Column ADC Circuit for Image Sensors With Differential X-Droop Suppression
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Solution Overview
Problem
Existing analog-to-digital converter (ADC) arrangements in image sensors face issues with 'x-droop' effects when imaging flat scenes with low contrast, causing ground plane voltage fluctuations and affecting column capacitors, which is not efficiently addressed by widening power tracks due to area inefficiency.
Innovation Solution
Incorporating a differential amplifier as the first stage of the output circuit, along with a correlated double sample circuit and sample capacitors, and using a fixed reference signal sampled during autozeroing to reduce noise and ground bounce, thereby mitigating x-droop effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a large number of inverters fire simultaneously during ADC conversion, then the conversion speed is maintained, but ground plane voltage rises causing x-droop and image distortion
Solution Approach 1:
A differential amplifier is introduced as an intermediary component between the inverter output and the column capacitor. This differential amplifier acts as a buffer that isolates the column capacitor from ground bounce effects while maintaining fast conversion speed. The differential amplifier's high input impedance prevents loading effects, and its differential structure rejects common-mode ground voltage fluctuations, thereby eliminating x-droop without sacrificing productivity
2Object-affected harmful factors
If power tracks are widened to reduce ground bounce, then ground plane stability improves, but device area increases reducing array density
Solution Approach 1:
The differential amplifier serves as an intermediary that electrically isolates the power distribution network from the sensitive capacitor nodes. This allows the use of standard-width power tracks while achieving ground bounce immunity through the differential amplifier's common-mode rejection capability, thus avoiding the area penalty of widened power tracks
Solution Approach 2:
The invention changes the electrical parameters at the capacitor nodes by using a differential amplifier with high input impedance and differential output. This parameter change allows the system to achieve ground stability without increasing physical power track dimensions, as the stability is achieved through electrical isolation rather than increased physical connectivity
3Reliability
If sample capacitors are continuously connected to ramp generator and pixel signal, then noise from switching is reduced, but circuit complexity increases
Solution Approach 1:
The sample capacitors are continuously connected to the ramp generator and pixel signal sources throughout the conversion process. This continuous connection eliminates the need for repeated switching operations, thereby reducing switching noise and maintaining signal integrity. The differential amplifier continuously drives the capacitors, ensuring uninterrupted signal flow and minimizing noise injection from discrete switching events
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively reduces x-droop by minimizing ground bounce and noise artifacts, maintaining simplicity and area efficiency while improving ADC performance in large arrays and high-speed imaging.
Implementation Method 1
the comparator circuit comprises a differential amplifier having one input receiving the combination of the analog pixel signal and the time variant reference signal, and a second input receiving a fixed reference signal
Implementation Method 2
two sample capacitors; and a comparator circuit that compares the time variant reference signal and the analog signal; wherein the analog signal from the pixel and the time variant reference signal are constantly read onto one of the two sample capacitors respectively
Implementation Method 3
an image sensor has a pixel array and comprises: a correlated double sample circuit comprising a first input for receiving an analog signal from a pixel of the pixel array, the signal varying during a pixel readout period
Data Source
AI summary
An image sensor has a per-column ADC arrangement including first and second capacitors for correlated double sampling, and a comparator circuit. The capacitors are continuously connected to, respectively, the analog pixel signal and a ramp signal without use of a hold operation. The comparator circuit comprises a differential amplifier having one input connected to the junction of the two capacitors and another input connected to a reference signal. The reference signal is preferably sampled and held from a reference voltage. The use of a differential amplifier as first stage of the comparator addresses problems arising from ground voltage bounce when a large pixel array images a scene with low contrast.