CMOS Image Sensor ADC Threshold Switching for Lower Reference Voltage
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Solution Overview
Problem
In CMOS image sensors, the successive approximation type AD converters face challenges with redundant voltage ranges, leading to increased power consumption and circuit area due to constant offset voltage variations with analog gain, requiring high reference voltages and inefficient power management.
Innovation Solution
A solid state imaging element with a converter that successively converts analog pixel signals into bit values based on threshold voltages set according to conversion history, using multiple voltage generation units to generate and select reference voltages, allowing dynamic adjustment of threshold voltages for efficient AD conversion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the number of bits in AD conversion is increased to handle high analog gain cases, then the redundant range is reduced, but the reference voltage must be increased which makes it difficult to reduce power consumption
Solution Approach 1:
The patent applies dynamics by making the number of effective bits in the AD converter variable rather than fixed. The control unit dynamically adjusts the number of effective bits based on the analog gain setting - using fewer bits when analog gain is low and more bits when analog gain is high. This dynamic adaptation allows the system to maintain appropriate measurement precision for each operating condition without always requiring the high reference voltage needed for maximum bit depth, thereby reducing power consumption.
Solution Approach 2:
The patent changes the parameter of bit depth in the AD converter based on operating conditions. By varying the number of effective bits according to the analog gain level, the system optimizes the balance between measurement precision and power consumption. When analog gain is low, fewer bits are used reducing the required reference voltage and power consumption; when analog gain is high, more bits are used to maintain precision.
2Device complexity
If a fixed number of bits is used in the AD converter, then the circuit design is simplified, but unnecessary redundant range is included when analog gain is low requiring high reference voltage
Solution Approach 1:
The patent introduces dynamic control of the bit depth parameter based on analog gain settings. The control unit monitors the analog gain level and adjusts the number of effective bits accordingly, creating a adaptive system that simplifies design constraints while optimizing power consumption. This dynamic approach eliminates the need to design for the worst-case scenario (maximum bits always active), allowing lower reference voltage and power consumption in low-gain conditions.
3Reliability
If the conversion range of the AD converter is increased to cope with characteristic variations, then reliability is improved, but the redundant range increases requiring high reference voltage
Solution Approach 1:
The patent applies local quality by providing different conversion ranges to different operational modes. Instead of uniformly increasing the conversion range for all cases, the system locally adapts the bit depth to match the specific analog gain condition. This allows the conversion range to be appropriately sized for each operating mode, maintaining reliability where needed while avoiding excessive power consumption in modes that require smaller ranges.
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 approach reduces the reference voltage requirement, improves power efficiency, and maintains high resolution while minimizing redundant voltage ranges, thereby optimizing power consumption and circuit performance.
Implementation Method 1
light entering from a subject is photoelectrically converted by a photoelectric conversion element provided in each pixel
Data Source
AI summary
A solid state imaging element according to an embodiment includes: a converter (14) that converts an analog pixel signal read out from a pixel into a bit value, successively for each of a plurality of bits, on the basis of a threshold voltage set according to a conversion history of the bit converted before a target bit; a plurality of voltage generation units (102a and 102b) that each generate a plurality of reference voltages; and a setting unit (12d) that sets the threshold voltage using the reference voltage selected from the reference voltages generated by each of the voltage generation units on the basis of a conversion result.