CMOS Image Sensor Reset Voltage Control for Noise Reduction
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Solution Overview
Problem
CMOS image sensors face challenges with reset noise, image lag, and fixed pattern noise due to unstable reset operations and varying detection circuit characteristics, which affect the signal-to-noise ratio.
Innovation Solution
A CMOS image sensor design incorporating a photodiode, reset switch, amplifier, selection switch, current source, comparator, and memory, where the current source decreases the photodiode node voltage, and a comparator controls the reset voltage based on a reference voltage to suppress noise, and a digital-to-analog converter converts digital values to analog signals for noise reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a reset switch is used to remove photo-induced charge from the photodiode, then the photodiode can be reset for the next detection cycle, but reset noise is generated due to unstable reset voltage and incomplete charge removal
Solution Approach 1:
The patent implements feedback control by comparing the pixel output voltage with a reference voltage through a comparator, and adjusting the reset voltage accordingly. The feedback control unit modifies the reset voltage based on the comparison result to ensure complete charge removal and stabilize the reset level, thereby reducing reset noise while maintaining fast detection cycling.
Solution Approach 2:
The patent dynamically adjusts the reset voltage parameter based on the pixel output voltage level. By changing the reset voltage parameter adaptively through the feedback control unit, the system ensures optimal charge removal at each reset operation, reducing reset noise without sacrificing detection speed.
2Adaptability or versatility
If different detection circuits are used in each pixel, then each pixel can independently detect light, but fixed pattern noise occurs due to different threshold voltages of amplifier transistors
Solution Approach 1:
The patent uses feedback control to adjust the reset voltage for each pixel based on its output voltage characteristics. This per-pixel feedback compensation corrects for variations in amplifier transistor threshold voltages, reducing fixed pattern noise while preserving independent detection capability of each pixel.
Solution Approach 2:
The system dynamically changes the reset voltage parameter for each pixel based on its specific characteristics. By adapting the reset voltage to each pixel's unique properties, the patent compensates for manufacturing variations in amplifier transistors, thereby reducing fixed pattern noise.
3Measurement precision
If photo-induced charge is accumulated for a certain time to increase detection signal voltage, then light sensitivity is improved, but image lag occurs when residual charge from previous images remains
Solution Approach 1:
The feedback control unit continuously monitors the pixel output voltage and adjusts the reset voltage accordingly. This ensures that all residual charge is completely removed during reset operations, preventing image lag while preserving the charge accumulation process needed for high light sensitivity.
Solution Approach 2:
The patent performs a preliminary reset operation before charge accumulation begins, ensuring the photodiode starts from a clean state with no residual charge. This preliminary action prevents image lag from previous frames while allowing full charge accumulation for the current frame, maintaining high light sensitivity.
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 design effectively reduces reset noise, image lag, and fixed pattern noise, enhancing the signal-to-noise ratio by stabilizing the reset voltage and compensating for detection circuit differences without requiring additional hardware like pin diodes or high driving voltages.
Implementation Method 1
A CMOS image sensor uses PN-junction photodiodes, which can be formed by a CMOS manufacturing process, to convert the intensity of incident light to an electrical signal.
Implementation Method 2
a comparator for comparing an output voltage of the amplifier with a reference voltage for controlling the current source to reset the photodiode
Data Source
AI summary
Disclosed is a CMOS image sensor that controls a reset voltage to reduce reset noise caused by a reset operation of the CMOS image sensor, fixed pattern noise caused by different characteristics of detection circuits, and image lag caused by the influence of a previous image signal upon the current output signal, thereby achieving a high signal to noise ratio.