Differential PPG Readout Circuit for AC/DC Signal Detection
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Solution Overview
Problem
Existing photoplethysmography (PPG) signal read-out circuitry struggles to maintain a high dynamic range while detecting both AC and DC components, as DC cancellation circuits prevent the detection of DC components, which are crucial for assessing physical conditions.
Innovation Solution
A read-out circuitry with a photodiode configured for fully differential amplification, using current sensing and a DC bias voltage to separate and detect AC and DC components without interference, allowing for a high dynamic range and fast response time, enabling the detection of both components in PPG signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a DC cancellation circuit is used to eliminate biasing voltages and improve dynamic range, then the dynamic range is improved, but the DC components of the PPG signal cannot be detected
Solution Approach 1:
The patent segments the signal processing into two independent paths: one path handles AC component detection with DC cancellation, while the other path preserves DC components for separate processing. This segmentation allows both functions to operate simultaneously without interference, resolving the contradiction between dynamic range improvement and DC component preservation.
Solution Approach 2:
The patent introduces an intermediary circuit that separates the AC and DC components before further processing. This intermediary structure allows the DC cancellation circuit to operate on AC signals while a separate path maintains DC information, thus resolving the conflict between improving dynamic range through DC cancellation and preserving DC component detection capability.
2Measurement precision
If amplifiers connected to both terminals of photodiode are used, then dynamic range is improved, but DC cancellation circuit prevents detection of DC components
Solution Approach 1:
The patent divides the signal processing into separate AC and DC paths after the photodiode amplification stage. The AC path uses DC cancellation for improved dynamic range, while the DC path preserves DC components for reliable detection. This segmentation resolves the contradiction between dynamic range improvement and DC detection reliability.
Solution Approach 2:
Instead of using a single processing path that either cancels DC or preserves it, the patent inverts the conventional approach by creating separate processing paths: one that actively cancels DC for AC signal optimization, and another that deliberately preserves DC components. This inverted dual-path architecture resolves the contradiction between dynamic range and DC detection reliability.
3Measurement precision
If long LED pulses are used to ensure sufficient light for detection, then detection sensitivity is improved, but power consumption increases
Solution Approach 1:
The patent employs periodic pulsed LED operation with optimized duty cycles rather than continuous illumination. By synchronizing the pulsed LED operation with the read-out circuitry's integration periods and utilizing the fast response of the photodiode-amplifier system, sufficient detection sensitivity is achieved during active pulses while power consumption is reduced during off-periods.
Solution Approach 2:
The patent ensures continuous useful action by maintaining the photodiode in a reverse-biased ready state with low noise, allowing immediate response to short LED pulses. The read-out circuitry continuously monitors and integrates signals, ensuring that no detection capability is lost during LED off-periods, thus maintaining sensitivity while reducing power consumption through pulsed operation.
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 provides a high dynamic range and fast response time, allowing for accurate detection of both AC and DC components in PPG signals, even in the presence of motion artifacts, and reduces power consumption by enabling the use of short LED pulses, making it suitable for wearable sensors.
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
AI summary
A device for read-out of a photoplethysmography (PPG) signal comprises: a photodiode, which is configured to detect a PPG signal, the photodiode comprising a first and a second terminal; and a read-out circuitry for reading out the PPG signal, wherein an input stage is connected to receive a first and a second input signal from the terminals and a DC bias voltage, and wherein the input stage 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 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 is configured to generate a differential output voltage through a gain component.