Mr. Nicolaas E. Nel Profile

Nicolaas E. Nel

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Proceedings Article | 24 January 2019 Paper

CMOS in-pixel optical pulse frequency modulator

Nicolaas Nel, M. du Plessis, T.-H. Joubert

Proceedings Volume 11043, 110430C (2019) https://doi.org/10.1117/12.2502455

KEYWORDS: Transistors, Capacitors, Capacitance, Photodiodes, Switches, Sensors, Biosensors, Switching, CMOS sensors, Diodes

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In this paper, a CMOS image detector array is presented that can convert a photocurrent generated by light falling onto a photodiode to a measurable output frequency. The designed array consists of 64 pixels, each containing an n⁺ -p⁺ /psubstrate photodiode along with all signal-to-frequency conversion circuitry. The 8x8 array was designed using the ams 350 nm process, with each individual pixel occupying an area of 30×30 μm² . Measurements on the fabricated array show that a dynamic range of 10 - 15 dB can be achieved with a maximum output frequency of 125 kHz. Work was done to increase the output driving capability of the array to increase the dynamic range of the pixel in simulations to 54 dB. Further research will also investigate ways of implem

Proceedings Article | 3 February 2017 Paper

CMOS in-pixel optical pulse frequency modulator

Nicolaas Nel, M. du Plessis, T-H. Joubert

Proceedings Volume 10036, 100360O (2017) https://doi.org/10.1117/12.2245449

KEYWORDS: Sensors, Capacitance, Capacitors, Photodiodes, Transistors, Diodes, Image processing, Switches, Switching, Image sensors

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This paper covers the design of a complementary metal oxide semiconductor (CMOS) pixel readout circuit with a built-in frequency conversion feature. The pixel contains a CMOS photo sensor along with all signal-to-frequency conversion circuitry. An 8×8 array of these pixels is also designed. Current imaging arrays often use analog-to-digital conversion (ADC) and digital signal processing (DSP) techniques that are off-chip¹. The frequency modulation technique investigated in this paper is preferred over other ADC techniques due to its smaller size, and the possibility of a higher dynamic range. Careful considerations are made regarding the size of the components of the pixel, as various characteristics of CMOS devices are limited by decreasing the scale of the components².

The methodology used was the CMOS design cycle for integrated circuit design. All components of the pixel were designed from first principles to meet necessary requirements of a small pixel size (30×30 μm²) and an output resolution greater than that of an 8-bit ADC. For the photodetector, an n⁺-p⁺/p-substrate diode was designed with a parasitic capacitance of 3 fF. The analog front-end stage was designed around a Schmitt trigger circuit. The photo current is integrated on an integration capacitor of 200 fF, which is reset when the Schmitt trigger output voltage exceeds a preset threshold. The circuit schematic and layout were designed using Cadence Virtuoso and the process used was the AMS CMOS 350 nm process using a power supply of 5V.

The simulation results were confirmed to comply with specifications, and the layout passed all verification checks. The dynamic range achieved is 58.828 dB per pixel, with the output frequencies ranging from 12.341kHz to 10.783 MHz. It is also confirmed that the output frequency has a linear relationship to the photocurrent generated by the photodiode.

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