PPG Analog Frontend Circuit for Ambient Light Rejection
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Solution Overview
Problem
Existing photoplethysmography (PPG) systems struggle to accurately filter out ambient light components from the current signal, which interferes with the analysis of blood-related AC and DC components, leading to inaccurate heart rate and oxygen saturation measurements.
Innovation Solution
An analog frontend circuit with a first current control circuit, monitoring circuit, and reading circuit that tracks and maintains a sub-current to filter out ambient light components, allowing for precise conversion of the filtered signal into an output voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If ambient light components are not filtered out, then the PPG measurement is simpler to obtain, but the accuracy of heart rate and blood oxygen saturation measurement deteriorates
Solution Approach 1:
The patent segments the current signal into multiple components using parallel current control circuits. Each circuit (first, second, third current control circuits) extracts specific signal components (first sub-current, second sub-current, third sub-current) corresponding to different ambient light conditions and physiological signals. This segmentation allows selective filtering of ambient light components while preserving physiological information.
Solution Approach 2:
The patent introduces intermediate processing circuits between the optical sensor and final measurement. The monitoring circuit and multiple current control circuits act as intermediaries that process the raw current signal, separating ambient light components from physiological signals through controlled current paths and sub-current extraction before the final measurement stage.
2Measurement precision
If complex filtering methods are used to remove ambient light, then measurement accuracy improves, but the device complexity and processing time increase
Solution Approach 1:
The patent performs preliminary signal component separation through multiple parallel current control circuits that simultaneously extract different sub-currents representing various signal components. By preparing these separated components in advance through the monitoring circuit and parallel processing paths, the system avoids time-consuming sequential filtering operations during the measurement phase.
Solution Approach 2:
The patent employs periodic switching between different current control circuits and signal paths. The monitoring circuit and switches alternately activate different current control circuits (first, second, third) to process different signal components in periodic cycles, enabling efficient time-multiplexed filtering that reduces overall processing time while maintaining accuracy.
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
Effectively filters out ambient light components, enabling accurate measurement of heart rate and blood oxygen saturation by isolating the relevant AC and DC components of the current signal.
Implementation Method 1
the light reflect by blood and human tissue of the user is measured and transformed to a current signal by an optical sensor
Data Source
AI summary
An analog frontend circuit for photoplethysmography includes a current control circuit and a monitoring circuit both coupled with a first node, and also includes a reading circuit coupled with the first node through the monitoring circuit. The current control circuit generates a first sub-current which is part of an input current flowing to the first node. In a tracking period, the monitoring circuit controls the current control circuit to determine magnitude of the first sub-current to control a voltage of the first node to track a reference voltage. In a sensing period after the tracking period, the monitoring circuit controls the current control circuit to keep the first sub-current determined in the tracking period. After the first sub-current is kept, the monitoring circuit provides a second sub-current which is part of the input current, and the reading circuit converts the second sub-current to an output voltage.