Image Sensor Column ADC With Differential Amplifier Against X-Droop
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Solution Overview
Problem
Existing ADC arrangements in image sensors face issues with 'x-droop' noise, particularly in large arrays or high-speed applications with low contrast scenes, where simultaneous inverter firing causes ground plane voltage rise, affecting column capacitors and resulting in image distortion, which is inefficiently addressed by widening power tracks at the cost of area efficiency.
Innovation Solution
Incorporating a differential amplifier as the first stage of the output circuit, with a long tail pair configuration, and continuously connecting the time variant reference signal to one sample capacitor and the analogue signal to another, while using a fixed reference signal sampled during autozeroing to reduce noise and ground bounce effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a large number of inverters fire simultaneously in low contrast scenes, then the ground plane voltage rises causing x-droop noise, but widening power tracks to counteract this effect is not area efficient in larger arrays
Solution Approach 1:
The patent extracts the problematic inverter circuit from the signal path and replaces it with a differential amplifier. This removes the source of ground plane interference while maintaining the necessary signal processing function, thereby eliminating x-droop noise without requiring wider power tracks
Solution Approach 2:
The patent introduces a differential amplifier as an intermediary component between the sample capacitors and the output. This mediator circuit provides a low-impedance drive that prevents ground plane voltage variations from affecting the column capacitors, thereby eliminating x-droop without increasing power track area
2Reliability
If sample capacitor size is reduced to minimize kT/C noise, then noise is reduced, but this limits the design when combined with hold operations
Solution Approach 1:
The patent performs correlated double sampling without requiring a hold operation. By continuously connecting the sample capacitors to the input signals and using a differential amplifier to subtract the reset signal from the pixel signal, the system achieves noise reduction with smaller capacitors and reduced design constraints
3Device complexity
If the inverter circuit is used for analog-to-digital conversion, then the circuit is simple, but ground bounce affects column capacitors differently across the array causing x-droop
Solution Approach 1:
The patent removes the inverter circuit from the signal path and replaces it with a differential amplifier. This extraction eliminates the ground bounce issue inherent to inverter circuits while maintaining circuit simplicity through the use of a standard differential amplifier configuration
Solution Approach 2:
The patent uses a differential amplifier that processes signals in a differential manner, effectively copying and subtracting the ground bounce effect from both signal paths. This cancellation approach maintains image uniformity without requiring complex circuitry
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
This configuration effectively reduces the impact of ground bounce and 'x-droop' noise, maintaining area efficiency and performance by minimizing the size of sample capacitors and eliminating noise from the reference voltage, thus improving image quality in high-speed and large array applications.
Implementation Method 1
a differential amplifier having one input receiving a combination of the analogue signal and the time variant reference signal, and another input receiving a fixed reference signal
Implementation Method 2
The use of a differential amplifier as first stage of the output circuit reduces the effect of ground bounce and hence reduces x-droop
Implementation Method 3
first and second sample capacitors; the time variant reference signal is constantly and continuously connected to the first sample capacitor and that the analogue signal is constantly and continuously connected to the second sample capacitor
Data Source
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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.