Ramp Signal Generator Offset Compensation for CMOS Image Sensors
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Solution Overview
Problem
The variability of common-mode voltage in Correlated Double Sampling (CDS) operations of CMOS image sensors due to changes in ramp signals generated by current digital-analog converters (IDACs) degrades image quality, particularly when the ramp signal changes based on gain, affecting the stability of CDS operations.
Innovation Solution
A ramp signal generator that includes a reference current generation unit, a ramp signal generation unit, a replica current supply unit, and an offset compensation unit, using current mirror paths to generate a constant output voltage or current by supplying a replica current, thereby maintaining a stable output voltage even at full code conditions without increasing circuit area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a ramp signal is changed according to the reference current based on a gain, then the ramp signal generation is flexible and adaptive, but the common-mode voltage of CDS operation changes, degrading image quality
Solution Approach 1:
The patent uses a current mirror to create a replica of the reference current path. This copy allows the ramp signal to be adjusted according to gain while maintaining a stable common-mode voltage through offset compensation, resolving the contradiction between adaptability and reliability
Solution Approach 2:
The offset compensation unit acts as an intermediary between the ramp signal generation and the CDS operation. It compensates for voltage changes by injecting a compensating current, thereby maintaining stable common-mode voltage while allowing ramp signal flexibility
2Reliability
If the common-mode voltage of CDS operation is maintained constant, then image quality is preserved, but the circuit complexity increases due to additional compensation components
Solution Approach 1:
The patent merges the offset compensation function with the existing ramp signal generation circuitry by using the same current mirror structure. This integration minimizes additional circuit complexity while maintaining constant common-mode voltage for image quality preservation
3Area of stationary object
If a current digital-analog converter is used for ramp signal generation, then the circuit area is reduced, but the output voltage varies with gain changes, affecting CDS stability
Solution Approach 1:
The patent implements a feedback mechanism where the output of the current digital-analog converter is monitored and compensated through the offset compensation unit. This feedback loop maintains stable common-mode voltage despite gain variations, ensuring CDS stability while keeping the circuit area small
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 solution ensures a constant output voltage or current is maintained, improving the stability and image quality of CMOS image sensors by compensating for offset variations, thus enhancing the constancy of analog circuit characteristics and preventing degradation in image quality.
Implementation Method 1
the ramp signal generator comprises a plurality of current mirror paths for supplying currents to be used to generate the ramp signal
Implementation Method 2
an offset compensation unit suitable for compensating for an offset of the ramp signal generated by the ramp signal generation unit using the replica current
Data Source
AI summary
A ramp signal generator may include a reference current generation unit suitable for generating a reference current based on a gain; a ramp signal generation unit suitable for generating a ramp signal according to the reference current; a replica current supply unit suitable for supplying a replica current using the reference current generation unit; and an offset compensation unit suitable for compensating for an offset of the ramp signal generated by the ramp signal generation unit using the replica current.