CCD vs. CMOS
`
`CCD vs. CMOS:
`CCD vs. CMOS:
`
`Facts and Fiction
`
`Choosing an imager means considering not only the chip, but
`also its manufacturer and how your application will evolve.
`
`by Dave Litwiller
`
`Much has been made in the
`
`past five years of the poten-
`tial for CMOS imagers and
`of the impending demise of the in-
`cumbent image-sensing technology,
`CCDs.
`Strong claims by the proponents
`of a resurgent CMOS technology
`have been countered by equally force-
`ful claims by CCD defenders. In a
`pattern typical of battling technolo-
`gies (both with significant merits but
`also lacking maturity in some re-
`gards), users have become leery of
`performance representations made
`by both camps. Overly aggressive
`promotion of both technologies has
`led to considerable fear, uncertainty
`and doubt.
`
`Imager basics
`For the foreseeable future, there
`will be a significant role for both types
`of sensor in imaging. The most suc-
`cessful users of advanced image cap-
`ture technology will be those who
`consider not only the base technol-
`ogy, but also the sustainability,
`adaptability and support. They will
`perform the best long term in a dy-
`namic technology environment that
`the battle between CCDs and CMOS
`promises to deliver.
`Both image sensors are pixelated
`metal oxide semiconductors. They
`accumulate signal charge in each
`pixel proportional to the local illu-
`mination intensity, serving a spatial
`
`Figure 1.On a CCD, most functions
`take place on the camera’s printed
`circuit board. If the application’s
`demands change, a designer can
`change the electronics without
`redesigning the imager.
`
`sampling function.
`When exposure is complete, a CCD
`(Figure 1) transfers each pixel’s
`charge packet sequentially to a com-
`mon output structure, which con-
`verts the charge to a voltage, buffers
`it and sends it off-chip. In a CMOS
`imager (Figure 2), the charge-to-volt-
`age conversion takes place in each
`pixel. This difference in readout tech-
`niques has significant implications
`for sensor architecture, capabilities
`and limitations.
`Eight attributes characterize image-
`sensor performance:
`• Responsivity, the amount of sig-
`nal the sensor delivers per unit of
`input optical energy. CMOS imagers
`are marginally superior to CCDs, in
`general, because gain elements are
`easier to place on a CMOS image sen-
`sor. Their complementary transis-
`tors allow low-power high-gain am-
`
`plifiers, whereas CCD amplification
`usually comes at a significant power
`penalty. Some CCD manufacturers
`are challenging this conception with
`new readout amplifier techniques.
`• Dynamic range, the ratio of a
`pixel’s saturation level to its signal
`threshold. It gives CCDs an advan-
`tage by about a factor of two in com-
`parable circumstances. CCDs still
`enjoy significant noise advantages
`over CMOS imagers because of qui-
`eter sensor substrates (less on-chip
`circuitry), inherent tolerance to bus
`capacitance variations and common
`output amplifiers with transistor
`geometries that can be easily adapted
`for minimal noise. Externally cod-
`dling the image sensor through cool-
`ing, better optics, more resolution or
`adapted off-chip electronics cannot
`make CMOS sensors equivalent to
`CCDs in this regard.
`
`Camera
`(Printed Circuit Board)
`
`Charge-Coupled Device
`Image Sensor
`
`Bias
`Generation
`
`Clock &
`Timing
`Generation
`
`Oscillator
`
`Line
`Driver
`
`Clock
`Drivers
`
`Gain
`
`To Frame
`Grabber
`
`Analog-to-Digital
`Conversion
`
`Photon-to-Electron
`Conversion
`Electron-to-Voltage
`Conversion
`
`Reprinted from the January 2001 issue of PHOTONICS SPECTRA © Laurin Publishing Co. Inc.
`
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`
`Magna 2041
`TRW v. Magna
`IPR2015-00436
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`

`
`CCD vs. CMOS
`
`Photon-to-Electron
`
`Conversion
`
`Electron-to-Voltage
`
`Conversion
`
`Camera
`(Printed Circuit Board)
`
`Complementary Metal Oxide Semiconductor
`Image Sensor
`
`Row Access
`
`Row Drivers
`
`Clock &
`Timing
`Generation
`
`Oscillator
`
`Generation
`
`Bias
`
`Bias Decoupling
`
`Connector
`
`Line
`Driver
`
`Gain
`
`Column Amps
`
`Column Mux
`
`To Frame
`Grabber
`
`Analog-to-Digital
`Conversion
`
`• Uniformity, the consistency of
`response for different pixels under
`identical illumination conditions.
