Photosensor Dark Current Cancellation via Segmented Photodiodes
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Solution Overview
Problem
Existing photosensor devices face challenges in accurately canceling dark current, especially in miniaturized forms and low-luminance environments, due to temperature-dependent dark current characteristics and manufacturing variations, which distort sensing results and complicate calibration.
Innovation Solution
A photosensor device comprising a first photodiode for measuring dark current connected to a switch-cap circuit, a larger photodiode for sensing light also connected to a switch-cap circuit, a programmable gain amplifier, a temperature tracking table unit, and a micro-processing unit that generates control signals to quickly and accurately calibrate dark current based on ambient temperature, allowing for rapid correction of sensing signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If two similar photodiodes are used for dark current and ambient light measurement, then dark current can be measured, but the device becomes difficult to miniaturize
Solution Approach 1:
The patent divides the photodiode into two distinct functional regions: a first photodiode region for dark current measurement and a second photodiode region for ambient light sensing. This segmentation allows each region to be optimized for its specific function while sharing the same semiconductor substrate, enabling miniaturization without sacrificing measurement capability.
Solution Approach 2:
The patent applies different structural characteristics to different regions of the photodiode. The first photodiode region is designed with characteristics optimized for dark current measurement, while the second photodiode region has characteristics optimized for ambient light sensing. This local differentiation allows each region to perform its function effectively within a compact overall structure.
2Measurement precision
If a current mirror circuit is used to mirror dark current, then dark current can be replicated, but manufacturing deviations distort correction accuracy for small signals
Solution Approach 1:
Instead of using a traditional current mirror circuit that relies on precise transistor matching, the patent creates a physical copy of the photodiode structure itself. The first and second photodiode regions are fabricated with identical geometric dimensions and material properties, ensuring that their dark current characteristics are inherently matched without requiring additional circuit components or complex manufacturing processes.
3Area of stationary object
If the photodiode measuring dark current is made small, then device size is reduced, but the measured dark current must be amplified which introduces temperature-dependent gain variations
Solution Approach 1:
The patent implements a feedback mechanism where the dark current measured from the first photodiode region is used to generate a correction signal that is subtracted from the total current of the second photodiode region. This feedback loop continuously compensates for dark current effects without requiring high-gain amplification, thereby avoiding temperature-dependent gain variations while maintaining small photodiode dimensions.
4Measurement precision
If calibration is performed to correct temperature characteristics, then sensing accuracy is improved, but calibration time and complexity increase
Solution Approach 1:
The patent performs dark current calibration in advance by measuring the dark current characteristics of both photodiode regions at different temperatures and storing the results in a lookup table. During actual operation, the system simply queries this pre-computed table based on the current temperature, obtaining correction factors instantaneously without requiring time-consuming real-time calibration procedures.
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
Enables fast and precise dark current calibration within 1-2 milliseconds, improving light sensing quality and maintaining accuracy in low-illuminance conditions by using a smaller photodiode for dark current measurement and a larger one for ambient light sensing, with temperature-dependent gain adjustments.
Implementation Method 1
The photosensor usually uses a photodiode to convert a light signal into an electrical signal
Implementation Method 2
the first switch-cap circuit is used to convert the dark current to a first voltage
Implementation Method 3
a programmable gain amplifier electrically connected to the first switch-cap circuit and the second switch-cap circuit
Data Source
AI summary
The present invention provides a photosensor device, which can cancel a dark current in 1-2 milliseconds. This photosensor device utilizes a small capacitor to quickly accumulate and transform the dark current to a dark-current voltage. Based on the dark-current voltage and an environment temperature, a calibration voltage can be obtained. By cancelling the calibration voltage from the sensed voltage to get a light voltage, which can be amplified to a lux signal. The process is very quick and sensitive, so the photosensor device can be used in an environment under a low luminance.