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Scanner front end does 40MS/s conversion

Posted: 17 May 2004 ?? ?Print Version ?Bookmark and Share

Keywords:scanner? flatbed? peripheral? wolfson? digital?

Flatbed scanners and multifunction peripherals appear to be settling out a bit on spatial and density resolution. But page-scanning speed is still a competitive issue in the end market, as is price. This competitive pressure is reflected in a new scanner analog front-end (AFE) chip - the interface between the CCD or CMOS image sensor and the digital electronics - from Wolfson Microelectronics plc.

The WM8214 is an integration of programmable-gain amplifier, analog signal-conditioning, sample/hold (S/H) and ADC circuits into a single 3.3V mixed-signal IC. The part is designed for a maximum conversion rate of 40MS/s, and has three analog input channels. It can be used in one-, two- or three-channel mode, conveniently serving the needs of both monochrome and color scanners.

Producing a 40MS/s, 16bit A/D for the part is a non-trivial exercise in itself, but that is only a portion of the interest in the 8214 design, said design project leader Ross Arnott. Each channel includes a programmable-gain stage, he said, allowing the chip to balance the signal levels coming from three color sensors before the signals get into the conversion process. The programmability also allows the AFE to compensate for issues such as lamp fade over the longer term. "Normally, the scanner would be designed to scan a reference white strip before each page to provide calibration levels," Arnott said. "Then the digital control chip would work out the best gain for each channel and set up the AFE for the page."

Another wrinkle comes from the way CCD sensors output their data. Rather than a string of analog voltages and a clock, many CCDs send out a string of AM pulses in which each pixel is represented by a pair of pulses - one for a reference voltage and one for the video level. The actual video level is then the difference between the amplitudes of the two.

That creates a number of issues, particularly as sampling rates increase. Accurate clocking is a must, as it is important to trigger the S/H circuit near the center of each pulse. Jitter or drift between the reference sample clock and the video sample clock can result in noise. To simplify this problem for the system designer, Wolfson has moved from a 2x clock input in previous chips to a 1x input - the designer need now provide the clock only at the sampling rate, not twice the sampling rate.

The WM8214 is already available. It consumes about 400mW at 3.3V, and is packaged in a 28-pin SSOP. The price is $3.49 each in thousands.

- Ron Wilson

EE Times

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