Dual Feedback PPG Front End for DC Offset Cancellation
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Solution Overview
Problem
Photoplethysmography (PPG) devices face challenges in accurately acquiring cardiac waveforms due to high power consumption, large circuit area, and motion artifacts from wrist-based wearable devices, with significant DC offset issues overwhelming the signal of interest, leading to sub-optimal signal amplification and noise.
Innovation Solution
The implementation of a dual feedback loop system, comprising a linear feedback loop and a sampled feedback loop, to cancel DC offset by generating a feedforward cancellation signal and using a sample and hold circuit to extend the dynamic range, allowing for real-time adaptation and reduced noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If amplification gain is increased to detect the small AC signal from blood vessels, then signal detection capability is improved, but amplifier saturation occurs due to the large DC component
Solution Approach 1:
The patent divides the feedback mechanism into two separate loops: a linear feedback loop for continuous DC offset cancellation and a sampled feedback loop for periodic correction. This segmentation allows each loop to be optimized for its specific function, preventing amplifier saturation while maintaining signal detection capability.
Solution Approach 2:
The patent implements dual feedback loops that continuously monitor and cancel DC offset components. The linear feedback loop provides continuous cancellation, while the sampled feedback loop periodically corrects residual offsets, ensuring the amplifier operates within its linear range and avoiding saturation.
2Reliability
If a low gain, high bandwidth amplifier chain with oversampling ADC is used to avoid clipping, then signal clipping is prevented, but power consumption increases and the approach becomes sub-optimal
Solution Approach 1:
The patent extracts and removes the DC offset component from the signal path using dedicated feedback loops. By separating the DC cancellation function from the main amplification chain, the system can use lower gain amplifiers without risking clipping, significantly reducing power consumption compared to high-gain oversampling approaches.
Solution Approach 2:
The patent dynamically adjusts the operating parameters of the amplifier by continuously canceling DC offsets, allowing the amplifier to operate at lower gain settings. This parameter adjustment enables efficient operation without requiring high-gain, high-power amplifier designs.
3Area of moving object
If device area is reduced for wearable application, then portability is improved, but circuit complexity increases to maintain performance
Solution Approach 1:
The patent merges the DC offset cancellation function with the main amplification circuit by integrating the feedback loops directly into the amplifier structure. This integration eliminates the need for separate DC cancellation circuits, reducing overall device area while maintaining the required functionality.
Solution Approach 2:
The feedback loops serve multiple functions: they cancel DC offsets, prevent amplifier saturation, and enable lower power consumption operation. This multi-functionality reduces the need for additional dedicated circuits, thereby reducing overall device area despite the added complexity of the feedback mechanism.
4Ease of operation
If wrist-based wearable device is used for PPG measurement, then portability and ease of use are improved, but motion artifacts are introduced making signal tracking difficult
Solution Approach 1:
The patent implements dynamic DC offset tracking using feedback loops that continuously adapt to changing conditions. This dynamic adjustment allows the system to maintain accurate signal tracking despite motion-induced variations in DC offset, enabling reliable PPG measurements during wrist movement.
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 results in a more efficient, low-power PPG device with enhanced dynamic range and transimpedance gain, effectively canceling DC offsets and improving the accuracy of heart rate and blood oxygen measurement, while being insensitive to process variations and leakage issues.
Implementation Method 1
The light is generated by a pulsed Light Emitting Diode (LED) which is placed against the skin (often a wrist) and detected by a photodiode also placed against the skin in near vicinity to the LED
Data Source
AI summary
Described is an apparatus which comprises: a current source to generate a current having AC and DC components; a current-to-voltage converter to convert the current or a copy of the current to a voltage proportional to a resistance, the voltage having AC and DC components that correspond to the AC and DC components of the current; a first sample-and-hold circuit to sample and filter the AC component from the voltage and to provide an output voltage with the DC component; a second sample-and-hold circuit to sample the output voltage; a voltage-to-current converter to convert the sampled output voltage to a corresponding current; and an amplifier to receive the output voltage.