`Ideally, behavior would be uniform,
`but spatial wafer processing varia-
`tions, particulate defects and ampli-
`fier variations create nonuniformi-
`ties. It is important to make a dis-
`tinction between uniformity under
`illumination and uniformity at or
`near dark. CMOS imagers were tra-
`ditionally much worse under both
`regimes. Each pixel had an open-
`loop output amplifier, and the offset
`and gain of each amplifier varied con-
`siderably because of wafer process-
`ing variations, making both dark and
`illuminated nonuniformities worse
`than those in CCDs. Some people
`predicted that this would defeat
`CMOS imagers as device geometries
`shrank and variances increased.
`However, feedback-based ampli-
`fier structures can trade off gain for
`greater uniformity under illumina-
`tion. The amplifiers have made the il-
`luminated uniformity of some CMOS
`imagers closer to that of CCDs, sus-
`tainable as geometries shrink.
`Still lacking, though, is offset vari-
`ation of CMOS amplifiers, which
`manifests itself as nonuniformity in
`darkness. While CMOS imager man-
`
`ufacturers have invested consider-
`able effort in suppressing dark
`nonuniformity, it is still generally
`worse than that of CCDs. This is a
`significant issue in high-speed ap-
`plications, where limited signal lev-
`els mean that dark nonuniformities
`contribute significantly to overall
`image degradation.
`• Shuttering, the ability to start
`and stop exposure arbitrarily. It is a
`standard feature of virtually all con-
`sumer and most industrial CCDs,
`especially interline transfer devices,
`and is particularly important in ma-
`chine vision applications. CCDs can
`deliver superior electronic shutter-
`ing, with little fill-factor compromise,
`even in small-pixel image sensors.
`Implementing uniform electronic
`shuttering in CMOS imagers requires
`a number of transistors in each pixel.
`In line-scan CMOS imagers, elec-
`tronic shuttering does not compro-
`mise fill factor because shutter tran-
`sistors can be placed adjacent to the
`active area of each pixel. In area-
`scan (matrix) imagers, uniform elec-
`tronic shuttering comes at the ex-
`pense of fill factor because the
`opaque shutter transistors must be
`placed in what would otherwise be
`an optically sensitive area of each
`
`Figure 2.ACMOS imager converts
`charge to voltage at the pixel, and
`most functions are integrated into the
`chip. This makes imager functions
`less flexible but, for applications in
`rugged environments, a CMOS
`camera can be more reliable.
`
`pixel. CMOS matrix sensor design-
`ers have dealt with this challenge in
`two ways:
`A nonuniform shutter, called a
`rolling shutter, exposes different lines
`of an array at different times. It re-
`duces the number of in-pixel tran-
`sistors, improving fill factor. This is
`sometimes acceptable for consumer
`imaging, but in higher-performance
`applications, object motion manifests
`as a distorted image.
`A uniform synchronous shutter,
`sometimes called a nonrolling shut-
`ter, exposes all pixels of the array at
`the same time. Object motion stops
`with no distortion, but this approach
`consumes pixel area because it re-
`quires extra transistors in each pixel.
`Users must choose between low fill
`factor and small pixels on a small,
`less-expensive image sensor, or large
`pixels with much higher fill factor on
`a larger, more costly image sensor.
`• Speed, an area in which CMOS
`arguably has the advantage over
`CCDs because all camera functions
`can be placed on the image sensor.
`
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`
`With one die, signal and power
`trace distances can be shorter,
`with less inductance, capacitance
`and propagation delays. To date,
`though, CMOS imagers have es-
`tablished only modest advantages
`in this regard, largely because of
`early focus on consumer appli-
`cations that do not demand no-
`tably high speeds compared with
`the CCD’s industrial, scientific
`and medical applications.
`• Windowing. One unique ca-
`pability of CMOS technology is
`the ability to read out a portion of
`the image sensor. This allows el-
`evated frame or line rates for
`small regions of interest. This is
`an enabling capability for CMOS
`imagers in some applications,
`such as high-temporal-precision
`object tracking in a subregion of
`an image. CCDs generally have
`limited abilities in windowing.
