Image Sensor ADC Auto-Zero Segmentation for Resolution and Power
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Solution Overview
Problem
Analog-to-digital converters (ADCs) used in image sensors face challenges in reducing power consumption while maintaining high resolution and efficiency in signal processing.
Innovation Solution
The proposed solution involves an analog-to-digital converting circuit that utilizes auto-zero period optimization. This circuit includes a first amplifier that equalizes voltage levels and compares ramp and reset signals, and a second amplifier that charges a capacitor during an auto-zero period and generates output signals based on the first amplifier's output. The auto-zero periods are optimized to reduce power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the ADC processes many signals within the same time or provides improved resolution for each signal, then the performance and resolution of signal processing are improved, but the power consumption of the ADC increases
Solution Approach 1:
The ADC operation is divided into distinct time segments: auto-zero periods and signal conversion periods. During auto-zero periods, only the first amplifier operates to calibrate offset errors, while the second amplifier is turned off. During signal conversion periods, both amplifiers operate as needed. This temporal segmentation allows high-resolution conversion when required while reducing power consumption during calibration phases.
Solution Approach 2:
The ADC employs periodic auto-zero periods interspersed between signal conversion operations. These periodic calibration intervals equalize voltage levels in the first amplifier to eliminate offset errors, improving measurement precision. By confining these power-intensive calibration operations to specific periodic intervals rather than continuous operation, the overall power consumption is reduced while maintaining high resolution during active conversion periods.
2Measurement precision
If auto-zero periods are extended to improve offset error correction, then measurement precision is improved, but the conversion speed and productivity are reduced
Solution Approach 1:
Offset error correction is performed in advance during dedicated auto-zero periods before signal conversion begins. The first amplifier equalizes voltage levels at its input and output nodes to pre-correct offset errors. This preliminary calibration ensures high measurement precision during subsequent conversion operations without extending the actual signal conversion time, thereby maintaining productivity.
Solution Approach 2:
The ADC dynamically adjusts its operation mode by switching between auto-zero periods and signal conversion periods. During auto-zero periods, the system focuses on precision calibration with the first amplifier active. During conversion periods, both amplifiers operate to maintain high conversion speed. This dynamic temporal allocation allows the system to optimize for precision when needed and for speed when needed, resolving the contradiction between offset correction and conversion speed.
Data Source
AI summary
A circuit includes a first amplifier that first compares a ramp signal and a reset signal of a pixel signal output from a pixel array in a first operation period, second compares the ramp signal and an image signal of the pixel signal in a second operation period, and generates a first output signal in the first and second operation periods based on first and second comparison results; and a second amplifier that charges a capacitor in response to a second auto-zero signal in a second auto-zero period, stops an operation of the second amplifier from a time point at which the second auto-zero period ends to a time point at which the first operation period starts, and generates a second output signal based on the first output signal in the first operation period and the second operation period.