SAR Current Cancellation Circuit for PPG Background Light Interference
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Solution Overview
Problem
Existing heart rate detection systems using photoplethysmograph (PPG) technology face saturation issues due to background light, leading to channel saturation and increased noise, which complicates the detection process and requires additional algorithmic circuits for cancellation, increasing complexity and cost.
Innovation Solution
A current cancellation circuit incorporating a successive approximation analog-to-digital converter (SAR ADC) with a current-voltage conversion circuit and a resistive DAC, which automatically cancels interference currents without additional algorithmic participation, using a SAR logic circuit to control the DAC to approximate the interference current, thereby reducing noise and complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an additional background light cancellation (BGC) circuit is configured to cancel the background light, then the normal operation of the channel is ensured, but the noise of the channel increases and the complexity and cost of the detection circuit increase
Solution Approach 1:
The patent merges the background light cancellation function with the existing SAR ADC circuit by integrating a DAC module and photoelectric conversion module into the ADC. The DAC generates a cancellation current that is subtracted from the photoelectric conversion current within the same circuit, eliminating the need for a separate BGC circuit while maintaining channel operation reliability.
Solution Approach 2:
The SAR ADC circuit is designed to perform multiple functions: photoelectric signal conversion, background light cancellation, and analog-to-digital conversion. The DAC module serves dual purposes by generating both the cancellation current and enabling the successive approximation conversion process, reducing overall circuit complexity while ensuring reliable operation.
2Reliability
If an additional background light cancellation (BGC) circuit is configured to cancel the background light, then the normal operation of the channel is ensured, but the noise of the channel increases
Solution Approach 1:
The circuit uses itself to cancel background light by generating the cancellation current internally through the DAC module rather than relying on an external BGC circuit. The photoelectric conversion module and DAC work together within the same circuit architecture, allowing the system to self-regulate and cancel background interference without introducing additional noise sources.
Solution Approach 2:
The patent replaces the mechanical/electronic BGC circuit with a software-controlled successive approximation algorithm implemented in the SAR ADC. The cancellation current is generated through digital control of the DAC, substituting a complex analog cancellation circuit with a more compact digital control mechanism that reduces noise while ensuring reliable operation.
3Reliability
If an external algorithm circuit is used to cancel the background light, then the background light cancellation is achieved, but the complexity and cost of the detection circuit increase
Solution Approach 1:
The patent combines the background light cancellation algorithm with the existing SAR ADC hardware by integrating a DAC module that generates cancellation currents based on digital control signals. The algorithm is implemented within the existing ADC architecture rather than requiring a separate external circuit, achieving effective cancellation while reducing overall system complexity and cost.
Solution Approach 2:
The DAC module serves as an intermediary between the digital control logic and the analog photoelectric conversion circuit. It translates digital cancellation signals into analog currents that can be subtracted from the photoelectric conversion current, enabling background light cancellation without requiring a complex external algorithm circuit while maintaining simplicity and cost-effectiveness.
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 effectively cancels interference currents, improving the accuracy and simplicity of heart rate detection by reusing existing PPG detection circuit components and reducing channel noise, without the need for external algorithmic circuits.
Implementation Method 1
a photoelectric sensor configured to receive the interference light signal and perform photoelectric conversion on the received interference light signal to obtain an interference current
Data Source
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Figure 3
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AI summary
A current cancellation circuit, a heart rate detection device and a wearable device. The current cancellation circuit includes: a current-voltage conversion circuit and a successive approximation SAR analog-to-digital converter ADC, where the SAR ADC includes a digital-to-analog converter DAC, an SAR logic circuit and a comparator; the current-voltage conversion circuit is configured to receive an analog current output by the DAC and an interference current output by a photoelectric sensor, calculate a difference between the analog current and the interference current, and output an analog voltage, where the interference current is obtained by photoelectric conversion of an interference light signal by the photoelectric sensor; the comparator is configured to receive the analog voltage output by the current-voltage conversion circuit, and output a comparison result according to the analog voltage; and the DAC is configured to output the analog current according to a digital signal corresponding to the comparison result that is output by the SAR logic circuit, and the analog current is used to cancel the interference current output by the photoelectric sensor.