SPAD Pixel Current Switching for Accurate Distance Imaging
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Solution Overview
Problem
Existing SPAD-based imaging systems face accuracy issues due to after-pulse generation and latching current phenomena, leading to prolonged dead time and noise resistance drops during distance measurement, with existing solutions failing to adequately address these problems.
Innovation Solution
The implementation of a pixel array section with an SPAD, a resistance component, an output section, and a pulse generation section, where a switch or pull-in section is used to suppress the input current through the SPAD, preventing prolonged dead time and improving noise resistance by synchronizing the pulse signal with the light reception signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If a capacitance is used to apply reverse voltage to suppress after-pulse generation, then after-pulse generation is suppressed, but noise resistance drops during detection period
Solution Approach 1:
A switch is introduced as an intermediary component between the capacitance and the SPAD. The switch controls the connection state, allowing the system to apply reverse voltage during refresh period (suppressing after-pulses) while disconnecting during detection period (maintaining noise resistance), thus resolving the contradiction between suppressing after-pulses and maintaining noise resistance
Solution Approach 2:
The system alternates between refresh period and detection period in a periodic manner. During refresh period, the switch connects the capacitance to apply reverse voltage and suppress after-pulses. During detection period, the switch disconnects to maintain noise resistance. This periodic switching resolves the contradiction by applying different voltage states at different time intervals
2Object-generated harmful factors
If a delay circuit is used to start refresh period after capacitance voltage drops, then after-pulse generation is suppressed, but dead time varies significantly
Solution Approach 1:
The system uses feedback control where the switch is controlled based on the state of the SPAD and capacitance. The control signal adjusts the switch timing to ensure the refresh period starts at the optimal moment, suppressing after-pulses while maintaining consistent dead time. This feedback mechanism resolves the contradiction between suppressing after-pulses and maintaining stable dead time
3Object-generated harmful factors
If quenching resistance is used to limit current, then avalanche multiplication is controlled, but dead time is prolonged due to latching current
Solution Approach 1:
The switch is activated in advance before the avalanche multiplication completes, preemptively cutting off the current path through the SPAD. This preliminary action prevents the latching current from developing, thereby controlling avalanche multiplication while avoiding the prolonged dead time that would result from waiting for natural current decay through the quenching resistance
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 solution effectively reduces the dead time of the SPAD, enhancing the accuracy of distance measurement and minimizing noise resistance drops, thereby improving the overall performance of the imaging system.
Implementation Method 1
the SPAD and a capacitance for biasing it are connected with a power source during a refresh period and disconnected during a detection period with a view to suppressing after-pulse generation
Implementation Method 2
the invention described in PTL 1 involves electrically charging the capacitance during the refresh period so that the capacitance applies to the SPAD a reverse voltage equal to or higher than a breakdown voltage
Implementation Method 3
When the voltage of the capacitance exceeds the breakdown voltage, a switch is turned off to disconnect the SPAD and the capacitance from the power source, thereby starting the detection period
Implementation Method 4
Upon elapse of a delay time with a delay circuit after the drop of the capacitance voltage below the breakdown voltage, the switch is turned on to connect the SPAD and the capacitance with the power source to start the refresh period
Data Source
AI summary
This technology relates to an imaging apparatus and an imaging system for improving the accuracy of distance measurement performed by use of SPADs.The imaging apparatus includes a pixel array section having pixel sections arrayed therein. Each pixel section includes: an SPAD (single photon avalanche photodiode); a resistance component configured to be connected serially with the SPAD; an output section configured to output a light reception signal indicating photon incidence on the SPAD; and a pulse generation section configured to output a pulse signal in synchronism with the output of the light reception signal. Each pixel sections further includes at least one of: a switch configured to be connected interposingly between the SPAD and the resistance component and turned off in synchronism with the pulse signal; or a pull-in section configured to pull in an input current flowing through the SPAD via the resistance component in synchronism with the pulse signal, thereby suppressing the input current flowing through the SPAD. This technology may be applied to cameras that capture range images, for example.