Switched Capacitor Ramp Voltage Generation for A/D Converter Stability
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Solution Overview
Problem
Existing A/D converters in solid-state image sensors face variations in resistance and capacitance values due to element variations during manufacturing, leading to fluctuations in the ramp voltage generation, which affect the accuracy of A/D conversion and result in image differences across chips.
Innovation Solution
A constant current source with a switched capacitor is used to generate a stable ramp voltage, where the switched capacitor and capacitive load are manufactured in the same process, minimizing the impact of element variations, and a stabilization capacitor is added to ensure stable current generation even at high sampling clock frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional ramp voltage generation circuit is used, then the A/D conversion can be performed, but the digital values vary due to element variations during manufacturing
Solution Approach 1:
The patent changes the physical state of the resistive load from a fixed resistor to a switched capacitor, which allows the equivalent resistance to be controlled by the ratio of two capacitors. This parameter change eliminates the dependency on precise resistor values and reduces sensitivity to manufacturing variations, thereby improving the reliability and consistency of digital values across different chips.
2Device complexity
If the ramp voltage generation circuit is simplified, then the device complexity is reduced, but the inclined angle becomes sensitive to element variations
Solution Approach 1:
The patent replaces the resistive load with a switched capacitor circuit, changing the parameter that determines the inclined angle from resistor values to capacitor ratios. This allows the inclined angle to be determined by the ratio of two capacitors (C1 and C2) rather than absolute resistor values, significantly reducing sensitivity to manufacturing variations while maintaining circuit simplicity.
Solution Approach 2:
The patent uses a switched capacitor to create an equivalent resistance that copies the functional behavior of a physical resistor but with improved precision. The switched capacitor circuit replicates the resistive function while using capacitor ratios instead of absolute resistance values, thereby achieving better manufacturing precision without increasing overall circuit complexity.
3Stability of the object's composition
If a constant current source is used to generate ramp voltage, then the voltage changes monotonically, but the current fluctuations cause digital value variations
Solution Approach 1:
The patent changes the parameter determining the constant current from a single resistor value to a ratio of two capacitor values (C1/C2). This parameter change makes the current less sensitive to manufacturing variations because capacitor ratios can be controlled more precisely than absolute resistance values, thereby improving digital value consistency while maintaining monotonic voltage change.
4Manufacturing precision
If the ramp voltage generation circuit is made robust against variations, then the manufacturing precision is improved, but the circuit complexity increases
Solution Approach 1:
The patent uses a switched capacitor circuit to replace the resistive load, which improves manufacturing precision by using capacitor ratios instead of absolute resistance values. The switched capacitor approach achieves better tolerance to element variations without significantly increasing circuit complexity, as it uses standard capacitor components and switching transistors that are already present in CMOS technology.
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 stabilizes the ramp voltage generation, reducing variations in A/D conversion results and ensuring consistent digital values, thereby producing accurate and consistent images across different chips.
Implementation Method 1
a resistive load which comprises a switched capacitor
Implementation Method 2
for charging and discharging a capacitive load
Data Source
AI summary
A ramp voltage generation circuit suitable for an A/D converter preventing a variation in a digital value obtained by an A/D conversion operation. The circuit comprises a stabilization voltage source Vref, an operation amplifier AMP1 having a non-inversion input terminal receiving a voltage VREF from the Vref and an inversion input terminal connected to a switched capacitor equivalent resistance Req, and a transistor MNSF for conducting a current Ick to the Req based on an output voltage of the AMP1. Both ends of a conductive load Cint charged and discharged based on a current Iint2 generated by a current mirror of the Ick are connected to an output terminal and an inversion input terminal of an operation amplifier AMPint, a voltage of a stabilization voltage source Vc is applied to a non-inversion input terminal, and an output voltage of the AMPint is outputted to the outside as a ramp voltage.