Photoplethysmographic Sensor Ambient Light Rejection Circuit
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Solution Overview
Problem
Existing biometric tracking devices face challenges in accurately measuring heart rate with minimal power consumption and in small form factors, while also effectively rejecting ambient light interference, such as sunlight, which affects signal quality and increases power usage.
Innovation Solution
The solution involves a method and device that capture ambient light signals and adjust output signals to isolate heart rate measurements by operating light emitters at different intensity levels, using a combination of low and high gain amplifiers, and differential oversampling to reject ambient noise, thereby reducing power consumption and improving signal-to-noise ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional heart rate sensors continuously emit light at high intensity to improve signal quality, then measurement precision is improved, but power consumption increases
Solution Approach 1:
The light emitter operates in periodic pulses rather than continuous emission, with multiple pulse sequences at different intensities. The sensor emits light at high intensity during measurement pulses and remains off during ambient light capture intervals, reducing overall power consumption while maintaining measurement accuracy through temporal sampling
Solution Approach 2:
The system dynamically changes the intensity parameter of the light emitter between at least two different intensity levels. By alternating between high and low intensity modes and processing the differential signals, the system achieves accurate heart rate measurements while reducing average power consumption compared to continuous high-intensity operation
2Measurement precision
If ambient light filtering methods are added to reject sunlight interference, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The system uses periodic alternating emission at different intensities to encode the photoplethysmographic signal, enabling ambient light rejection through differential processing without requiring additional hardware filters or complex ambient light sensors
Solution Approach 2:
The system captures ambient light signals separately during intervals when the light emitter is off, creating a copy of the ambient light component. This ambient light copy is then subtracted from the total detected signal to isolate the heart rate component, achieving ambient rejection without additional hardware
3Object-affected harmful factors
If multiple light intensity levels and differential processing are implemented, then ambient light rejection is improved, but device complexity increases
Solution Approach 1:
The system implements periodic pulse sequences at multiple intensity levels with specific timing relationships. By capturing signals during different pulse phases and using differential processing of these temporally-separated measurements, the system rejects ambient light while using minimal processing circuitry
Solution Approach 2:
The light emitter itself serves dual purposes: it provides the measurement signal and simultaneously enables ambient light characterization through its periodic off-periods. The same photodetector that measures heart rate also captures ambient light during emitter off-periods, eliminating the need for separate ambient light sensors
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 optical heart rate measurements with lower power consumption, reduced circuit board space requirements, and enhanced dynamic range, allowing for the use in small form factor devices that are battery-powered and effective in varying light conditions.
Implementation Method 1
These heart rate sensors typically operate by emitting light into the skin of the user and then measuring the light reflected or diffused back after the emitted light interacts with the user's skin
Data Source
AI summary
Some embodiments relate to a device, method, and/or computer-readable medium storing processor-executable process steps to remove a component of a signal corresponding to ambient light in a photoplethysmographic sensor device, including capturing a first detected light signal representing an ambient light at a first time, causing a light emitter to generate a source light signal driven at a first level, capturing a second detected light signal representing the source light signal after interacting with a user's tissue plus the first detected light signal, generating a first output signal based on the second detected light signal adjusted by the first detected light signal, causing the light emitter to generate a source light signal driven at a second level, capturing a third detected light signal representing the source light signal driven at the second level after interacting with the user's skin plus the first detected light signal, and generating a second output signal based on the third detected light signal adjusted by the first detected light signal.