Differential PPG Readout Circuit for DC-Preserving Dynamic Range
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Solution Overview
Problem
Existing PPG signal read-out circuitries face challenges in achieving high dynamic range while maintaining the detection of DC components, which are essential for accurate physiological monitoring, often getting affected by motion artifacts and requiring high power consumption.
Innovation Solution
A read-out circuitry with a photodiode configured for fully differential amplification, incorporating an input stage for current sensing and an output stage with a transimpedance amplifier mode or integration mode, allowing detection of both AC and DC components without interference from DC bias voltage, enabling high dynamic range and fast response time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a DC cancellation circuit is used to eliminate biasing voltages and improve dynamic range, then the dynamic range is improved, but the DC component of the PPG signal cannot be detected
Solution Approach 1:
The read-out circuitry is divided into separate functional stages: an input stage that performs current sensing and differential amplification while preserving DC components, and an output stage that generates the differential output voltage. This segmentation allows the input stage to maintain DC information while the overall system achieves high dynamic range through the differential architecture.
Solution Approach 2:
The patent introduces an intermediate current sensing stage that converts photodiode current to voltage while maintaining DC components. This intermediary stage acts as a bridge between the photodiode and the output stage, allowing DC preservation while enabling the differential amplification needed for high dynamic range performance.
2Stability of the object's composition
If amplifiers are connected to both terminals of the photodiode to improve dynamic range, then the dynamic range is improved, but the circuit complexity increases
Solution Approach 1:
The patent combines multiple functions into integrated circuit stages. The input stage merges current sensing and differential amplification functions, while the output stage combines differential voltage generation and gain control. This merging reduces overall circuit complexity compared to using separate discrete components for each function.
Solution Approach 2:
The differential amplifier stage serves multiple functions simultaneously: it provides impedance matching, performs differential amplification, preserves DC components, and enables high dynamic range operation. This multi-functionality reduces the need for additional dedicated circuit stages, thereby reducing overall complexity.
3Stability of the object's composition
If a high dynamic range read-out circuitry is used to detect motion artifacts with large amplitude, then the dynamic range is improved, but the settling time increases
Solution Approach 1:
The patent employs dynamic gain control in the output stage, where the gain component can be adjusted based on signal conditions. This dynamic adjustment allows the circuit to respond quickly to transient signals while maintaining high dynamic range for larger amplitude variations, thereby reducing settling time without sacrificing dynamic range performance.
Solution Approach 2:
The read-out circuitry is designed to process PPG signals in periodic measurement cycles, with the light source emitting light in periodic pulses. This periodic operation allows the circuit to settle between measurement cycles while maintaining readiness for rapid response to motion artifacts, effectively managing the trade-off between dynamic range and settling time.
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 provides a PPG signal with high dynamic range and short settling time, allowing for accurate detection of both AC and DC components, reducing power consumption, and enabling effective monitoring of heart activity and blood oxygen saturation.
Implementation Method 1
a photodiode, which is configured to detect a PPG signal as intensity of light based on interaction of an emitted light signal with a blood flow of a subject
Data Source
Figure 1~2
Figure 3a~3b
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AI summary
A device for read-out of a photoplethysmography (PPG) signal comprises: a photodiode (106), which is configured to detect a PPG signal, the photodiode (106) comprising a first and a second terminal (114; 116); and a read-out circuitry (110; 210) for reading out the PPG signal, wherein an input stage (112) is connected to receive a first and a second input signal from the terminals (114, 116), and a DC bias voltage, and wherein the input stage (112) is configured for current sensing to provide a fully differential amplification of the input signals to a first and a second current signal, and wherein an output stage (134; 234) is configured to receive the current signals, wherein the current signals comprise an AC and a DC component of the PPG signal, and wherein the output stage (134; 234) is configured to generate a differential output voltage through a gain component (136).