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Proceedings Paper

CMOS in-pixel optical pulse frequency modulator
Author(s): Nicolaas E. Nel; M. du Plessis; T-H. Joubert
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Paper Abstract

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.

Paper Details

Date Published: 3 February 2017
PDF: 15 pages
Proc. SPIE 10036, Fourth Conference on Sensors, MEMS, and Electro-Optic Systems, 100360O (3 February 2017); doi: 10.1117/12.2245449
Show Author Affiliations
Nicolaas E. Nel, Univ. of Pretoria (South Africa)
M. du Plessis, Univ. of Pretoria (South Africa)
T-H. Joubert, Univ. of Pretoria (South Africa)

Published in SPIE Proceedings Vol. 10036:
Fourth Conference on Sensors, MEMS, and Electro-Optic Systems
Monuko du Plessis, Editor(s)

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