Download 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
!‫תודה רבה‬