Double-Ramp ADC Comparator With Auto-Zero Kickback Reduction
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Solution Overview
Problem
Conventional image sensors face challenges in achieving increased analog to digital conversion range while minimizing noise, particularly due to noise injection from comparators used in the conversion process.
Innovation Solution
The implementation of a comparator design with cascode devices and an auto-zero switch scheme that adjusts the biasing and enables/disables cascode devices based on analog gain, allowing for increased voltage range and reduced noise by ensuring operation in the saturation region, thereby minimizing kickback and noise propagation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional comparator is used for analog to digital conversion, then the conversion process can be completed, but noise is injected into the output and errors are introduced
Solution Approach 1:
The comparator employs dynamic cascode devices that can be enabled or disabled based on operating conditions. During the integration phase, cascode devices are disabled to minimize noise and kickback. During the comparison phase, they are enabled to ensure proper operation and reduce output errors, thus dynamically adapting to different operational requirements
Solution Approach 2:
The auto-zero switch scheme changes the biasing parameters of the comparator inputs by periodically adjusting the reference voltage level. This parameter change allows the comparator to compensate for offset errors and noise by comparing against a dynamically adjusted reference, thereby improving measurement precision while managing noise injection
2Adaptability or versatility
If the voltage range is increased to handle larger image charge voltage signal values, then the analog to digital conversion range is enhanced, but noise and kickback effects are amplified
Solution Approach 1:
The cascode devices are dynamically controlled based on the operating phase. During integration, they remain disabled to minimize noise and kickback. During comparison, they are activated to provide proper voltage level handling and reduce output errors. This dynamic control allows the system to adapt to different voltage ranges while minimizing harmful effects
Solution Approach 2:
The auto-zero switch acts as an intermediary element that mediates between the high voltage range requirements and noise reduction needs. By introducing this intermediate control mechanism, the system can adjust biasing conditions and reference levels to handle larger voltage signals while compensating for noise and kickback effects
3Measurement precision
If cascode devices are continuously enabled to ensure saturation region operation, then output errors are reduced, but noise kickback is increased
Solution Approach 1:
The cascode devices are periodically enabled and disabled according to the conversion phases. They are disabled during integration to minimize kickback noise, then enabled during the comparison phase to ensure saturation region operation and reduce output errors. This periodic action pattern optimizes both precision and noise performance
Solution Approach 2:
The system dynamically adjusts the state of cascode devices based on real-time operational requirements. The auto-zero control signal dynamically switches the cascode devices between enabled and disabled states, allowing the system to optimize for low noise during integration and for high precision during comparison, thus resolving the contradiction between noise reduction and error minimization
Data Source
AI summary
A comparator includes a first stage coupled to compare a reference voltage to an image charge voltage signal. The first stage includes first and second NMOS input transistors coupled between an enabling transistor and respective first and second cascode devices to receive the reference voltage and the image charge voltage signal. A first auto-zero switch is between a gate of the first NMOS input transistor and a first node. The first node is between the first NMOS input transistor and the first cascode device. A second auto-zero switch is between a gate of the second NMOS input transistor and a second node. The second node is between the second cascode device and a second PMOS transistor. A voltage difference between the first and second nodes during an auto-zero period reduces an amount of kickback that occurs during an ADC period.