Abstract
Extending CMOS Image Sensors’ dynamic range is of fundamental importance in applications, such as automotive, scientific, or X-ray, where a broad variation of incoming light should be measured. The typical logarithmic pixels suffer from poor performance under low light conditions due to a leakage current, usually referred to as the dark current. In this paper, we propose a logarithmic pixel design capable of reducing the dark current through low-voltage photodiode biasing, without introducing any process modifications. The proposed pixel combines a high dynamic range with a significant improvement in the dark response compared to a standard logarithmic pixel. The reported experimental results show this architecture to achieve an almost 35 dB improvement at the expense of three additional transistors, thereby achieving an unprecedented dynamic range higher than 160 dB.
Highlights
Technological advances in CMOS image sensors (CIS) have boosted their utilization in a variety of applications
In this paper, starting from the observation that enforcing a zero potential across the photodiode leads to a dark current reduction, a pixel circuit able to maintain a zero potential across the photodiode was presented
The proposed circuit consists of six transistors, has a logarithmic response and the advantage of reducing the dark current by enforcing a low voltage potential across the photodiode
Summary
Technological advances in CMOS image sensors (CIS) have boosted their utilization in a variety of applications. Alternative readout schemes have been proposed to reduce power consumption [9], while other analog encoding voltage techniques have been introduced to boost dynamic range (DR) [10]. Integrating CMOS Active Pixel Sensors have a limited dynamic range of 2–3 decades. This means that any significant improvement at the dark end of the response may lead to poor performance in the brighter regions. The subthreshold logarithmic pixel architecture is intended for large formats and meets the needs of scientific imaging where low power, low dark current and wide dynamic range may be employed and where the demand of speed or pixel scaling is limited
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