Chipworks published a report from 2013 Image Sensor Conference held on March 19-21 in London, UK. The report is written by Ray Fontaine and goes into great details on two presentations - Howard Rhodes' (Omnivision) and Junichi Nakamura's (Aptina) ones.
Omnivision develops its third generation BSI pixel based on 55nm process and prepares the next generation 45nm process to be ready sometime in 2014. A switch from Bayer CFA to Omnivision's proprietary "Clear Process Technology" with panchromatic pixels is said to improve the low light SNR by 6dB.
Aptina's presentation talked about high frame rate sensors and advantages it gives to the camera. The latest 10.8MP 1-inch sensor with 80fps speed features 3.4um dual conversion gain pixels and 13b column parallel SAR ADC.
hi ISW blog contributors,
ReplyDeleteIs there a circuit model for pinned-photodiodes that explains the charge-transfer process? Recently I have been discussing with circuit designers, teaching them the operation of the 4-T pixel. Since they love to think in a circuit based approach, it is becoming difficult for me to explain them charge-transfer operation (they think in terms of charge-sharing; I dont know whether that is quite representative.) Any pointers will be appreciated..
No, there is no simple circuit model. One can write it in Verilog-A/AMS, although it's quite a big investment of time if you want to include all the complex effects.
DeleteI think the hard part for them to understand is pinning. If you can model that, they can simulate and be happy :-) I don't remember exactly what I used now, but a voltage control resistor (or a voltage control switch?) should work. The control voltage is on the photodiode side and once this voltage reaches Vpin, it shuts off the path between photodiode and the transfer gate. Another trouble is the models of different Vth from select/reset FET. To a first order, you can also use a voltage source on the gate to simulate Vth shift, if you don't want the trouble of poking into model cards.
DeletePhotodiode can be modeled as a cap (or diode if you want leakage) and an adjustable current source to ground. Make sure that there is initial node set to something between ground and Vpin.
This should be a good first order model, not with all the details, but good enough for entertaining circuit designers.
Anybody knows what is the most common column ADC used in commercial imagers? I am quite attracted by the cyclic ADC. Is that just used in niche applications?
ReplyDeleteThanks in advance for the answer!