SAR ADC Current Cancellation for PPG Background Light Saturation

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Solution Overview

Problem

Existing PPG detection circuits face saturation issues due to background light, leading to failed detections and increased noise and complexity when using additional background light cancellation circuits.

Innovation Solution

A current cancellation circuit incorporating a successive approximation SAR ADC, current-voltage conversion circuit, and digital-to-analog converter (DAC) that automatically adjusts an analog current to cancel interference currents without requiring an external algorithm circuit, simplifying the circuit structure and reducing costs.

Engineering Contradictions & Design Principles

VSEngineering 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

Engineering Contradiction:

Improvenormal operation of the channelVSAvoidnoise of the channel

Core Design Contradiction:

ReliabilityVSObject-generated harmful factors

Solution Approach 1:

The patent combines the background light cancellation function with the existing SAR ADC circuit by integrating a DAC module within the ADC. This merging allows the cancellation circuit to share the same structural framework and signal processing path, ensuring reliable background light cancellation while minimizing additional noise through unified circuit design and shared components.

Inventive Principle:

Principle #5Merging (Combining)

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 complexity and cost of the detection circuit increase

Engineering Contradiction:

Improvenormal operation of the channelVSAvoidcomplexity and cost of the detection circuit

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent merges the BGC function with the SAR ADC circuit by integrating a DAC module within the existing ADC structure. This integration eliminates the need for separate external algorithm circuits, reducing overall circuit complexity and cost while maintaining reliable background light cancellation capability.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The SAR ADC circuit is designed to perform multiple functions: both analog-to-digital conversion and background light cancellation. The integrated DAC module within the ADC enables the circuit to generate cancellation currents while maintaining its primary conversion function, achieving multi-functionality without increasing device count or complexity.

Inventive Principle:

Principle #6Universality (Multi-functionality)

3Measurement precision

If a photoelectric sensor detects an intensity of reflected light, then heart rate detection is enabled, but the channel becomes saturated due to too much background light

Engineering Contradiction:

Improveheart rate detectionVSAvoidbackground light saturation

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The patent applies preliminary anti-action by generating a cancellation current that opposes the background light current before the saturation occurs. The integrated DAC module produces an analog current that is subtracted from the photoelectric sensor output, preemptively counteracting the harmful effect of background light and preventing channel saturation while preserving heart rate detection accuracy.

Inventive Principle:

Principle #9Preliminary anti-action

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 reliability of heart rate detection while reducing circuit complexity and noise, thereby enhancing the overall performance and cost-effectiveness of PPG detection systems.

Implementation Method 1

the interference current is obtained by photoelectric conversion of an interference light signal by the photoelectric sensor

Methodology Applied

Scientific EffectPhotoelectric conversion: Photoelectric Effect

Data Source

PatentUS11350836B2Current cancellation circuit, heart rate detection device and wearable device

Publication Date: 2022.06.07 SHENZHEN GOODIX TECH CO LTD

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 SAR ADC, where the SAR ADC includes a 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; 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.