image sensor
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Transcript image sensor
Miriam Pekar
Alex Liberchuk
Supervisors:
Dr. Alexander Fish
Mr. Arthur Spivak
P-2011-130
10/2011
What is an Image Sensor?
An image sensor is a device that
converts an optical image into an
electronic signal. It is used mostly in
digital cameras and other imaging
devices.
The two most popular kinds of image
sensors are:
Charge-coupled device (CCD).
Complementary Metal–Oxide–
Semiconductor (CMOS).
Why CMOS and not CCD?
CMOS is implemented using less components.
CMOS sensors consume less power.
This is important in portable devices.
Provides faster readout.
Cheaper to manufacture.
CMOS Drawbacks:
CMOS sensors, traditionally, are more susceptible to noise.
Light sensitivity of a CMOS chip tends to be lower because
several transistors are located next to each photodiode.
CMOS sensors tend to have Low Dynamic Range.
Effects of Low Dynamic Range Imaging:
Low DR Imaging
Wide DR Imaging
Dynamic Range quantifies the ability
of a sensor to image highlights and shadows.
Goal of Our Project: Improve the Dynamic Range of
the CMOS Sensor
What is a CMOS Sensor?
It is an image sensor produced by a CMOS
semiconductor process.
It consists of a photodiode and extra circuitry next
to each photodiode converting the light energy to a
voltage, later the voltage is converted to a digital
signal.
What is a Comparator?
a comparator is a device that compares two
voltages and switches its output to indicate
which is larger.
A good comparator implementation can be an Operational
Amplifier connected in open loop.
The Use of the Comparator in a
WDR Sensor:
If a pixel value exceeds the threshold - i.e. the pixel is expected to be saturated at the end of the
exposure time - the reset is given at that time to that pixel. The binary information concerning the reset
(i.e., if it is applied or not) is saved in a digital storage for later calculation of the scaling factor. Thus,
we can represent the pixel output in the following floating- point format: M⋅2EXP. Here, the mantissa (M)
represents the digitized pixel value, and the exponent (EXP) represents the scaling factor.
This way, the maximal signal value the sensor can process is raised – higher DR.
Project Process Flow
Specifications
Choose Suitable
Comparator
Topologies
Design Procedures
Set-up to determent
W/L (each Topology)
Full SPECTRA
simulation
Remaining Tasks
Our Project:
Design a High Precision Comparator to
Implement a WDR Sensor
Technology - TOWER 180nm
The Comparator’s Design Requirements:
Gain = 1000
Bandwidth = 1 - 2 MHz
GBW = 1-2 GHz
Slew Rate > 1.8 V/µsec
Power Dissipation < 100nW
CLoad = 150 fF
0V < Vout < 3.3V
0.2V < Vin < 2V
Project Process Flow
Specifications
Choose Suitable
Comparator
Topologies
Design Procedures
Set-up to determent
W/L (each Topology)
Full SPECTRA
simulation
Remaining Tasks
Comparator Topologies
Simple One-Stage
Two-Stage
Folded Cascode
Gain Boosted Folded Cascode
Project Process Flow
Specifications
Choose Suitable
Comparator
Topologies
Design Procedures
Set-up to determent
W/L (each Topology)
Full SPECTRA
simulation
Remaining Tasks
Simple One-Stage Comparator
The topology resulted in poor performance,
due to poor gain and bandwidth
Two-Stage Comparator
Active
Load
Bias
Current
Enable
Switch
Current
Mirror
Differential
Pair
Common
Source
Amplifier
Two-Stage Comparator cont.
Results:
Gain,
BW
Power
Dissipation
Slew
Rate
ENABLE=OFF
ENABLE=ON
dVout
3.275[V
]
Sec
GBW = Gain*BW= (62.03dB)*1.4MHzdt = 1.769GHz
SR
All the design requirements were met!
Folded Cascode Comparator
Current
Source
Differential
Pair
Cascode
Transistors
Common
Source
Amplifier
Bias
Circuit
Current
Mirror
Folded Cascode Comparator cont.
Results:
PowerSlew
Dissipation
Gain,
Rate
BW
ENABLE = ON
ENABLE=OFF
dVout
SR
2.53[V
]
Sec
dt
GBW = Gain*BW= (60.12dB)*1.36MHz = 1.379GHz
All the design requirements were met!
Project Process Flow
Specifications
Choose Suitable
Comparator
Topologies
Design Procedures
Set-up to determent
W/L (each Topology)
Full SPECTRA
simulation
Remaining Tasks
Full SPECTRA simulation
DC analysis – make sure all transistors
are in saturation mode
AC analysis – find a suitable W/L for the
desired Gain, BW and GBW.
Transient analysis – checks the Slew
Rate, and Power Dissipation.
Now, Corners were checked.
Project Process Flow
Specifications
Choose Suitable
Comparator
Topologies
Design Procedures
Set-up to determent
W/L (each Topology)
Full SPECTRA
simulation
Remaining Tasks
Remaining Tasks
Create and check Gain Boosted Folded
Cascode topology.
Comparison of all topologies designed in
this project.
Layout Implementation of the best
topology and post layout simulations.
Questions
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