CMOS Image Sensor Test Circuit for Reference Voltage Stabilization
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Solution Overview
Problem
CMOS image sensors face challenges in efficiently reading and resetting pixels due to the slow stabilization time of reference cells, which can lead to a significant slowing of image acquisition rates, especially in applications like motor vehicles where integrity checking is required.
Innovation Solution
The implementation of a test circuit with series-connected elementary reference cells and a control method that stabilizes reference voltages before the actual reading phase, allowing for simultaneous control of pixels and reference cells without interrupting image acquisition.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If reference cells are used for integrity checking during image acquisition, then measurement precision is improved, but productivity deteriorates due to slow stabilization time
Solution Approach 1:
The patent applies preliminary action by stabilizing the reference voltage before the actual reading phase. The control circuit first allows the reference cell to stabilize during an initialization period, then performs the integrity checking and image acquisition. This ensures that when measurements are taken, the reference voltage is already stable, maintaining measurement precision without delaying the actual image capture process.
2Reliability
If integrity testing is performed during image acquisition, then reliability is improved, but loss of time increases due to testing duration
Solution Approach 1:
The patent implements continuity of useful action by enabling simultaneous operation of integrity testing and image acquisition. The control circuit is designed to perform both functions concurrently - while the reference cells are stabilizing and being tested, the image sensors continue to capture images. This eliminates the need to stop image acquisition for testing, maintaining continuous productive operation while ensuring sensor reliability.
3Measurement precision
If reference cells are stabilized before reading phase, then measurement precision is improved, but duration of action increases
Solution Approach 1:
The stabilization of reference cells is performed as a preliminary action during an initialization period before the actual reading and image acquisition phases. This preliminary stabilization ensures that when measurements are eventually taken, the reference voltage is already stable and accurate, without extending the duration of the main productive operations.
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 time dedicated to integrity testing, enabling continuous image acquisition without significant slowing, while maintaining noise-resistant and accurate voltage readings.
Implementation Method 1
Each pixel comprises a photodiode, having its junction capacitance discharged by a photocurrent according to a received light intensity
Data Source
AI summary
A CMOS image sensor including: an array of M×N pixels, the pixels of a same column being connected to a same output track, each pixel including a photodiode, a sense node, a transfer transistor, a reset transistor, and a read circuit; and a test circuit including an assembly of N elementary reference cells respectively connected to the N output tracks of the sensor, each cell including a resistor, a sense node, a transfer transistor, a reset transistor, and a read circuit, the N resistors being series-connected between first and second nodes of application of a reference voltage.