Dual-Ramp ADC Comparator Layout for Image Sensor Linearity
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Solution Overview
Problem
Existing dual ramp analog-to-digital converters for image sensors suffer from column-to-column fixed pattern noise and nonlinearity due to signal-dependent input offset voltage in the comparator, leading to suboptimal performance.
Innovation Solution
Applying the analog signal and coarse ramp to the same input of a comparator, with a fine ramp applied to the opposite input, and using a second comparator to compensate for switch feedthrough, thereby reducing nonlinear input offset errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the analog signal and coarse ramp are applied to different inputs of the comparator, then the conversion process is simpler, but signal-dependent input offset voltage causes column-to-column fixed pattern noise and nonlinearity
Solution Approach 1:
The patent inverts the conventional approach by applying the analog signal and coarse ramp to the same comparator input, rather than different inputs. This inversion eliminates the signal-dependent input offset voltage that causes nonlinearity, as the offset becomes independent of the input signal amplitude. The fine ramp is applied to the opposite input to complete the dual-ramp comparison process.
2Device complexity
If a single comparator is used for dual ramp conversion, then the device complexity is reduced, but switch feedthrough effects increase nonlinear input offset errors
Solution Approach 1:
The patent segments the comparison function into two separate comparators: a first comparator that handles the coarse ramp comparison with the analog signal, and a second comparator that handles the fine ramp comparison. This segmentation isolates the switch feedthrough effects to specific stages, allowing each comparator to operate with reduced nonlinear offset errors while maintaining overall system simplicity.
Data Source
AI summary
Dual ramp analog-to-digital converters and methods allow for performing analog-to-digital conversion of an analog signal. Various dual ramp analog-to-digital converters and methods allow for applying the analog signal and a coarse ramp to a same input of a comparator, and applying a fine ramp to another input of the comparator. Some dual ramp analog-to-digital converters and methods allow for applying the analog signal, a coarse ramp, and a fine ramp to a same input of a comparator. Various dual ramp analog-to-digital converters and methods allow for applying the analog signal to an input of a first comparator, applying a coarse ramp to the input of the first comparator through a coarse ramp switch, applying the analog signal to an input of a second comparator, and applying a fine ramp to another input of the second comparator.