A wide-dynamic-range time-based CMOS imager

This thesis describes a novel dual-threshold time-based current sensing algorithm suitable for use in wide-dynamic-range CMOS imagers. A prototype 150 x 256 pixel imager employing this algorithm experimentally achieves 95.5 dB dynamic range, 37 dB peak signal-to-noise ratio (SNR), and a highly-linear transfer characteristic while consuming 1.79 nJ/pixel/frame, making it one of the most energy-efficient wide-dynamic-range imagers reported. The individual pixels experimentally achieve 98.8 dB dynamic range and 44 dB peak SNR. The array performance lags slightly behind that of the individual pixels due to the additional noise power contributed to the array data by pixel-to-pixel mismatch effects, attributed primarily to gain and dark-current fixed pattern noise (FPN). The dualthreshold algorithm is also shown to improve low-illumination SNR by 6.1 dB and overall array dynamic range by more than 6.0 dB compared with auto-zeroing alone. The prototype imager implements pixels and their associated 18-bit timing memories in separate on-chip arrays linked by a 200 MHz time-domain-multiplexed communication bus, enabling a pixel pitch of 12.5 pm with 42.7% fill factor in a 0.18-lm 1.8-V CMOS process. Four innovations are contributed by this thesis over previous work, leading to the performance outlined above. First, a novel dual-threshold time-based current sensing algorithm is proposed that forces each single-slope integrating pixel to cross two threshold levels per frame - once just after reset and a second time after a near-optimal amount of photogenerated charge has been collected. This differential measurement technique eliminates offset FPN and pixel reset noise, and reduces comparator 1/f noise. Second, synchronous threshold detection is employed, yielding significant power savings compared with asynchronous approaches in this application, and the resulting time-domain quantization noise introduced by the synchronous detection is analyzed.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Electrical Engineering and Computer Science.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2008

Author dc:creator

• O'Halloran, Micah G. (Micah Galletta), 1978-

Advisor dc:contributor.advisor

• Rahul Sarpeshkar.

• Electrical Engineering and Computer Science.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/43058
• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Identifier URI

http://dspace.mit.edu/handle/1721.1/43058

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/43058