Abstract
This paper analyzes and compensates for process and temperature dependency among a (Complementary Metal Oxide Semiconductor) CMOS image sensor (CIS) array. Both the analysis and compensation are supported with experimental results on the CIS’s dark current, dark signal non-uniformity (DSNU), and conversion gain (CG). To model and to compensate for process variations, process sensors based on pixel source follower (SF)’s transconductance gm,SF have been proposed to model and to be compared against the measurement results of SF gain ASF. In addition, ASF’s thermal dependency has been analyzed in detail. To provide thermal information required for temperature compensation, six scattered bipolar junction transistor (BJT)-based temperature sensors replace six image pixels inside the array. They are measured to have an untrimmed inaccuracy within ±0.5 °C. Dark signal and CG’s thermal dependencies are compensated using the on-chip temperature sensors by at least 79% and 87%, respectively.
Highlights
The pixels in a CMOS image sensor (CIS) array, as all semiconductor devices, are sensitive to process, voltage, and temperature (PVT) variations, which give rise to (Fixed Pattern Noise) FPN, including dark current or degraded conversion gain (CG) with temperature [1]
This research paper analyzes and compensates for the process and the temperature dependency in a CIS image sensor, facilitated with the temperature and the process sensors implemented inside the image pixel array
The conclusions are that if one can afford a cascode current biasing, ASF ’s temperature dependency would be less than 0.5% over a temperature range of 100 ◦ C, due to the loop gain around 100 that biases the source follower (SF)
Summary
The pixels in a CMOS image sensor (CIS) array, as all semiconductor devices, are sensitive to process, voltage, and temperature (PVT) variations, which give rise to (Fixed Pattern Noise) FPN, including dark current or degraded conversion gain (CG) with temperature [1]. In contrast to the aforementioned publications, this paper explores the possibilities to predict and compensate for process and temperature dependency without requiring any accurate input voltage or light source, in the following aspects: (1) Process sensors based on pixel. The column readout circuits are 14 bit 1st-order delta-sigma ADCs (DSADC), which are less sensitive to process and temperature variations, due to their feedback loop, equaling their digital bit stream (bs) outputs to the analog inputs. Both the resolution and noise of the DSADC are less than 10 μV.
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