ISSCC 2017 (2)

Wootaek Lim of University of Michigan talked about “A sub-nW 80mlx-to-1.26Mlx self-referencing light-to-digital converter with AlGaAs photodiode”.  The work is focusing on a wearable image sensor for instance to acquire a measurement of the cumulative light exposure a person gets over a long period of time (e.g. UV radiation exposure).  Crucial parameters for this application are low power consumption, wide dynamic range and low relative error.  These requirements are realized by using a special ring oscillator and counter as an integrating ADC, use a photodiode voltage as the input in combination with a divider to extend the measurable voltage range, and linearly coding the light intensity in the log-log domain.  All various techniques were explained in detail including circuit diagrams.  As a result, with these news techniques, the power was reduced over 1000 x, the dynamic range was extended up from 1.26 Mlx (starting from 80mlx), all combined with the lowest conversion energy of 4.13 nJ/conv. at 50klx.  The sensor is fully functional between -20 and +85 deg.C.


“A 1.8 e temporal noise over 110dB dynamic range 3.4 um pixel pitch global shutter CMOS image sensor with dual-gain amplifiers, SS-ADC and multiple accumulation shutter” by Masahiro Kobayashi of Canon.  This was a great paper with a great presentation of the obtained results, but I did have serious doubts about the novelty of the work (and I was not the only one).  What is done is the implementation of a global shutter with a storage node in the charge domain.  This results in the so-called 6T transistor architecture.  To increase the fillfactor of the pixels, 2-by-1 sharing is applied.  In a classical GS pixel, the charge needs to be stored on the PPD, on the SG and on the FD.  If they are all equal to each other in capacitive value, a particular full well is obtained which is pretty limited.  The idea now is to make the PPD smaller and the SG larger.  In that case the full well would be determined by the small PPD, but during the exposure the PPD can be emptied multiple times and then the weakest link in the chain is shifted to the larger SG.  This is not new, Canon themselves introduced this already at IEDM 2016, but also Aptina published a similar solution at the IISW in 2009.  Nevertheless, besides this general idea, the presented sensor has a funnel-shaped light guide structure above the pixels, an optimized light shield to keep the PLS low.  To enhance the dynamic range of the sensor, the columns are provided with a gain stage that automatically choses between a gain of 1x or 4x.  With some clever timing of the transfer of the PPD and with an increased readout speed of the sensor, extra new option can be added, such as wider dynamic range and in-pixel coded exposure.

Results and images were shown during the presentation, despite of the fact that not everything is/was new, the results were impressive.  5 Mpixels, up to 120 fps, 450 mW, pixel pitch 3,4 um, 130 nm 1P4M +LS process, 1.8 e noise floor, maximum 79 dB dynamic range and in the HDR mode 111 dB, 20 e/s dark current at 60 deg.C.

Albert, 8-2-2017.

Leave a Reply