TOF Image Sensor ADC Sharing for Differential Pixel Conversion
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Solution Overview
Problem
The indirect Time-of-Flight (TOF) method for image sensors requires two analog-to-digital converters, increasing the area burden and complexity, as it measures distance by detecting phase differences between modulated light signals from two pixels with different activation times.
Innovation Solution
An image sensor design that includes a comparison operation using an operation amplifier to generate a digital code from the voltage difference between pixel signals of two tabs, reducing the need for multiple analog-to-digital converters by performing auto-zeroing and analog-to-digital conversion operations with a single counter circuit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If two separate analog-to-digital converters are used to convert pixel signals from two tabs in the indirect TOF method, then the distance measurement can be performed by detecting phase difference, but the area burden and device complexity increase
Solution Approach 1:
The patent merges the functionality of two separate analog-to-digital converters into a single ADC by using an operation amplifier to compare pixel signals from two tabs and generate a differential signal. This single ADC then converts the voltage difference directly, eliminating the need for two separate converters and reducing the overall circuit area while maintaining distance measurement precision.
Solution Approach 2:
The operation amplifier is designed to perform multiple functions: it compares pixel signals from two different tabs, generates a differential voltage signal, and prepares the signal for conversion by the single ADC. This multi-functional approach allows one component to replace what would traditionally require two separate converters, reducing area burden.
2Measurement precision
If two separate analog-to-digital converters are used to convert pixel signals from two tabs, then the phase difference detection can be performed, but the device complexity increases
Solution Approach 1:
The patent combines two separate converter circuits into one by using the operation amplifier to create a differential signaling path. The amplifier compares the pixel signals and generates a single differential output that feeds into one ADC, simplifying the overall device architecture while preserving the ability to detect phase differences for accurate distance measurement.
3Area of stationary object
If the voltage difference between pixel signals is analog-to-digital converted using a single converter, then the area is reduced, but the conversion process requires auto-zeroing operation
Solution Approach 1:
The operation amplifier performs an auto-zeroing operation before the actual analog-to-digital conversion to eliminate offset errors and ensure accurate differential measurement. This preliminary calibration step prepares the converter for precise operation, enabling area reduction through single-ADC architecture while maintaining measurement accuracy through proper error compensation.
Data Source
AI summary
An image sensor includes: a pixel of a first tab; a pixel of a second tab; an operation amplifier suitable for comparing, in a comparison operation, a pixel signal of the first tab and a pixel signal of the second tab with each other to produce a comparison result, receiving, in an auto-zeroing operation, a ramp voltage and a selected pixel signal which is selected based on the comparison result between the pixel signal of the first tab and the pixel signal of the second tab, and receiving, in an analog-to-digital conversion operation, the ramp voltage and an unselected pixel signal which is not selected from the pixel signal of the first tab and the second pixel signal of the second tab; and a counter circuit suitable for generating a digital code based on an output of the operation amplifier.