# 320 × 240 SPAD Direct Time-of-Flight Image Sensor and Camera Based on In-Pixel Correlation and Switched-Capacitor Averaging

## TL;DR

A new 320 × 240 SPAD camera uses in-pixel correlation to capture depth images with high precision over a 6-meter range.

## Contribution

First demonstration of CA-dToF on a large SPAD array with on-chip timing support and in-pixel processing.

## Key findings

- In-pixel processing avoids data communication and storage bottlenecks.
- Depth-sense images achieved sub-cm precision over a 6 m range.
- Exponential moving averaging reduced noise from ambient photons and dark counts.

## Abstract

Correlation-Assisted Direct Time-of-Flight (CA-dToF) is demonstrated for the first time on a large 320 × 240-pixel SPAD array sensor that includes on-chip high-speed timing support circuitry. SPAD events are processed in-pixel, avoiding data communication over the array and/or storage bottlenecks. This is accomplished by sampling two orthogonal triangle waves that are synchronized with short light pulses illuminating the scene. Using small switched-capacitor circuits, exponential moving averaging (EMA) is applied to the sampled voltages, delivering two analog voltages (VQ2, VI2). These contain the phase delay, or the time of flight between the light pulse and photon’s time of arrival (ToA). Uncorrelated ambient photons and dark counts are averaged out, leaving only their associated shot noise impacting the phase precision. The QVGA camera allows for capturing depth-sense images with sub-cm precision over a 6 m range of detection, even with a small PDE of 0.7% at an 850 nm wavelength.