CMOS Image Sensor Noise Reduction via Analog CDS
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Solution Overview
Problem
Existing CMOS image sensors face challenges in reducing noise during low illumination without increasing oversampling times, leading to increased chip area and noise multiplication due to gain setting requirements.
Innovation Solution
A CMOS image sensor with a Δ∑ modulator and decimation filter circuit configuration that includes an amplifier at the input stage to adjust the AD input range to a constant level, reducing noise by relaxing the noise specification of the Δ∑ AD converter and allowing gain setting without changing the Δ∑ AD converter's numerical constants.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If gain setting is performed to secure output digital value in low illumination imaging, then the output range is improved, but noise is multiplied by the gain
Solution Approach 1:
The patent applies preliminary action by performing analog CDS (Correlated Double Sampling) before the Δ∑ AD conversion to remove noise components in advance. The CDS circuit subtracts a reference signal from the input signal to eliminate kTC noise and other low-frequency noise, so that subsequent digital processing operates on a cleaner signal, preventing noise multiplication by gain
2Object-generated harmful factors
If capacity value is increased to reduce noise influence, then noise reduction is improved, but chip area increases
Solution Approach 1:
The patent replaces the mechanical approach of increasing capacitor capacity with a circuit-level solution using analog CDS. Instead of enlarging capacitors to reduce noise, the system uses a CDS circuit that actively subtracts noise components from the signal, achieving noise reduction without increasing the physical area of storage elements
3Object-generated harmful factors
If oversampling times are increased to reduce noise, then noise reduction is improved, but processing time and complexity increase
Solution Approach 1:
The patent introduces an intermediary analog CDS circuit between the pixel output and the Δ∑ AD converter. This CDS circuit acts as a noise filter that removes low-frequency noise components before digital processing, enabling effective noise reduction through a single sampling operation rather than requiring multiple oversampling operations
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 configuration achieves high image quality with reduced noise in low illumination conditions without increasing oversampling times, simplifying the circuit and reducing chip area and power consumption.
Implementation Method 1
the pixel including a photodiode converting an optical signal into an electrical signal
Data Source
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AI summary
There is provided a solid-state image sensor including a pixel array unit in which pixels are arrayed, the pixel including a photodiode converting an optical signal into an electrical signal, and a readout unit which reads out an analog image signal from the pixel to a signal line and processes the read out analog pixel signal in a unit of column. The readout unit includes a Δ∑ modulator which has a function to convert the analog pixel signal in to a digital signal, and an amplifier which is arranged on an input side of the Δ∑ modulator and amplifies the analog pixel signal read out to the signal line using a set gain to input the signal to the Δ∑ modulator.