Image Sensor ADC Timing With Reversible Counters and Double Sampling
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Solution Overview
Problem
The conversion speed of analog-to-digital converters in image sensors is a limiting factor in operating efficiency, necessitating an improvement without increasing instantaneous power consumption.
Innovation Solution
An analog-to-digital conversion method that compares ramp signals with analog signals across different conversion periods, utilizing two counters that count in alternating directions to generate comparison signals, thereby improving precision and reducing conversion time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the conversion speed of the analog-to-digital converter is increased, then the operating efficiency of the image sensor is improved, but the conversion precision may deteriorate
Solution Approach 1:
The conversion process is divided into multiple conversion periods, with each period handling a portion of the conversion task. The analog-to-digital converter performs sequential conversions across different periods, allowing the system to maintain higher overall speed while ensuring adequate precision through the structured multi-period approach
Solution Approach 2:
Ramp signals are pre-generated and prepared before the actual conversion measurement begins. The ramp signals are generated in advance during setup phases, allowing the conversion process to proceed more quickly without compromising the precision of the actual measurement phase
2Loss of time
If the conversion period is shortened, then the conversion efficiency is improved, but the counting accuracy may deteriorate
Solution Approach 1:
The first and second counters operate continuously throughout the conversion period without interruption. The counters run in parallel, with the first counter operating during the first conversion period and the second counter operating during the second conversion period, ensuring that the useful counting action continues without gaps, thereby maintaining accuracy despite the shortened overall period
Solution Approach 2:
The conversion process uses periodic ramp signals with specific frequencies and duty cycles. The ramp signals are generated periodically with controlled characteristics, allowing the system to perform multiple conversion cycles in a structured manner that maintains counting accuracy while improving overall conversion efficiency
3Measurement precision
If double sampling is implemented to eliminate systematic offset, then the measurement precision is improved, but the device complexity increases
Solution Approach 1:
The first and second counters are merged into a single counter structure that can operate in different modes. The same physical counter resource is utilized twice - first during the first conversion period, then during the second conversion period - thereby achieving double sampling and systematic offset elimination without requiring two completely separate counter systems
Solution Approach 2:
The analog-to-digital converter is designed with multi-functionality, where the same converter circuitry serves multiple purposes: it performs conversion during the first conversion period, then performs conversion again during the second conversion period with reversed count directions. This universal design eliminates systematic offset while avoiding the complexity of dedicated separate circuits for each function
Data Source
AI summary
An analog-to-digital conversion method, an analog-to-digital converter and an image sensor, are provided. The analog-to-digital conversion method includes a first conversion period and a second conversion period; in the first conversion period and the second conversion period, a first counter and the second counter have different effective clock edges and work in a time-sharing way using the first count clock signal and the second count clock signal respectively; in the second conversion period, count directions of the first counter and the second counter are reversed, and the count results in the first conversion period are used as an initial value of the second conversion period; and the conversion result is output based on the first count result and the second count result.