Wednesday morning, Feb. 23rd, the imaging session was schedule. Find here the first reviews.
M.W. Seo ( Shizuoka University) presented : “An 80mVrms Temporal Noise 82 dB Dynamic Range CMOS Image Sensor with a 13-to-19b Variable Resolution Column-Parallel Folding-Integration/Cyclic ADC”. This is not the first time that this group is presenting a column-level ADC for CMOS image sensors, but they keep improving the noise performance. During the presentation the author showed a nice overview of the work done by this group and how the performance was constantly improved. The target spec for this device was : < 1 electron noise, 80 dB dynamic range and 18 bits ADC. What was realized : 0.95 electron noise (at 128 times sampling), 82 dB (with 64 times sampling and up to 19 bits ADC. The ADCs do have the option for multiple sampling and that results in an improved noise performance. Unfortunately that goes together with a reduction of the frame rate. The author explained the circuitry and working principle of the ADC, but that is too complex (for me) to explain in this short review. During the Q&A the author mentioned that the limitation of the noise floor is now the 1/f noise, as well as the power dissipation being 415 mW at 19 bits. The device is a 1Mpixel test imager with a pixel pitch of 7.5 mm.
Next in line was C. Lotto (Heliotis) to present a nice pixel based on 4T (1 nMOS and 3 pMOS). The pixel can switch between two modes by clever re-coupling of the circuitry : closed loop reset mode (to keep the effect of FPN and spread on threshold low) and open loop amplifier mode (to add amplication in the pixel). Also this pixel realizes a noise floor lower than 1 electron, namely 0.86 electron (at RT and at 60 fr/s of 256 x 256 pixels), with a conversion gain of 300 mV/electron. Despite the extra gain in the pixel, the PRNU could be kept down to 2.5 % and the linearity error could be kept below 1.7 %. Also in this presentation is was stated that the limiting noise floor for the pixel is the flicker noise.
The two devices presented so far, both with a noise level below 1 electron, were both fabricated in 0.18 mm technology. The imager session started with two papers indicating the psychological barrier of 1 electron noise is not that far away anymore.
Albert, 24-02-2011.