|Number of Channels||3||3||3||1||1||1|
|Sampling Rate, MHz||6||6||6||21||18||18|
|Differential nonlinearity, LSBs||0.75 max||1.0 max||0.5 max||0.5 typ||0.5 typ||0.5 typ|
|No Missing Codes||Guaranteed||Guaranteed||Guaranteed||Guaranteed||Guaranteed||Guaranteed|
|Output Noise, rms (LSBs)||0.3||0.5||0.1||0.85||0.85||0.85|
|Internal voltage reference||Yes||Yes||Yes||Yes||Yes||Yes|
|Supply Voltage, V||+5||+5||+5||+3||+3||+3|
|Price, USD (1000s)||$25||$9.50||$9.50||**||$5.95||$8.50|
For scanner applications, the AD9807 and AD9805 (see Table) were introduced in late 1996. These devices feature three input channels for processing color linear CCDs, with input clamping, CDS, offset control, PGA, and a 12- or 10-bit ADC. Additional operating modes allow direct connection with contact image sensors (CIS), another type of image sensor that is gaining popularity. The latest product in this series is the AD9816 (Figure 3). This second-generation product functions like the AD9807, but it is housed in a smaller package and costs less.
Figure 3. The AD9816 features 3-channel simultaneous sampling, individual per-channel gain and offset adjustment, internal voltage reference, and a 6-MHz, 12-bit A/D converter. The on-board registers are programmed using a 3-wire serial interface.
For digital still camera (DSC) designs, the AD9801 was introduced in early 1997. Though it includes the same basic functions as the AD9807 family, it is tailored for use with area CCD arrays. A single-channel, 18-MHz architecture is used, with a 30-dB programmable gain amplifier, black level clamp loop, and 10-bit ADC. The input range is smaller, to accommodate the lower output voltages of area CCDs, and the programmable gain range is wider in order to be compatible with the broad range of lighting conditions in which a camera is used (scanners operate under more uniform lighting conditions). Battery operation demands lower power, so the AD9801 operates from a single 3-volt supply.
The AD9802, introduced in the fall of 1997, is intended to be used for both DSC and camcorder designs. Shown in Figure 4, the AD9802 has the features of the AD9801, and also includes a multiplexed direct input to the 10-bit ADC. A direct ADC input is required in camcorder applications, to digitize analog video signals from a tape or external VCR. The AD9803, now being sampled (at this writing), adds a serial digital interface for programming the internal registers--and features a higher sampling rate.
Two important characteristics of especial interest in imaging applications are noise and nonlinearity.
Noise in the AFE consists of wideband noise from all of the analog circuitry, wideband noise from the ADC, and quantization noise from the ADC. Stand-alone A/D converters usually specify a signal-to-noise ratio (SNR) or signal-to-noise-and-distortion (SINAD), but these types of measurements are not entirely useful in imaging applications. Converter SINAD is tested with a sine-wave input, and includes the effects of distortion of the analog signal, converter distortion due to integral and differential nonlinearity (INL and DNL), quantization noise, and thermal noise. In some cases, to reduce the contribution of thermal noise, multiple data records are averaged.
The distortion numbers are not of interest in imaging applications because CCD signals are not sinusoidal in nature, and the front-end of the ADC samples the CCD signal only during a relatively slow-moving portion of the waveform. Instead of using a traditional converter SNR measurement, CCD system designers consider the contributions from wideband noise, quantization noise, and DNL errors. Wideband noise can be measured using a "grounded-input histogram" test, in which the inputs to the device are grounded, and a histogram is taken of the output data. The standard deviation of the histogram will give the rms noise level of the device (not including the ADC quantization noise). A low-noise AFE can have a thermal noise level comparable to or less than the rms quantization noise of its on-board ADC.
AFE noise is important because of its impact on the system's dynamic range. Dynamic range is determined by comparing the maximum signal that can be processed to the minimum signal level that can be resolved in the system. Noise from the CCD and from the AFE (which includes the analog signal processing and A/D converter) will contribute to overall system noise level. The CCD random noise is usually specified by the CCD manufacturer as "noise floor" or "random noise" in mV or electrons rms; the kT/C and 1/f noise contributions will be reduced by the CDS. Fixed pattern noise due to variations in the dark current of each pixel can be very objectionable in images and should be included in the noise calculation if it is not reduced through calibration techniques. Noise will also be introduced by the amplifier used to buffer the CCD's output signal, though this can be minimized by amplifier choice and circuit techniques. The noise contribution from the AFE can be found on the product's data sheet, or measured using the grounded input histogram test. The ADC's resolution will determine the quantization noise level, which is calculated by dividing the weight of one LSB by square root of 12. Adding all the noise sources in a given bandwidth (referred to the same point in the signal chain) by root-sum-of-squares gives:
This equation can be used in approximating the achievable dynamic range, to see if the AFE being considered is a good match for the CCD. If the largest noise source is three times the next largest, it will be dominant. Understanding which noise sources are dominant will help in the selection of an appropriate AFE.
The AFE's linearity will also affect system performance. The nonlinearities of a real ADC can cause artifacts in the digitized image. Differential nonlinearity (DNL) is very important, because the human visual system is good at detecting edges or discontinuities in an image. DNL is the variation in code width for the ADC, with poor DNL causing uneven gradations or "steps" in adjacent luminosity levels. A true 10-bit system demands DNL of better than 1 LSB at the 10-bit level (0.5 LSB is preferable) to avoid degradation of image quality. DNL that is poor enough to cause missing codes can cause image artifacts in the digital processing. Integral nonlinearity (INL) is also important, but a given amount is less perceptible than a comparable amount of DNL. The human visual system is less adept at distinguishing gradual nonlinearity which is spread out over the entire grey-scale range. However, large INL can contribute to errors in the color processing algorithms of a particular system, resulting in color-related artifacts in the image.
Although the integrated approach does not have the advantage of allowing each separate processing stage to be evaluated, the AFE can be thoroughly evaluated under the operating conditions of a specific application. Evaluation boards, conveniently available for the AD980x family, simplify this step of the design.
Integration road map: Increased scope of on-chip integration for decreased size and cost is becoming a way of life in systems-on-a chip development. Now that good analog performance is possible with standard CMOS processes, it should become feasible to integrate some or all of the back-end digital processing of the imaging system onto a single chip to meet the needs of a specific application. Indeed, Analog Devices is currently producing an ASIC to meet the needs of a major scanner manufacturer for a chip that successfully integrates the AFE, digital image processing, SRAM, timing generation, CPU, and SCSI/EPP interfaces on a single chip. At this level of complexity, power and ground management on the chip is critical to minimize coupling of digital noise into the analog circuitry. Because of the large driver currents required, the solution to the problem of including the SCSI interface on-chip has been an especially challenging exercise.