Pinned Photodiode PPG Circuit With Transfer Gate DC Removal
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Solution Overview
Problem
Current photoplethysmography (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 due to complex circuitry and high DC component levels.
Innovation Solution
A PPG sensing device utilizing a pinned photodiode (PPD) with a transfer gate transistor and sink transistor configuration, allowing for shorter illumination pulses and eliminating the DC component at the point of readout, thereby reducing power consumption and enhancing signal-to-noise ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If conventional PN or PIN diodes are used in PPG systems, then light detection can be achieved, but power consumption becomes excessively high making them incompatible with portable or wearable devices
Solution Approach 1:
The patent changes the fundamental operating parameters of the photodetector by transitioning from conventional PN or PIN diodes to a pinned photodiode structure with a transfer gate. This structural parameter change enables the system to achieve the same light detection function with dramatically reduced power consumption, making wearable compatibility achievable
2Measurement precision
If conventional photodiodes with complex circuitry are used, then signal processing can be performed, but signal-to-noise ratio deteriorates due to high DC component levels
Solution Approach 1:
The transfer gate mechanism extracts and removes the DC component of the photodetector signal at the point of readout. By separating the DC component removal function from complex downstream circuitry, the system achieves superior signal-to-noise ratio with simplified circuit architecture, as the AC component containing the physiological information is preserved while the large DC offset is eliminated
Solution Approach 2:
The DC component elimination is performed preliminarily at the photodetector readout stage rather than requiring complex processing later in the signal chain. This preliminary action of removing the DC offset at the source simplifies subsequent circuitry and improves signal-to-noise ratio before further processing occurs
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
The solution significantly reduces power consumption and improves signal-to-noise ratio, making PPG systems compatible with wearable devices and enabling reliable, low-power health monitoring.
Implementation Method 1
a sense node (SN), said sense node being a n+-p junction capacitance whose role is to convert the photo-generated electrons in the pinned photodiode (PPD) well into a voltage, by the means of the conversion gain of said capacitance
Implementation Method 2
a pulsed light source, adapted to be pulsed-on or pulsed-off
Implementation Method 3
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
Data Source
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.