LM9831CCVJDX datasheet(21/41 Pages) NSC

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Applications Information

1.0 OVERVIEW

The LM9831 is a USB, 1200dpi, 14 bit (42 bit color) scanner-on-

a-chip.

2.0 ANALOG SIGNAL PROCESSING

One channel of the LM9831’s analog front end is shown in Figure

3. The gain through each channel can be set between 0.93V/V

and 9.0V/V using registers 3B, 3C, and 3D. The offset DAC pro-

vides up to ±278mV of offset correction using registers 38, 39,

and 3A. The offset DAC and gain stages should be adjusted dur-

ing coarse calibration so that the input signal is a maximum of

1.9Vp-p at the ADC input.

3.0 DIGITAL SIGNAL PROCESSING

3.1 ADC

The digital pixel data comes from a 6MHz 14 bit pipelined ADC.

The output data is formatted as a 16 bit word. The pixel data is in

the upper 14 bits, and the lower two bits are set to 0.

3.2 Pixel Processing Block

The Pixel Processing stage is used to digitally reduce the optical

resolution of the sensor. The optical resolution can be reduced by

a factor of 1, 1.5, 2, 3, 4, 6, 8, or 12. For a 1200 dpi (optical) sys-

tem, this would produce resolutions of 1200, 800, 600, 400, 300,

200, 150, and 100. A 600 dpi (optical) system would be capable

of 600, 400, 300, 200, 150, 100, 75, and 50 dpi. (Resolution in the

vertical direction is controlled by the stepper motor speed.)

Horizontal resolution reduction is accomplished by averaging

adjacent pixels. Averaging produces better image quality and

reduces aliasing versus the traditional technique of simply dis-

carding pixels to reduce resolution. For example, to get 100 dpi

from a 300dpi optical sensor, you would average 3 300dpi pixels:

The number of pixels coming out of the Pixel Processing block is

equal to the integer portion of the number of pixels going in to the

Pixel Processing block divided by the “Divide By” setting, from the

table shown in Figure 4.

This equation also applies to the divide by 1.5 function.

If there are not enough pixels at the end of a line to form a com-

plete pixel, the last pixel will be eliminated. For example, if a line

is 35 pixels wide and the Horizontal DPI setting is set to divide by

6, then the output of the Pixel Processing block will be 5 pixels

(the integer portion of 35/6). The last 5 pixels will be discarded,

since 6 pixels would be required to form a new pixel in this mode.

The output of this stage is sent to the Pixel Rate Offset Correction

Block.

3.3 Pixel Rate Offset Correction Block

Offset correction words for every pixel of the CCD are stored in

the external DRAM and accessed at the pixel rate. A digital sub-

tractor subtracts the 16 bit offset word (corresponding to that

pixel’s offset error) from each pixel.

Figure 3: Analog Front End (AFE) Model

VDAC

DAC

Offset

GPGA

14 Bit

ADC

VOS3

VOS2

DOUT

VOS1

VIN

Gain Boost

1V/V or

3V/V

PGA

0.93V/V to

3V/V

DOUT = (((VIN + VOS1)GB + VDAC + VOS2)GPGA + VOS3)C

simplified, with all offsets = 0, this is:

DOUT = (VINGB + VDAC)GPGAC

C is a constant that combines the gain error through the AFE, reference voltage variance, and analog voltage

to digital code conversion into one constant. Ideally, C = 8192 codes/V (16384codes/1.9V).

Manufacturing tolerances widen the range of C. See Electrical Specifications.

pixel

100dpi

n-2

n-1

--------------------------------------------

Divide

DPI

(1200

DPI

system)

DPI

(800

DPI

system)

DPI

(600

DPI

system)

DPI

(300

DPI

system)

1200

800

600

300

1.5

800

533

400

200

600

400

300

150

400

267

200

100

300

200

150

200

133

100

150

100

37.5

100

Figure 4: Decreasing Horizontal Resolution

Pixels

OUT

INT

Pixels

Divide By

-------------------------





LM9831CCVJDX Datasheet(PDF) 21 Page - National Semiconductor (TI)

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