Image Sensor Comparator Circuit for Low-Voltage ADC Linearity
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Solution Overview
Problem
Image sensors face challenges in maintaining linearity of analog-to-digital conversion when the power supply voltage is lowered, leading to potential signal level exceedance and loss of conversion linearity.
Innovation Solution
Incorporating differential pair transistors with load transistors and capacitance units in the comparator circuit, allowing for variable capacitance and isolation circuits to manage signal levels and reduce kickback, thereby maintaining linearity and reducing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the power supply voltage of the image sensor is lowered to achieve lower power consumption, then power consumption is reduced, but the signal level of the pixel signal may exceed the input dynamic range of the comparator, causing loss of linearity in analog-to-digital conversion
Solution Approach 1:
The comparator is divided into two separate amplification units: a first amplification unit that performs initial amplification of the pixel signal, and a second amplification unit that performs further amplification. This segmentation allows the total gain to be distributed across multiple stages, enabling the comparator to maintain adequate signal levels and linearity even when the overall power supply voltage is reduced.
Solution Approach 2:
The patent introduces a time dimension by implementing a two-stage amplification process that occurs sequentially rather than simultaneously. The first amplification unit operates in an initial phase, and the second amplification unit operates subsequently, allowing the system to achieve the required gain through temporal separation of amplification stages.
2Use of energy by moving object
If the power supply voltage is lowered, then power consumption decreases, but the input dynamic range of the comparator is reduced, causing pixel signal levels to exceed the acceptable range
Solution Approach 1:
The amplification function is segmented into two distinct units, each contributing to the overall signal amplification. This allows the comparator to maintain adequate input dynamic range by distributing the gain requirement across multiple stages, rather than requiring a single high-gain stage that would demand higher voltage.
Solution Approach 2:
The first amplification unit performs preliminary amplification of the pixel signal before it enters the second amplification unit. This preliminary action ensures that the signal is already boosted to an appropriate level before the final comparison stage, allowing the comparator to operate effectively even with reduced power supply voltage.
3Measurement precision
If a single-stage high-gain amplifier is used to maintain linearity at low voltage, then linearity is preserved, but the circuit complexity and difficulty of achieving stable operation increase
Solution Approach 1:
Rather than using a single complex high-gain amplifier stage, the patent segments the amplification function into two simpler units. Each unit can be designed with moderate gain requirements, making them easier to stabilize and less prone to oscillation, while collectively achieving the total gain needed for linear operation.
Solution Approach 2:
The patent transforms the problem from a spatial dimension (single high-gain stage) to a temporal dimension (multi-stage sequential amplification). By distributing the amplification across time-separated stages, the system achieves stable operation that would be difficult to obtain in a single simultaneous stage.
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
The solution ensures linearity of analog-to-digital conversion even at lower power supply voltages, reduces power consumption, and minimizes streaking artifacts in image capture.
Implementation Method 1
a capacitance unit connected between a common connection node of the first transistor of the differential pair and the first load transistor, and a node of a predetermined voltage
Data Source
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AI summary
An image sensor including a pixel array section in which a plurality of pixels including photoelectric conversion units is disposed, and a comparator that compares an analog pixel signal output from the pixels to a predetermined reference signal, and outputs a comparison result according to a signal level of the pixel signal. The comparator includes differential pair transistors, a first load transistor connected in series with a first transistor of the differential pair, and a second load transistor connected in series with a second transistor of the differential pair. The first transistor of the differential pair accepts a signal obtained by combining the pixel signal and the predetermined reference signal as a gate input, the second transistor of the differential pair accepts a predetermined voltage as a gate input. In addition, a capacitance unit is connected between a common connection node of the first transistor of the differential pair and the first load transistor, and a node of the predetermined voltage.