Logarithmic Oximetry Front End for Offset-Independent Tissue Sensing
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Solution Overview
Problem
Current oximetry measurement technologies are inadequate in providing satisfactory results due to limitations in analyzing optical properties of tissue.
Innovation Solution
A regional oximetry system is developed using a pair of logarithmic current-to-voltage amplifiers coupled to a difference amplifier, with two pairs of light emitters and photodetectors to calculate the difference in optical absorbance along multiple paths, mitigating temperature variations and offset issues through matched transistors and ambient current removal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional oximetry systems are used to measure optical properties of tissue, then basic oximetry measurement is achieved, but measurement precision and reliability are inadequate
Solution Approach 1:
The system divides the measurement into multiple optical paths (first and second paths) with different tissue penetration depths. By segmenting the measurement into shallow and deep path components, the system can calculate regional oximetry that is independent of ambient light and offset errors, thereby improving both precision and reliability
Solution Approach 2:
The patent introduces logarithmic current-to-voltage amplifiers as intermediary components that convert photodetector currents to voltages with logarithmic scaling. This intermediary transformation linearizes the relationship between optical absorbance and measured signal, improving measurement precision while the differential measurement approach eliminates reliability issues from offsets and ambient light
2Measurement precision
If multiple optical paths are used to improve measurement accuracy, then regional oximetry precision is improved, but device complexity increases
Solution Approach 1:
The patent combines multiple photodetector signals through logarithmic amplifiers and differential amplification into a single regional oximetry measurement. By merging the first and second path measurements in the logarithmic domain and subtracting them differentially, the system achieves high precision regional oximetry while keeping the circuit architecture relatively compact and manageable
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 enables accurate measurement of regional oximetry by providing a tissue characteristic, such as a hemoglobin index, independent of amplifier offsets, effectively addressing the limitations of existing oximetry systems.
Implementation Method 1
a photodetector can provide an electric current that is proportional to the light intensity
Implementation Method 2
A difference between a first and a second output signal of the pair of logarithmic current-to-voltage amplifiers can be provided to a non-inverting input terminal of a difference amplifier
Data Source
AI summary
An apparatus includes a first amplifier having a first input coupled to a first optical detector. The first amplifier includes a first output corresponding to a logarithm of the first input. The apparatus includes a second amplifier having a second input coupled to a second optical detector and having a second output corresponding to a logarithm of the second input. The apparatus includes a differential amplifier configured to amplify a difference between the first output and the second output.