Introduction
- DToF sensors measure long distances and have low Tx power consumption due to the high sensitivity of SPADs. But, spatial resolution is limited due to large pixel size, high bias voltage is required, and short distance detection is difficult due to high noise floor due to high-sensitive SPAD pixel circuits
- iToF sensors use common CIS-based pixel structure with small pixel pitch and do not reuqire high-voltage power supply. However, measurement range is shorter and pixels can be easily saturated at high ambient light conditions.
- Here, a diffractive optical element (DOE) to focus the Tx laser power on the selected spots is used.
- Only the valid pixel groups with high confidence values are selected by sweeping the kernel.
- Among the pixels in a group, the central pixel is selected as the spot.
- Depth temporal noise results are shown in Fig. 3 at indoor (only room LED lighting) and outdoor (illumination of ~70-klx halogen lamp on the chart) conditions.
- Depth noise was reduced by 78% and 87%
- The focused optical power leads to the SNR improvement because the “spot” pixels receive more optical power
- DC values are almost similar regardless of the Tx types in the indoor conditions, because intensity and amplitude are changed at a same rate if the light power comes only from the Tx
- Howeve, the DC with the patterned light source is enhanced in the outdoor condition, because higher optical power density increases amplitude more than intensity
- SNR and DC are merged as the depth-SNR (D-SNR) as follows:
- D-SNR values of the patterned light source are about 3.6 times and 6.2 times higher than the those of the conventional Tx
- The black circles are shared floating diffusion nodes
- 2×2 digital binning: Each dedicated readout circuit reads out the charges in each tap, then are averaged in a digital domain
- 2×2 analalog binning: Sums up the charges from each adjacent tap in a single FD node Only a quarter of FWC of the FD node can be used, limiting exposure time in the outdoor condition and consequently increasing the depth noise
- Read noise of the analog binning (1 readout circuit) is reduced compared to that of the digital binning (4 readout circuits)
- An adaptive binning is proposed to choose the best binning mode depending on ambient light
- Indoor condition: Select analog binning to minimize read noise
- Outdoor condition: Select digital binning to maximize exposure time to overcome ambient light
