anti-aliasing filters, then device complexity reduces, but power consumption increases
Solution Approach 1:
The patent changes the input parameter of the ADC from voltage to current. The continuous-time sigma-delta ADC is designed to accept current input directly from the down-converter, eliminating the need for separate voltage buffering and anti-aliasing filtering stages. This parameter change reduces overall system complexity while the current-based operation maintains power efficiency through direct coupling.
Data Source
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AI summary
A method and apparatus are described for bio-impedance measurement using voltage to current conversion. In one example, a bio-impedance transducer includes an input stage to receive a bio-impedance signal having an oscillating voltage from two electrodes, the electrodes being coupled to a body, a resistance across the two electrodes to determine an alternating current of the bio-impedance signal, a gain stage coupled to the resistance to amplify the alternating current, a down converter coupled to the gain stage to convert the amplified alternating current to a direct current bio-impedance signal, and an analog-to-digital converter coupled to the down converter to convert the direct current bio-impedance signal to a digital bio-impedance signal.