`• Antiblooming, the ability to
`gracefully drain localized overex-
`posure without compromising the
`rest of the image in the sensor.
`CMOS generally has natural
`blooming immunity. CCDs, on the
`other hand, require specific en-
`gineering to achieve this capabil-
`ity. Many CCDs that have been
`developed for consumer applica-
`tions do, but those developed for
`scientific applications generally do
`not.
`• Biasing and clocking. CMOS im-
`agers have a clear edge in this re-
`gard. They generally operate with a
`
`CCD vs. CMOS
`
`Choose Your Imager
`
`CMOS imagers offer superior integration,
`power dissipation and system size at the
`expense of image quality (particularly in
`low light) and flexibility. They are the tech-
`nology of choice for high-volume, space-
`constrained applications where image
`quality requirements are low. This makes
`them a natural fit for security cameras, PC
`videoconferencing, wireless handheld de-
`vice videoconferencing, bar-code scan-
`ners, fax machines, consumer scanners,
`toys, biometrics and some automotive in-
`vehicle uses.
`CCDs offer superior image quality and
`flexibility at the expense of system size.
`They remain the most suitable technol-
`ogy for high-end imaging applications,
`such as digital photography, broadcast
`television, high-performance industrial
`imaging, and most scientific and medical
`applications. Furthermore, flexibility
`means users can achieve greater system
`differentiation with CCDs than with CMOS
`imagers.
`Sustainable cost between the two tech-
`nologies is approximately equal. This is
`a major contradiction to the traditional
`marketing pitch of virtually all of the solely
`CMOS imager companies.
`G
`
`single bias voltage and clock level.
`Nonstandard biases are generated
`on-chip with charge pump circuitry
`isolated from the user unless there is
`some noise leakage. CCDs typically
`require a few higher -voltage
`biases, but clocking has been sim-
`plified in modern devices that op-
`erate with low-voltage clocks.
`
`Reliability
`Both image chip types are equally
`reliable in most consumer and in-
`dustrial applications. In ultrarugged
`environments, CMOS imagers have
`an advantage because all circuit
`functions can be placed on a sin-
`gle integrated circuit chip, mini-
`
`Figure 3.Are they really stars? For an
`ideal detector, each pixel’s response to
`a photon would be identical, and the
`“starlight”would be confined to the area
`of the star.
`
`mizing leads and solder joints,
`which are leading causes of cir-
`cuit failures in extremely harsh
`environments.
`CMOS image sensors also
`can be much more highly inte-
`grated than CCD devices.
`Timing generation, signal pro-
`cessing, analog-to-digital con-
`version, interface and other
`functions can all be put on the
`imager chip. This means that
`a CMOS-based camera can be
`significantly smaller than a
`comparable CCD camera.
`The user needs to consider,
`however, the cost of this inte-
`gration. CMOS imagers are
`manufactured in a wafer fab-
`rication process that must be
`tailored for imaging perfor-
`mance. These process adapta-
`tions, compared with a non-
`imaging mixed-signal process,
`come with some penalties in
`device scaling and power dis-
`sipation. Although the pixel
`portion of the CMOS imager al-
`most invariably has lower
`power dissipation than a CCD,
`the power dissipation of other
`circuits on the device can be
`higher than that of a CCD
`using companion chips from
`optimized analog, digital and
`mixed signal processes. At a system
`level, this calls into question the no-
`tion that CMOS-based cameras have
`lower power dissipation than CCD-
`based cameras. Often, CMOS is bet-
`ter, but it is not unequivocally the
`case, especially at high speeds (above
`about 25-MHz readout).
`The other significant considera-
`tions in system integration are adapt-
`ability, flexibility and speed of
`change. Most CMOS image sensors
`are designed for a large, consumer
`or near-consumer application. They
`are highly integrated and tailored for
`one or a few applications. A system
`designer should be careful not to in-
`vest fruitlessly in attempting to adapt
`a highly application-specific device
`for a use to which it is not suited.
`CCD image sensors, on the other
`hand, are more general purpose. The
`pixel size and resolution are fixed in
`the device, but the user can easily
`tailor other aspects such as readout
`
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`
`CCD vs. CMOS
`
`speed, dynamic range, binning,
`digitizing depth, nonlinear ana-
`log processing and other
`customized modes of operation.
