Image Sensor ADC Handover Between Ramp Slopes for Faster Conversion
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current analog-to-digital converters (ADCs) in image sensors face challenges with increased conversion time and power consumption as bit resolution increases, particularly due to the limitations of ramp ADCs in achieving high-speed and low-power performance.
Innovation Solution
The proposed solution involves an analog-to-digital converter that utilizes multiple ramp signals with different slopes, allowing for dynamic switching between these ramps during the conversion process, with a control stage managing the counter stage based on comparison outputs to optimize the conversion period and reduce power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the clock speed is increased to reduce quantisation error, then the Least Significant Bit (LSB) value is reduced, but the power consumption increases and the conversion time increases
Solution Approach 1:
The conversion process is segmented into multiple phases: a first conversion phase using a first ramp signal with a first slope, and a second conversion phase using a second ramp signal with a second slope. This segmentation allows the system to use different quantisation steps for different signal ranges, reducing the need for continuously high clock speeds while maintaining precision.
Solution Approach 2:
The system dynamically switches between different ramp signals based on the signal level. A control circuit determines when to switch from the first ramp signal to the second ramp signal based on comparison outputs. This dynamic adaptation allows the converter to optimize between precision and power consumption by using appropriate quantisation steps for different input signal conditions.
2Measurement precision
If the clock speed is increased to reduce quantisation error, then the Least Significant Bit (LSB) value is reduced, but the conversion time increases
Solution Approach 1:
The conversion process is divided into multiple phases with different ramp signals. The first conversion phase uses a first ramp signal for initial conversion, and the second conversion phase uses a second ramp signal for refined conversion. This segmentation enables the system to achieve high precision without requiring a single high-speed clock throughout the entire conversion process, thereby reducing total conversion time.
Solution Approach 2:
The system employs periodic switching between different ramp signals based on comparison outputs. The control circuit enables switching between the first and second ramp signals at specific periods during the conversion process, allowing the system to achieve high precision conversion within a reduced time frame by utilizing multiple quantisation steps periodically.
3Measurement precision
If the steepness of the ramp is reduced to reduce quantisation error, then the Least Significant Bit (LSB) value is reduced, but the input range of the ADC is reduced
Solution Approach 1:
The input range is segmented into multiple segments, each handled by a different ramp signal. The first ramp signal with a first slope handles one segment of the input range, while the second ramp signal with a second slope handles another segment. This segmentation allows the system to maintain a wide overall input range while using reduced steepness (lower slope) in specific segments to achieve lower quantisation error for those ranges.
Solution Approach 2:
Different parts of the input range are assigned different ramp signals with different slopes optimized for their specific ranges. The control circuit enables selective use of the first or second ramp signal based on the input signal level. This local optimization allows each segment to have the appropriate steepness for its required precision, maintaining overall input range versatility while reducing quantisation error in specific regions.
4Productivity
If multiple ramp signals with different slopes are used, then conversion speed and power efficiency are improved, but the device complexity increases
Solution Approach 1:
The converter is designed to perform multiple functions using a unified architecture. The same converter circuitry processes signals from multiple pixel columns, and the control circuit manages switching between different ramp signals based on comparison outputs. This multi-functionality allows the system to achieve high conversion speed and power efficiency without proportionally increasing device complexity, as the same hardware serves multiple purposes.
Solution Approach 2:
The control circuit automatically determines when to switch between ramp signals based on comparison outputs from the converter itself. The system uses its own internal signals (comparison outputs) to control the switching between different ramp signals, eliminating the need for external control logic. This self-service mechanism reduces the overall device complexity while enabling the benefits of multiple ramp signals.
Data Source
AI summary
An analog-to-digital converter (110) for an imaging device comprises an analog signal input (123) for receiving an analog signal from a pixel array of the imaging device and N ramp signal inputs (121, 122) for receiving N ramp signals, where N is an integer ≥2. The N ramp signals have different slopes. The ADC has a clock input (143) for receiving at least one clock signal. A comparison stage (120) is connected to the ramp signal inputs and to the analog signal input. The comparison stage (120) is configured to compare the ramp signals with the analog signal to provide comparison outputs during the conversion period. A control stage (130) is configured to control a counter stage (140) based on the comparison outputs and a selection input indicative of when at least one handover point has been reached during the conversion period.