Pinned Photodiode PPG Readout for Low-Power Wearable Sensing
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current PPG systems based on conventional PN or PIN diodes face challenges with high power consumption, making them incompatible with portable or wearable devices, and struggle with signal-to-noise ratio and dynamic range, which affects the accuracy and reliability of health monitoring.
Innovation Solution
The use of pinned photodiodes allows for shorter illumination pulses and eliminates the DC component of the PPG signal at the point of readout, significantly reducing power consumption and improving the signal-to-noise ratio without the need for complex signal processing, enabling more efficient and accurate health monitoring in wearable devices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional PN or PIN diodes are used in PPG systems, then the system can detect light signals, but the power consumption becomes excessively high making it incompatible with wearable devices
Solution Approach 1:
The patent changes the fundamental parameter of the photodetector device from conventional PN or PIN diodes to pinned photodiodes integrated in CMOS technology. This parameter change enables the system to achieve the same light detection function with dramatically reduced power consumption, making wearable PPG devices feasible.
Solution Approach 2:
The patent replaces the conventional separate LED and photodetector configuration with an integrated CMOS-based pinned photodiode system. This substitution eliminates the need for complex external circuitry and high-power components, achieving both detection capability and low power consumption through integrated circuit technology.
2Measurement precision
If standard PPG systems use broadband photodiodes with complex circuitry, then they can process signals, but the device complexity increases and battery lifetime decreases
Solution Approach 1:
The patent merges the photodetector function with CMOS circuit technology by using pinned photodiodes that can be directly integrated into standard CMOS processes. This combining of optical detection and electronic processing in a single integrated structure simplifies the overall device architecture while maintaining signal processing precision.
Solution Approach 2:
The pinned photodiode structure serves multiple functions: it acts as both the light-sensitive element and the signal generation source, eliminating the need for separate broadband photodiodes and complex external processing circuitry. This multi-functionality reduces device complexity while preserving measurement precision.
3Quantity of substance
If longer illumination pulses are used to compensate for tissue absorption, then enough photons reach the detector, but the LED power consumption increases significantly
Solution Approach 1:
The patent extracts the DC component of the photodetector output signal, which contains the majority of the power information. By removing this DC portion, the system can operate with shorter illumination pulses since the AC coupling naturally blocks the DC component and only processes the pulsatile signal, thereby reducing LED power consumption while maintaining sufficient photon detection.
Solution Approach 2:
The patent employs periodic AC coupling to selectively pass only the time-varying pulsatile blood flow signal while blocking the static DC component. This periodic action allows the use of shorter illumination pulses because the system is designed to detect only the AC variations caused by blood flow, not the total light intensity.
4Loss of information
If the DC portion of the PPG signal is not eliminated at the photodetector output, then the full signal is preserved, but the dynamic range constraints and noise issues worsen
Solution Approach 1:
The patent extracts and removes the DC component from the photodetector output signal through AC coupling. This extraction eliminates the large DC offset that would otherwise dominate the signal dynamic range, thereby improving the signal-to-noise ratio for the smaller AC pulsatile components without losing the essential blood flow information.
Solution Approach 2:
The patent applies preliminary AC coupling to preemptively block the DC component before it can degrade the signal quality. By anticipating and preventing the DC offset problem at the source, the system maintains optimal dynamic range and signal-to-noise ratio for subsequent processing of the pulsatile blood flow signal.
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 solution results in a massive reduction of power consumption, making PPG systems suitable for wearable devices while enhancing the signal-to-noise ratio and reducing dynamic range constraints, leading to more reliable and precise health monitoring.
Implementation Method 1
Once at the detector the light is converted into a photo-generated current
Implementation Method 2
Taking advantage of the different absorption properties of the molecules circulating in the blood and particularly of the oxygenated haemoglobin (HbO2), and the deoxygenated haemoglobin (Hb)
Data Source
Figure 1~2A
Figure 2B~3
Figure 4A~4B
AI summary
The invention relates to a photoplethysmography (PPG) sensing device comprising - a pulsed light source, - at least one pixel to create photo-generated electrons, synchronized with said pulsed light source. It is mainly characterized in that each pixel comprises: - a pinned photodiode (PPD) having two electronic connection nodes, - a sense node (SN), to convert the photo-generated electrons into a voltage, and - a Transfer Gate (TGtransfer) transistor, having its source electronically connected to one electronic connection node of said pinned photodiode (PPD), and being configured to act as a transfer gate (TG) between said pinned photodiode (PPD) and said sense node (SN), allowing the photo-generated electrons to sink when the light is pulsed-off, the photo-generated electrons integration when the light is pulsed-on and the transfer of at least part of the integrated photo-generated electrons to said sense node for a read-out.