Dual Conversion Gain Readout for Wide Dynamic Range Image Sensors
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Solution Overview
Problem
Conventional image sensors face challenges in achieving a wide enough voltage range for analog-to-digital conversion, leading to incorrect digital representation of image charge, especially when the generated charge exceeds the voltage range of the ADC circuits.
Innovation Solution
The implementation of dual conversion gain (DCG) pixels with a new readout scheme that includes high conversion gain (HCG) and low conversion gain (LCG) signals, combined to achieve high dynamic range (HDR), using small-sized comparators and a specific readout sequence that includes HCG reset followed by HCG and LCG readouts, and LCG reset, along with a ramp circuit to auto-zero and normalize input voltages for accurate ADC operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional ADC circuits are used with fixed voltage range, then device complexity is reduced, but measurement precision deteriorates when generated charge exceeds the voltage range
Solution Approach 1:
The patent implements dynamic adjustment of conversion gain in the ADC circuit by switching between first and second conversion gain modes. The circuit automatically selects appropriate gain levels based on the magnitude of the charge signal, enabling accurate conversion across a wide dynamic range without requiring multiple fixed-range ADC circuits. This dynamic adaptation resolves the contradiction by making the voltage range flexible rather than fixed.
Solution Approach 2:
The patent changes the conversion gain parameter of the ADC circuit adaptively. By adjusting the conversion gain between two distinct levels, the system can accurately represent both small and large charge values within the same ADC hardware, eliminating the need for multiple specialized circuits while maintaining measurement precision across different signal conditions.
2Measurement precision
If multiple ADC circuits with different voltage ranges are used, then measurement precision is improved, but device complexity and area increase
Solution Approach 1:
The patent makes a single ADC circuit universal by enabling it to operate in multiple conversion gain modes. The same physical circuit can accurately convert both small charges (using first conversion gain) and large charges (using second conversion gain), eliminating the need for separate ADC circuits for different charge ranges. This multi-functionality reduces the total sensor area while maintaining measurement precision.
Solution Approach 2:
By dynamically switching the conversion gain parameter, a single ADC circuit replaces what would traditionally require multiple static circuits. The dynamic reconfiguration of the same hardware resources provides the functionality of multiple circuits without the area overhead, resolving the contradiction between measurement precision and sensor area.
3Measurement precision
If conversion gain is increased to expand voltage range, then measurement precision for large charges is improved, but noise increases and measurement precision for small charges deteriorates
Solution Approach 1:
The system dynamically adjusts conversion gain based on charge magnitude. For small charges, low conversion gain is used to minimize noise and maintain precision. For large charges, high conversion gain is activated to expand the voltage range and maintain accuracy. This dynamic adaptation allows the system to optimize the signal-to-noise ratio for each measurement condition, resolving the contradiction between handling large and small charges.
Solution Approach 2:
The conversion gain parameter is changed adaptively based on the charge signal magnitude. By switching between two conversion gain values, the system optimizes the output voltage range for different input conditions while maintaining an acceptable noise floor. This parameter adaptation resolves the contradiction by matching the conversion gain to the signal level.
4Measurement precision
If dual conversion gain readout is implemented, then measurement precision and dynamic range are improved, but device complexity increases
Solution Approach 1:
The patent implements dynamic conversion gain switching within a unified readout architecture. The readout circuit automatically selects the appropriate conversion gain mode based on the charge signal characteristics, providing dual conversion gain functionality without requiring completely separate readout paths. This dynamic approach achieves high measurement precision while keeping the readout circuit complexity manageable through intelligent resource sharing.
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 allows for a wider dynamic range and accurate digital representation of image charges by effectively increasing the voltage range of the comparator, reducing noise and errors, and maintaining the comparator within operational limits, thus enhancing image sensor performance.
Implementation Method 1
The image sensor includes a plurality of photosensitive elements such that each photosensitive element absorbs a portion of incident image light. Photosensitive elements included in the image sensor, such as photodiodes, each generate image charge upon absorption of the image light.
Data Source
AI summary
A method includes reading a first analog reference signal from a first storage node in a dual conversion gain pixel, and converting the first analog reference signal to a first digital reference signal using a comparator coupled to the dual conversion gain pixel. The method also includes reading a first analog image signal from the first storage node, and converting the first analog image signal to a first digital image signal using the comparator. A second analog image signal may be read from the first storage node and a second storage node in the dual conversion gain pixel, and the second analog image signal may be converted to a second digital image signal. A second analog reference signal may be read from the first storage node and the second storage node, and the second analog reference signal may be converted to a second digital reference signal using the comparator.