Depth Pixel Read Circuit with Switchable Integration for Bias Stability

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Solution Overview

Problem

Existing read circuits for depth image acquisition systems using unpinned photodiodes, particularly those based on III-V materials like InGaAs, face challenges in maintaining biasing stability, signal-to-noise ratio, and bandwidth limitations, especially at high operating frequencies, making it difficult to achieve optimal performance.

Innovation Solution

A read circuit design incorporating a switching circuit with integration capacitors and amplifier stages that alternates capacitor connections during sampling phases to improve signal-to-noise ratio and compensate for transistor imbalances, using a switching circuit with switch elements to reverse capacitor connections and rebiasing units to maintain photodiode biasing.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If a read circuit uses unpinned photodiodes for depth image acquisition, then the device can operate at high frequencies and use III-V materials like InGaAs, but the biasing stability deteriorates and signal-to-noise ratio decreases

Engineering Contradiction:

Improveoperating frequencyVSAvoidbiasing stability

Core Design Contradiction:

SpeedVSReliability

Solution Approach 1:

The circuit performs preliminary charging of the integration capacitor during a reset phase before the actual sampling phase. This preliminary action ensures that the capacitor is properly biased and ready for signal integration, which maintains biasing stability even at high operating frequencies with unpinned photodiodes

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The read circuit operates in periodic cycles alternating between reset/charging phases and sampling phases. This periodic operation allows the circuit to re-establish proper biasing conditions regularly, maintaining stability throughout continuous high-frequency operation

Inventive Principle:

Principle #19Periodic action

2Measurement precision

If integration capacitors are used to sample charges during sampling phases, then the signal-to-noise ratio can be improved, but transistor imbalances and mismatches cause performance degradation

Engineering Contradiction:

Improvesignal-to-noise ratioVSAvoidtransistor matching

Core Design Contradiction:

Measurement precisionVSManufacturing precision

Solution Approach 1:

The circuit inverts the polarity of the integration capacitor connections between different sampling phases. By switching the capacitor connections to opposite polarities and then combining the results, the circuit cancels out the effects of transistor imbalances and mismatches, maintaining measurement precision despite manufacturing variations

Inventive Principle:

Principle #13The other way round (Inversion)

Solution Approach 2:

The circuit changes the connection parameters of the integration capacitor dynamically during operation. By switching between different connection configurations (reversing capacitor connections), the circuit adapts to compensate for fixed transistor mismatches, effectively improving signal-to-noise ratio despite manufacturing imperfections

Inventive Principle:

Principle #35Parameter changes

3Power

If the read circuit uses amplifier stages to read signals from depth pixels, then the signal can be amplified, but amplifier saturation occurs reducing measurement accuracy

Engineering Contradiction:

Improvesignal amplificationVSAvoiddistance measurement accuracy

Core Design Contradiction:

PowerVSMeasurement precision

Solution Approach 1:

The circuit uses multiple integration capacitors that sample the signal at different phases. By distributing the signal integration across multiple capacitors and then combining their outputs, the circuit achieves the necessary signal amplification without any single amplifier stage becoming saturated, thereby maintaining measurement accuracy

Inventive Principle:

Principle #16Partial or excessive action

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

The solution enhances signal-to-noise ratio and compensates for transistor mismatches, improving the performance of depth image acquisition systems with unpinned photodiodes, particularly at high frequencies, by maintaining biasing stability and reducing amplifier saturation.

Implementation Method 1

each depth pixel of the set comprising a read circuit associated with a photodetector... the photodetector being an unpinned photodiode

Methodology Applied

Scientific EffectPhotoelectric effect: Photoelectric Effect

Implementation Method 2

a storage element including an integration capacitor configured to acquire samples of charges from the photodetector during sampling phases

Methodology Applied

Scientific EffectCapacitance: Capacitance

Data Source

PatentUS20250294269A1Depth pixel with switchable integration capability

Publication Date: 2025.09.18 COMMISSARIAT A LENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES

AI summary

An image acquisition device provided with a set of depth pixels, each depth pixel comprising a read circuit associated with a photodetector (PD), the read circuit having a switching circuit (120) configured to:adopt a first configuration so as to couple a first electrode (101a) of an integration capacitor (Ca) with a first read node (NL1) and a second electrode (102a) of the integration capacitor (Ca) with a detection node (ND), then,adopt a second configuration so as to couple the first electrode (101a) of the integration capacitor (Ca) with the detection node (ND) and the second electrode (102a) of the integration capacitor (CINT1) with the read node (NL).