Image Sensor Double Integration Time Conditional Sampling
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing image sensors face challenges in accurately determining the need for a third sampling step due to uncertainties in threshold values caused by manufacturing variations, temperature changes, and offset voltages, leading to reduced dynamic range and increased current consumption.
Innovation Solution
The use of a ramp analog-digital converter with a differential amplifier to test the differential voltage between samples after storage, applying a short linear voltage ramp to determine if a third sampling is necessary, and performing the final conversion regardless of the third sampling's occurrence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a comparator is used to test the potential level for determining saturation risk, then the saturation detection capability is improved, but the device complexity increases due to requiring a bulky comparator for each column
Solution Approach 1:
The patent combines the saturation detection function with the existing analog-to-digital converter by using its differential amplifier and ramp generator. Instead of adding separate comparators for each column, the solution merges the detection functionality into the shared ADC resources, allowing one ADC to serve multiple columns while maintaining saturation detection capability.
Solution Approach 2:
The analog-to-digital converter is designed to perform multiple functions: it serves as both the primary conversion device for pixel data and as a saturation detection mechanism. The differential amplifier within the ADC is utilized to compare the sampled potential against reference levels, enabling the same hardware to handle both conversion and saturation assessment tasks.
2Measurement precision
If the column conductor is powered continuously to enable testing and sampling, then the measurement accuracy is improved, but the current consumption increases
Solution Approach 1:
The column conductor is powered periodically rather than continuously. The power is activated only during specific phases when sampling occurs, allowing both the test sampling and the final sampling to be performed with controlled power bursts. This periodic activation ensures measurement accuracy is maintained during critical moments while minimizing overall current consumption during idle periods.
3Reliability
If a third sampling is always performed to ensure accurate saturation detection, then the measurement reliability is improved, but the productivity decreases due to additional sampling steps
Solution Approach 1:
Instead of always performing the third sampling step, the system performs it only partially - specifically, only when saturation is detected during the initial sampling phase. The decision to perform the third sampling is conditional, based on the results of the first sampling. This approach ensures measurement reliability when needed (when saturation risk is present) while maintaining high productivity when saturation is not an issue, avoiding unnecessary additional sampling steps.
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 reduces uncertainties in threshold determination, minimizes current consumption, and maintains dynamic range by allowing the third sampling only when necessary, thereby enhancing the sensor's ability to measure a wider range of illuminations efficiently.
Implementation Method 1
each pixel comprising a photodiode connected by a transfer transistor to a storage node
Implementation Method 2
means for carrying out a first sampling, in a first capacitor sampling of the reading circuit, of the level of potential taken by the column conductor
Implementation Method 3
the read circuit comprises a ramp analog-digital converter comprising a differential amplifier connected to the two capacitors and means for applying a linear voltage ramp to one of the capacitors
Data Source
Figure 1
Figure 2
Figure 3
AI summary
The invention relates to image sensors, and more particularly to those designed to capture images at both low and high illumination levels. The sensor operates with a double integration time (Ti1, Ti2) for each frame. Two successive charge transfers (TRa, TRb) are performed from a photodiode to a storage node, the first after a first duration (Ti1), the second after a second duration (Ti2) different from the first. The potential of the storage node after the first charge transfer is sampled (shs1) in a first capacitor of the readout circuit. The potential after a reset of the storage node is sampled (shr) in a second capacitor. A potential level taken after the second transfer is then conditionally resampled (shs2) in the first capacitor. The resampling condition is a signal level condition after the first transfer.This level is applied to the differential amplifier of a ramp converter for a short ramp duration (RMP1). Depending on the state of the amplifier output at the end of the provisional ramp, a decision is made whether or not to resample (shs2), and a full analog-to-digital conversion is then performed with a final ramp (RMP2) of the differential signal level sampled in the capacitors.