Double-Ramp ADC Comparator Biasing to Reduce Kickback Noise
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Solution Overview
Problem
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 biasing to ensure operation in the saturation region, reducing kickback noise and increasing the voltage range for more accurate digital representation of image charge signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a comparator is used to convert analog charge signals to digital representations, then the conversion function is achieved, but noise is injected into the output and kickback noise affects the input
Solution Approach 1:
The comparator is divided into multiple independent stages: a first comparator stage performs the initial comparison between the charge signal and reference voltage, while a second comparator stage processes the output of the first stage. This segmentation isolates the noise-generating switching action from the input signal path, reducing kickback noise while maintaining conversion accuracy.
Solution Approach 2:
An intermediate processing stage is introduced between the input charge signal and the final digital output. This intermediate stage includes the first comparator that conditions the signal before it reaches the second comparator, acting as a buffer that prevents direct noise coupling from the output back to the input while preserving the analog-to-digital conversion function.
2Device complexity
If the comparator operates with a limited voltage range, then the circuit design is simpler, but the analog to digital conversion range is restricted
Solution Approach 1:
The reference voltage is made dynamic through a ramp generator that continuously varies the reference voltage over time. This dynamic reference voltage allows the comparator to handle a wider range of input charge signals without requiring multiple comparators or complex switching networks, thereby extending the conversion range while maintaining relatively simple circuit architecture.
Solution Approach 2:
The reference voltage parameter is changed over time using a ramp waveform instead of using a fixed reference voltage. This parameter change enables the single comparator stage to effectively cover a broader input voltage range by comparing against different reference levels at different times, increasing adaptability without proportionally increasing device complexity.
Data Source
AI summary
Apparatuses and method for an image sensor with increased analog to digital conversion range and reduced noise are described herein. An example method may include disabling a first auto-zero switch of a comparator, the first auto-zero switch coupled to auto-zero a reference voltage input of the comparator, adjusting an auto-zero offset voltage of a ramp voltage provided to the reference voltage input of the comparator, and disabling a second auto-zero switch of the comparator, the second auto-zero switch coupled to auto-zero a bitline input of the comparator.