Document Type : Original Article
Author
Electrical Engineering Department Faculty of Engineering, Assiut University Assiut 71715, Egypt
Abstract
Highlights
Based on the obtained results, the effect of adding diode
connected transistors M7 and M8 improves a lot in the
dynamic range and readout noise of CMOS image sensor. As
shown in the traditional WDR CIS the use of overflow
integration capacitance reduces the bandwidth of the sensor
and so its speed is reduced. In the proposed WDR CIS by
using diode connected transistors the readout noise reduced to
20 μVrms (0.6 e-rms) compared with in traditional CIS, which
reached to 94.5 uVrms also, bandwidth in the proposed
circuitry extendes to 5 MHz compared to the traditional one in
which reached to 43 KHz. As this reduction in readout noise
in the proposed CIS, the dynamic range is extended to 140 dB
after adding a diode connected transistors M7 and M8
compared to traditional in which dynamic range reached to
83.7 dB.
Keywords
The active pixel sensors (APS) are implemented in a
commonly used CMOS technology, these sensors which
fabricated using CMOS technology are very useful in many
scientific, commercial and consumer applications [1].
Recently, CMOS image sensors allow integration of all
functions required for timing, exposure process, color
processing, compression of the image and analog to digital
converters (ADCs) on the same die. In addition, CMOS image
sensors provide additional advantages over CCD in terms of
small consumption of power, small operating voltage, smaller
noise and high sensitivity.
Low noise CMOS Image Sensor (CIS) plays an important
role in the image efficiency and clearance [2]. As, low noise
sensor increases the sensor’s ability to see the image by only a
few photons. This sensor has a wide application in different
fields.
Wide dynamic range CMOS image sensor, also considered
very important parameter in determining the sensor
performance. Wide dynamic range increases the sensor’s
ability to capture the details of the image under low and high
light conditions [3].
The image sensor is considered the main part of a digital
camera, which converts optical information of the light
photons into electrical signals. Nowadays, CMOS based
imaging arrays have a great growing stage. Besides the low
cost and low power consumption advantages, the continual
development of CMOS image sensor publicity goes to other
parameters such as the capability to integrate the sensors with
electronics and the ability to achieve fast, customizable frame
rates [4]. For all image sensors the low readout noise and high
dynamic range are considered two important factors define
the sensor quality.
Nowadays, the development of CMOS image sensor
technology is requiring a lot of efforts to be made to get
CMOS image sensor with factors suitable with the existing
requirements for imaging technology. Dynamic range of CIS
is considered one of the most important parameters of the
sensor which must be improved to increase the sensor quality
in capturing the image details [5], [6], [7]. In this decade,
increasing the dynamic range of CMOS image sensor became
the main target even though there are initial methods were
already proposed in most commercial sensors in which the
dynamic range of the sensor is limited to about 72 dB [8].
There is a more common implementation using multiple
exposures which achieved a high dynamic range reaches to
112 dB [9]. Reference [10] provides a new method to increase
dynamic range by reducing the input referred noise as the
temporal read-out noise. The CIS in [11] introduces a method
that achieved a 40% reduction in the input–referred temporal
noise.
This paper focuses on increasing the dynamic range of
voltage-mode CMOS image sensor and reducing its readout
noise by providing a new technique that uses a diode
connected transistors in pixel circuitry, and get CIS with high
speed.
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