Ramp-Slope ADC for CMOS Image Sensor Gain Without PGAs
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Solution Overview
Problem
The existing CMOS image sensors face challenges in reducing chip area while maintaining performance due to the need for programmable gain amplifiers (PGAs) and accurate analog-to-digital conversion, which increases complexity and design difficulty, particularly in compact devices like smartphones.
Innovation Solution
An optical sensing system with a ramp signal generator, digital logic circuit, and column circuits that generate and process photosensitive electric signals without PGAs, using a ramp signal to control signal amplification and ensure accurate analog-to-digital conversion by coupling photosensitive electric signals with ramp signals to reduce common-mode range dependency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If PGAs are arranged to perform signal gain processing on each pixel unit, then signal amplification is achieved, but circuit complexity and chip area increase
Solution Approach 1:
The patent extracts the signal amplification function from the traditional PGA circuit and implements it through a ramp signal-based analog-to-digital conversion mechanism. The ramp signal's slope is used to represent the amplification factor, eliminating the need for separate PGA circuits at each pixel unit while maintaining signal gain capability.
Solution Approach 2:
The patent replaces the mechanical/electrical PGA amplification system with an optical-ramp signal-based system. Instead of using programmable gain amplifiers to amplify signals, the system uses a ramp signal with variable slopes to achieve signal gain during the analog-to-digital conversion process, substituting a complex electrical amplification mechanism with a simpler time-varying signal approach.
2Reliability
If PGAs are arranged to perform signal gain processing on each pixel unit, then signal amplification is achieved, but chip area increases
Solution Approach 1:
The patent extracts the signal amplification function from the traditional PGA circuit and implements it through a ramp signal-based analog-to-digital conversion mechanism. The ramp signal's slope is used to represent the amplification factor, eliminating the need for separate PGA circuits at each pixel unit while maintaining signal gain capability.
Solution Approach 2:
The ramp signal generator serves multiple functions: it provides the timing reference for analog-to-digital conversion, encodes the signal amplification factor through its slope, and enables gain processing without requiring separate PGA circuits. This multi-functional approach reduces the overall chip area by consolidating what would otherwise require multiple dedicated components.
3Productivity
If a comparator is used for analog-to-digital conversion with varying analog signal voltages, then conversion is achieved, but the input common-mode range requirement increases design difficulty
Solution Approach 1:
The patent changes the operating parameters of the comparator by introducing a ramp signal with a specific slope. The ramp signal's voltage changes over time in a controlled manner, allowing the comparator to operate with a moving target rather than a static voltage level. This parameter change (from static to dynamic voltage reference) simplifies the common-mode range requirements while maintaining conversion accuracy.
Solution Approach 2:
The patent introduces dynamics into the analog-to-digital conversion process by using a time-varying ramp signal instead of a static reference voltage. The ramp signal's slope represents the amplification factor, and its continuous change over time allows the comparator to handle varying analog signal voltages without requiring a wide fixed common-mode range, thereby simplifying the comparator design.
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 simplifies the chip structure, reduces area overhead, and ensures accurate analog-to-digital conversion, broadening application scenarios and ensuring effective image processing without the need for additional PGA circuits.
Implementation Method 1
photoelectric conversion is performed in an exposure stage by a photodiode to generate signal charges
Data Source
AI summary
The present disclosure relates to an optical sensing system and an analog-to-digital converter. The optical sensing system includes a ramp signal generator, a digital logic circuit and a plurality of column circuits. The ramp signal generator is configured to generate a ramp signal and transmit the ramp signal to the column circuits. The ramp signal includes a signal of which a signal intensity varies with time, and a signal intensity variation rate of the ramp signal corresponds to an amplification factor of a photosensitive electric signal. Each column circuit is configured to receive photosensitive electric signal, and output a photosensitive gain signal to the digital logic circuit according to ramp signal and photosensitive electric signal. Photosensitive gain signal includes a digital signal corresponding to photosensitive electric signal of which signal intensity is amplified. The digital logic circuit is configured to calculate a digital sampling signal according to photosensitive gain signals.