Auxiliary Sensor Noise Cancellation in PPG Systems
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Solution Overview
Problem
Plethysmography (PPG) systems face challenges in reducing noise, particularly 1/f noise, which limits the ability to measure low-frequency signals such as heart rate and blood oxygen saturation, especially in battery-powered devices with space constraints, leading to increased power consumption and reduced battery life.
Innovation Solution
Incorporating an auxiliary sensor to directly measure light emitted by LEDs, allowing for noise reduction in PPG signals by comparing measurements from the PPG sensor and the auxiliary sensor, thereby generating signals with lower noise and reducing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If noise reduction techniques are applied to LED drivers and receivers, then measurement precision is improved, but power consumption increases
Solution Approach 1:
An auxiliary photodetector is introduced as an intermediary element to directly measure LED optical output. This auxiliary sensor acts as a mediator between the LED and the main PPG photodetector, providing reference measurements that enable noise cancellation algorithms to remove 1/f noise from the PPG signal without requiring additional power-consuming hardware filtering or averaging circuits
Solution Approach 2:
The system implements feedback by continuously monitoring LED output with the auxiliary photodetector and using this information to compensate for LED-induced noise in the main PPG signal. The processed auxiliary signal feeds back into the noise cancellation algorithm, dynamically adjusting for LED variations and improving measurement precision without increasing power consumption
2Volume of moving object
If small photodiodes are used to collect diffuse light, then device size is reduced, but signal quality deteriorates due to increased noise
Solution Approach 1:
The auxiliary photodetector serves as an intermediary that measures LED output directly, providing reference data for noise cancellation. This allows the main PPG photodetector to be small while still achieving high signal quality, as the noise cancellation process compensates for the reduced light collection capability of the smaller sensor
Solution Approach 2:
The auxiliary photodetector creates a copy of the LED output signal that can be used for noise cancellation. This copied reference signal allows the main PPG measurement to be cleaned of LED-induced noise, enabling the use of smaller photodiodes without sacrificing measurement precision
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 improves signal-to-noise ratio (SNR) by reducing 1/f noise, potentially leading to a 3 dB to 6 dB improvement, resulting in substantial savings in power consumption and more accurate measurements.
Implementation Method 1
Incorporating an auxiliary sensor to directly measure light emitted by LEDs
Implementation Method 2
measuring changes in transmission or diffuse reflectance from the body tissue or subject under active illumination
Implementation Method 3
measuring changes in transmission or diffuse reflectance from the body tissue
Data Source
AI summary
A plethysmography (“PPG”) measurement system may include at least one source of PPG radiation and at least one auxiliary sensor for detection of PPG radiation. The radiation source emits a portion of the PPG radiation toward a subject and another portion along an optical path for direct communication between the PPG radiation source and the auxiliary sensor. The auxiliary sensor may develop a profile against which measurements from primary PPG sensors, which receive light returning from the subject, may be compared. From this comparison, new PPG signals may be generated that exhibit lower noise than the PPG signals output by PPG sensors. These noise mitigation techniques may be used advantageously by a PPG system to generate more accurate measurements and also to reduce power consumption by the radiation sources.