Imaging and video systems mixed-signal design simplified

Electronic News, August 12, 1996 by Raphael Horton

The rapidly growing and combining markets of television, movies, personal computers and the Internet fuel an increasing demand for cost effective and accurate imaging and video systems. These systems fulfill consumer expectations for highly graphical user interfaces and virtual environments.

Many new products bring the visual world into our television and personal computers, including scanners, digital cameras, video teleconferencing, video camcorders and video capture boards. The drive to make these products affordable to a large consumer base is a strong function of integrated mixed-signal design.

Exar, a supplier of mixed-signal semiconductor products for the document imaging industry, has a range of products for applications from imaging to video. Most imaging and video applications use a transducer called a CCD, charge coupled device, that converts light to electronic signals. Exar's mixed-signal expertise includes the conditioning and quantization of CCD signals using CMOS technology. Conceptually, the typical CCD signal path of a scanner, video camera or digital camera application remains relatively constant from imaging to video, while speeds and resolutions vary by application.

The application of a digital still camera follows the typical circuit layout with the exception of a novel imaging subsystem, the MP8831, developed at Exar. Digital still cameras capture images using a CCD and store them digitally in memory instead of using film. In a stand-alone system like a digital still camera, the captured images are manipulated and stored by the camera's electronics. This is unlike a scanner that can rely on the processing power of the attached computer CPU and memory.

Using the digitally controlled references in the MP8831, the designer can avoid some DSP post processing reducing the size of the digital application-specific integrated circuit (ASIC). For example, light level adjustments of the CCD image can be made during the analog-to-digital conversion (ADC) using the digitally controlled gain adjustment digital-to-analog conversion (DAC). The MP8831 can be used to adjust the gain and offset pixel-by-pixel at the full rated speed of the converter. The pixel-by-pixel adjustment capability allows for separate control of each pixel color.

The single-channel XRD4411 or dual XRD4412 provide CDS (Correlated Double Sampling) and PGA (Programmable Gain Amplification) to match the MP8831 input range for a digital still camera. The XRD4411 and XRD44112 incorporate several features to aid in the design of the overall system. The analog path is differential relative to the positive power supply (VDD) in order to provide a means for coupling and rejecting the power supply noise of the CCD.

VDD for the XRD4411/12 should be coupled to and/or generated from the CCD power supply. These parts also offer two timing modes. A line mode samples the black level at the beginning of a line only. The second mode updates the black level at every pixel for greater accuracy. The combination of these two mixed-signal CMOS circuits provides a highly integrated two-chip solution for low-cost and portable power-sensitive designs.

If a complete one-chip CMOS solution is needed for the CCD analog signal path of a scanner, then use the XRD4417. The XRD4417 is a complete CCD digitizing subsystem running off a single 5V power supply. It includes a flexible correlated double sampling circuit, a 6-bit programmable gain amplifier, a 10-bit ADC, a 6-bit utility DAC and a single bidirectional three wire serial interface. It is manufactured on a state-of-the-art, submicron CMOS process for high speed, low noise and low power. The XRD4417 provides all of the functions required to digitize the output of a single-line CCD.

Exar's position in the document imaging industry has resulted in many pioneering mixed-signal and data converter designs. PC peripherals that use these designs include flatbed scanners, sheet-fed scanners, business card scanners, photo and film scanners and hand-held scanners.

The MP8830, XRD4433 and MP8832 are imaging subsystems that quantize CCD signals pixel-by-pixel using a novel approach with digitally controlled references.

The RGB color signals from a CCD are digitized and adjusted on a pixel-by-pixel basis in the simultaneous sampling triple MP8830. Digitally controlling the gain and offset references in these imaging subsystems allows designers to counteract the effects of lamp and CCD photo response non-uniformity. The resolution of the systems varies from the 8-bit output of the MP8832 to 10 bits of the MP8831, MP8830 and XRD4433. While the MP8830 converts at 1.25 MSPS, page scan rates are increased by the 3-MSPS conversion of the XRD4433.

The MP8832 provides gamma correction and pixel-by-pixel adjustment at the quantization process saving time and processing steps. The proprietary internal gamma correction circuit adjusts the transfer function of the ADC to provide an eight-segment approximation of a gamma function. The result is a gamma corrected 8-bit output with near 12-bit resolution in the dark region that requires no postprocessing for gamma.