Delta-Sigma Light-to-Digital Conversion With Unbiased Photodiodes
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Solution Overview
Problem
High-performance analog front-ends in PPG sensor systems require significant power and area, and are vulnerable to noise coupling due to the need for bias voltage in photodiodes, which complicates the design and scaling of system-on-chip devices.
Innovation Solution
A light-to-digital converter that eliminates the need for bias voltage by using unbiased photodiodes and delta-sigma conversion, with multiple units arranged in an array to reduce noise and power consumption, directly outputting digital signals for simplified configuration and noise resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a pre-amplifier IC is used to amplify small current output from the photodiode, then the signal can be amplified, but the power consumption and area increase significantly
Solution Approach 1:
The patent extracts and eliminates the pre-amplifier IC from the system by using an unbiased photodiode configuration that directly outputs detectable signals without requiring external amplification circuitry, thereby reducing power consumption and simplifying the overall system configuration
Solution Approach 2:
The photodiode system serves itself by generating sufficient output signals intrinsically without requiring external bias voltage or pre-amplification stages, making the system self-sufficient and reducing the need for additional power-consuming components
2Reliability
If bias voltage is applied to the photodiode, then the photodiode can operate, but noise coupling increases and design becomes more complex
Solution Approach 1:
The patent inverts the conventional approach by using an unbiased photodiode instead of a biased one, eliminating the need for bias voltage circuits and thereby reducing noise coupling and design complexity while maintaining operational reliability
Solution Approach 2:
The patent changes the operating parameter of the photodiode from biased to unbiased state, which fundamentally alters the noise characteristics and simplifies the design by removing bias voltage requirements while maintaining detection capability
3Ease of operation
If analog signals are transmitted over long distances (7-10 cm), then the photodiode can be positioned optimally, but noise coupling becomes very vulnerable
Solution Approach 1:
The patent replaces the mechanical/electrical analog signal transmission system with a digital signal processing approach, where light signals are directly converted to digital data through delta-sigma modulation, eliminating the vulnerability of long-distance analog transmission while preserving positioning flexibility
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 low-power, low-noise PPG sensor system with reduced noise coupling and simplified design, achieving six times less power consumption and twenty-five times less noise compared to traditional systems.
Implementation Method 1
a photodiode (PD) 101 converting an optical signal into a current signal
Implementation Method 2
performing delta-sigma conversion on a photodiode voltage created from an unbiased photodiode
Data Source
AI summary
A light-to-digital converter of the present invention includes: a plurality of light-to-digital conversion units each creating a first clock signal by performing delta-sigma conversion on a photodiode voltage created from an unbiased photodiode, and outputting a unit output signal obtained by synchronizing the first clock signal with a second clock signal that is a reference clock signal; and an adder outputting a digital output signal by adding up the unit output signals of the plurality of light-to-digital conversion units.