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Flexible timing key to CCD design

Posted: 16 Oct 2002 ?? ?Print Version ?Bookmark and Share

Keywords:timing generator? ccd design? signal conditioning? clock pulses? consumer imaging?

Charge-coupled devices (CCDs) are the image sensor of choice for digital imaging applications. The exploding demand for products like digital still cameras and camcorders has made the CCD familiar to everyday consumers. The volume of digital still cameras has been growing at the rate of more than 40 percent a year, with total shipments expected to reach approximately 20 million units this year. This staggering volume has contributed to a decrease in the average price of a CCD, making digital-imaging products more affordable. But it has also increased the challenge for the designer to develop a product that distinguishes itself from the myriad others in the market.

Compounding this challenge, much of a CCD sensor's functionality and performance depend wholly on another component--the timing generator, which produces a variety of clock pulses. The pulses drive charges accumulated in the CCD pixel array to a signal-conditioning circuit, where they can be digitized and processed into an image. The clock pulses determine how the image is acquired and reconstructed (as a still or moving image), so the timing generator is a key component in the imaging signal chain.

Whether the CCD is used in consumer, scientific or industrial imaging fields, the market dictates the resolution and performance needed by the CCD in the application. As such, the designer has a relatively limited selection of CCD imagers from which to choose. Also, since competitors are likely to use similar or identical sensors, the innovative designer has almost no flexibility in differentiating his design through the CCD if the accompanying timing generator functionality is fixed. In other words, the designer is limited to what he can actually do with the CCD as an image sensor.

Several other issues exist with the fixed-function, discrete timing generator. In many cases, the designer may have to introduce a rapid succession of products in series, each with a different CCD. And, it would help to use a standardized design, with the same components and PCB layout as much as possible. With fixed-function timing generators, standardization of the AFE portion of the design would not be possible because a different timing generator would be needed for each CCD.

The discrete timing generator, separated from the analog front-end and CCD, produces high-speed clocks that increase the likelihood of digital crosstalk and create EMI shielding issues in sensitive image systems.

The fixed-function timing generator typically is the same size as the AFE, both frequently packaged in 48-pin TQFPs. In space-sensitive applications, these two chips would take up a lot of precious board space.

Fixed-function timing generators are susceptible to temperature-induced timing variability, which adversely affects image quality by the subtle shift in the edges of sampling clocks. This can be unacceptable in many situations, since users expect higher resolution and performance imaging systems that are stable over a wide range of environmental conditions. In all of those cases, resolving the issues results in time-consuming changes to the system design and layout. Those changes subsequently lead to rising development costs and delays in product introduction.

An integrated, programmable timing generator can resolve all of the problems. It is flexible enough for the designer to configure the timing specifications to the type of CCD the application requires. That lets the designer program specific sequences that were not present on the standard, fixed-function timing generator, which in turn, allows innovative functions not previously imagined and maximizes the overall performance of the CCD. Thus, the designer can easily differentiate an application, even if the same image sensor was used in a competing product.

In addition, a programmable timing generator allows CCD imagers of differing resolutions to be obtained from a variety of manufacturers and used with the same PCB, since the timing generator can be configured via software changes, to the timing specification that the CCD requires. That greatly reduces development time and expense, since much of the work needed to lay out and build the circuit boards with a discrete, fixed-function timing generator is eliminated.

- Yuzo Shida

Product Marketing Manager, High-Speed Converters

Analog Devices Inc.

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