Multi-Ramp ADC Switching for Faster Image Sensor Conversion
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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 inefficiencies in ramp ADCs, which become slow and power-inefficient at higher resolutions.
Innovation Solution
The proposed solution involves an analog-to-digital converter that utilizes multiple ramp signals with different slopes, allowing for simultaneous comparison and dynamic switching between these ramps during the conversion period, enabling efficient control of a counter stage based on handover points, thereby optimizing conversion speed and reducing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the clock speed is increased to reduce quantisation error, then the conversion speed is improved, but the power consumption increases
Solution Approach 1:
The patent applies dynamics by making the ramp slope variable rather than fixed. The ramp generator dynamically adjusts the ramp slope based on the input signal amplitude, using a steeper ramp for larger signals and a shallower ramp for smaller signals. This dynamic adaptation allows the converter to achieve high conversion speed for large signals while maintaining low power consumption for small signals, resolving the contradiction between speed and power consumption.
Solution Approach 2:
The patent changes the parameter of ramp slope dynamically during operation. By varying the ramp slope parameter according to signal conditions (through the selection of different ramp generators or adjustable slope control), the system optimizes the trade-off between conversion speed and power consumption for different input signal levels.
2Measurement precision
If the bit resolution is increased to reduce quantisation error, then the measurement precision is improved, but the conversion time increases
Solution Approach 1:
The patent uses dynamic ramp slope adjustment to resolve the contradiction between resolution and conversion time. For small input signals, a shallower ramp is used which allows more clock cycles to count, achieving higher effective resolution without increasing the maximum conversion time. For large signals, a steeper ramp is used to maintain fast conversion. This dynamic approach allows high resolution to be achieved adaptively without proportionally increasing conversion time for all signal levels.
3Device complexity
If a single ramp with fixed slope is used, then the device complexity is reduced, but the adaptability to different signal levels is limited
Solution Approach 1:
The patent segments the single ramp function into multiple ramp generators, each producing a ramp with a different fixed slope. Instead of one complex adjustable ramp, the system uses multiple simpler ramp generators operating in parallel or sequentially. This segmentation maintains relatively simple individual circuit structures while providing adaptability through selection among the segmented ramp options.
Solution Approach 2:
The patent achieves multi-functionality by having multiple ramp generators serve different signal level ranges. Each ramp generator is specialized for a particular slope, but collectively they provide universal coverage for adapting to various input signal amplitudes. The system can universally handle different signal levels by selecting the appropriate ramp generator, making the ADC adaptable without requiring complex adjustment mechanisms in each individual ramp circuit.
Data Source
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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.