`Even when it makes economic
`sense to pay for sensor cus-
`tomization to suit an applica-
`tion, time to market can be an
`issue. Because CMOS imagers
`are systems on a chip, devel-
`opment time averages 18
`months, depending on how
`many circuit functions the de-
`signer can reuse from previous
`designs in the same wafer fab-
`rication process. And this
`amount of time is growing be-
`cause circuit complexity is out-
`pacing design productivity. This
`compares with about eight
`months for new CCD designs in
`established manufacturing
`processes. CCD systems can
`also be adapted with printed cir-
`cuit board modifications,
`whereas fully integrated CMOS
`imaging systems require new
`wafer runs.
`
`Figure 4. Shuttering is a concern in military target
`acquisition applications. A“rolling shutter” can start
`and stop exposure on a CMOS device, but the
`technique can result in a distorted image.
`
`tainability. Many CMOS start-
`ups are dedicated to high-vol-
`ume applications. Pursuing
`the highest-volume applica-
`tions from a small base of
`business has meant that
`these companies have had to
`price below their costs to win
`business in commodity mar-
`kets. Some start-ups will win
`and sustain these prices.
`Others will not and will have
`to raise prices. Still others will
`fail entirely.
`CMOS users must be aware
`of their suppliers’ profitabil-
`ity and cost structure to en-
`sure that the technology will
`be sustainable. The cus-
`tomer’s interest and the ven-
`ture capitalist’s interest are
`not well-aligned: Investors
`want highest return, even if
`that means highest risk,
`whereas customers need sta-
`bility because of the high cost
`of midstream system design
`change.
`Increasingly, money and
`talent are flowing to CMOS
`imaging,in large part because
`of the high-volume applica-
`tions enabled by the small
`imaging devices and the high
`digital processing speeds. Over
`time, CMOS imagers should
`be able to advance into
`higher-performance applications.
`For the moment, CCDs and CMOS
`remain complementary technologies
`— one can do things uniquely that
`the other cannot. Over time, this
`stark distinction will soften, with
`CMOS imagers consuming more and
`more of the CCD’s traditional appli-
`cations. But this process will take
`the better part of a decade — at the
`very least.
`G
`
`Meet the author
`Dave Litwiller is Vice President,
`Corporate Marketing at DALSA in
`Waterloo, Ontario, Canada.
`
`Which costs less?
`One of the biggest misunder-
`standings about image sensors
`is cost.
`Many early CMOS proponents
`argued that their technology
`would be vastly cheaper be-
`cause it could be manufactured
`on the same high-volume wafer pro-
`cessing lines as mainstream logic
`and memory devices. Had this as-
`sumption proved out, CMOS would
`be cheaper than CCDs.
`However, the accommodations re-
`quired for good electro-optical per-
`formance mean that CMOS imagers
`must be made on specialty, lower-
`volume, optically adapted mixed-sig-
`nal processes and production lines.
`This means that CMOS and CCD
`image sensors do not have signifi-
`cantly different costs when produced
`in similar volumes and with compa-
`rable cosmetic grading and silicon
`
`area. Both technologies offer appre-
`ciable volumes, but neither has such
`commanding dominance over the
`other to establish untouchable
`economies of scale.
`CMOS may be less expensive at
`the system level than CCD, when
`considering the cost of related cir-
`cuit functions such as timing gen-
`eration, biasing, analog signal pro-
`cessing, digitization, interface and
`feedback circuitry. But it is not
`cheaper at a component level for the
`pure image sensor function itself.
`The larger issue around pricing,
`particularly for CMOS users, is sus-
`
`DALSA is a leader in the design, development, manufacture, and sale of high-performance digital imaging
`solutions. DALSA’s image sensor chip and electronic camera products are based on core competencies in
`charge-coupled device (CCD) technology and CMOS imagers. DALSA sells to original equipment manu-
`facturers (OEMs) requiring high performance imaging products for their vision systems. Our products are
`high speed, high resolution and highly light sensitive. We serve markets in the United States, Europe, Japan
`and Asia. For more information contact us at sales@dalsa.com or visit our web site at www.dalsa.com.
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