Active Pixel Circuit for Time-of-Flight Systems
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Solution Overview
Problem
Conventional Time-of-Flight (ToF) pixel systems face challenges in achieving high dynamic range due to large integration capacitors, which result in low conversion gain, increased noise, and reduced fill factor, leading to poor performance in low-light conditions and high background light scenarios.
Innovation Solution
The proposed pixel circuit operates in both passive and active modes, utilizing an integrator circuit with an integration capacitor and amplifier, where the capacitor is reset and connected in a negative feedback loop, minimizing parasitic capacitance effects and allowing self-biasing with photocurrent to reduce power consumption and avoid saturation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a large integration capacitor is used to handle high background light signals, then the dynamic range is improved, but the conversion gain is significantly reduced and readout noise increases
Solution Approach 1:
The patent segments the integration process into two distinct phases: a first integration phase where both differential ToF signal and common mode background light are integrated, and a second integration phase where only the differential signal is integrated. This segmentation allows the use of a large integration capacitor for handling high background light while maintaining high conversion gain for the differential signal through separate processing.
Solution Approach 2:
The patent discards the common mode background light component during the second integration phase by integrating only the differential signal, while recovering and preserving the differential ToF signal throughout both phases. This selective discarding and recovering enables the system to handle high background light levels without sacrificing differential signal quality.
2Object-affected harmful factors
If a large integration capacitor is used to accommodate maximum photo charge, then background light robustness is improved, but the fill factor is reduced and dark signal sensitivity deteriorates
Solution Approach 1:
The patent segments the integration process into two phases: first integrating both differential signal and common mode background light with a large capacitor for robustness, then separately integrating only the differential signal to maintain high conversion gain and dark signal sensitivity. This segmentation allows the large capacitor to handle background light without degrading dark performance.
Solution Approach 2:
The patent applies different integration strategies to different signal components: the large integration capacitor handles common mode background light during the first phase, while the differential signal receives dedicated high-gain integration during the second phase. This local quality approach ensures each signal component is processed optimally.
3Stability of the object's composition
If the frame is split into several microframes with short integration times, then the dynamic range is improved, but the power consumption and ADC bandwidth requirements increase
Solution Approach 1:
The patent implements periodic action through two sequential integration phases within a single frame: first integrating both differential and common mode signals, then integrating only the differential signal. This periodic two-phase approach achieves high dynamic range for background light without requiring multiple microframes, thereby reducing power consumption and ADC bandwidth requirements compared to conventional microframe approaches.
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 effectively suppresses background light while maintaining high conversion gain for differential signals, reducing power consumption and area, and minimizing signal loss through parasitic capacitance, thereby enhancing the overall performance of the ToF sensor.
Implementation Method 1
an optical sensor (10) arranged for receiving a reference modulation signal (20) and a light signal and arranged for outputting a photocurrent signal depending on said light signal and on a phase shift
Implementation Method 2
an integrator circuit with an integration capacitor (Cint)
Data Source
Figure 1~2
Figure 3
Figure 3A~4
AI summary
The present invention relates to a pixel circuit for performing Time of Flight measurements comprising - at least one optical sensor (10) arranged for receiving a reference modulation signal (20) and a light signal and arranged for outputting a photocurrent signal depending on the light signal and on a phase shift corresponding to a phase difference between the light signal and the reference modulation signal, - an integrator circuit comprising an integration capacitor (Cint), an amplifier (15) and switching means (R0,R1,S0,S1, RS), whereby said switching means are arranged for resetting the integration capacitor in a reset mode, for connecting the integration capacitor between the at least one optical sensor and a voltage reference signal (Vref) in a passive mode, for connecting in an active mode the integration capacitor in a negative feedback loop comprising said amplifier, said negative feedback loop being fed with the photocurrent signal of the at least one optical sensor, and for connecting a signal output by the integrator circuit to an output bus in a readout mode.