Analog front-ends ease camera design
Designed to ease the development of high-performance cameras, the AD984X family of analog front-end (AFE) ICs from Analog Devices Inc. (ADI; Norwood, MA) reduces the amount of glue logic required to interface to CCD and CMOS sensors. The first member of the family, the AD9840, is a 10-bit, 36-MSPS (million samples per second) device designed to support large CCD pixel arrays. Last December, ADI released two other devices—the AD9843 and the AD9844. The AD9843 is a 10-bit, 18-MSPS part and produces one-fourth the noise and consumes one-third the power of the previous generation AD9803. Also an 18-MSPS AFE, the AD9844 increases resolution to 12 bits.
"Despite the usefulness of such parts as the AD9844 in monochrome camera designs," says Paul Chatwell, president of Losp GmbH (Emmendingen, Germany), "camera designers require an on-board pixel gain amplifier in such AFEs." With this on-chip amplifier, pixel sensitivity can be changed at pixel rates. This is especially useful in color camera designs where the gain of the blue-striped pixels is lower than that of either the red or green channels.
Fortunately for Chatwell, ADI plans to introduce such a device, the AD9842, this month. Losp will use this part, which includes a 12-bit, 18-MHz, on-chip pixel gain amplifier, in a series of cameras based on several megapixel CCDs from Sony Semiconductor (San Jose, CA). Currently, Chatwell is developing several cameras based on devices that include the ICX2025AL and ICX085AL 11-mm, interline CCD image sensors. While both devices are progressive interline devices with a 2/3-in. sensor, the ICX2025AL and ICX085AL feature 1360 x 1024-pixel and 1360 x 1034-pixel resolutions, respectively.
"Using a clock rate of 14.31 MHz, the ICX205AL, when interfaced to ADI's AD9842 AFE, allows all the pixels' signals to be output in approximately 1/7.5 s," says Chatwell. "Interestingly," he adds, "the ICX085AL can be clocked at 20.25 MHz, allowing all pixels to be output independently at an even faster rate—1/12 s."
At this time, such designs are limited by the speed of the AFE. "We could build such an interface with off-the-shelf analog-to-digital converters and antialiasing filters, " says Chatwell. "But such designs would be more susceptible to noise," he says. Although at the present time the fastest 12-bit AFE devices clock at 18 MHz, Analog Devices is developing a higher-speed true 12-bit AFE that will enable designers such as Chatwell to optimize their higher-end designs.
To support its camera development, Losp plans to introduce a low-voltage differential signaling (LVDS) interface using LVDS drivers from National Semiconductor (Santa Clara, CA). This 12-bit interface will allow the camera to be placed at 10 to 15 m from the host frame grabber, making the subsystem suitable for machine-vision applications. To support this interface, Losp also plans to introduce an LVDS PCI frame grabber and driver software for OEMs and systems integrators later this year.
—ANDREW WILSON