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• Set the Multiplier Gain Range to 1:3 (register 9, bits 2 and 3 =

00)

4.2 Setting the Static Offset DACs

• Scan one line of a pure black test pattern, or scan one line with

the light source turned off. (Caution: turning the light source on

and off may cause thermal drift in light intensity as the light

source warms up and cools down.) Scanning multiple black

lines and averaging each pixel from each line will reduce noise

and increase the accuracy of this step.

• Find the minimum pixel value of the scanned black image

(BlackMIN) for each color (Figure 8).

• Reduce the Offset DAC value for each color so that the

minimum black pixel is close to 0, but still greater than 0

(Figure 9). One offset DAC code = ~4.2 ADC LSBs, so change

the DAC code to (31 - BlackMIN/4.2).

• Because the above equation is based on typical LM9812

performance, the new DAC settings should be verified. Scan

another black line(s) with the new DAC values and verify that

all the pixels are still above 0. If any pixels are equal to 0,

increment the DAC value by 1. If the minimum pixel is above 8,

consider decrementing the DAC value by 1. Repeat this step

until the minimum pixel value of the scanned black image

(BlackMIN) for each color is greater than 0 but less than 9.

BlackMIN values greater than 9 are acceptable (they will be

removed by the Pixel Rate Offset Subtractor), but will reduce

the dynamic range of the system.

4.3 Setting the Static PGAs

• Decide on the Target Code for your scanner. This is the

maximum output code from the ADC for a pure white image.

This code may be as large as 1023, but this value could cause

clipping if the light source drifts and becomes brighter after

calibration. A value of 1000 allows for moderate light source

drift between calibration and image scan.

• Scan one line of a pure white test pattern. Scanning multiple

white lines and averaging each pixel from each line will reduce

noise and increase the accuracy of this step.

• Find the maximum white pixel value of the image data

(WhiteMAX) for each color (Figure 10).

• Program the PGA value for each color so that the maximum

white pixel is close to, but still less than the Target Code

(Figure 11). The PGA gain required to meet this condition is

equal to the Target Code divided by WhiteMAX. Using the PGA

gain equation (Equation 1), the PGA code required is:

If the Blue channel requires a gain greater than 3 (Target

Code/WhiteMAX is >3), then set register 2, bit 7=1 and use

Equation 14:

• Program the calculated PGA values for each color into

configuration registers 3, 4, and 5.

• Because the above equations are based on typical LM9812

performance, the new PGA settings should be verified. Scan

another white line(s) with the new PGA values and recalculate

WhiteMAX and WhiteMIN (the minimum ADC output code for the

white image) for each color. Verify that WhiteMAX is near but

still below the Target Code. If WhiteMAX is greater than or

equal to the target code, decrement that PGA value by 1.

•

4.4 Determining the Multiplier Correction Range Setting

• For best performance, repeat Steps 4.2 and 4.3 before

proceeding. This will maximize the available dynamic range of

the ADC and ensure that all the pixels are in range.

• Digitize another white line (or multiple lines) and calculate the

minimum and maximum pixels from that line (WhiteMIN and

WhiteMAX). Verify that WhiteMAX is less than the Target Code.

• The correction range required (Range) = WhiteMAX/WhiteMIN

• If Range < 1.5, then set the Multiplier Range bits (register 9,

bits 3 and 2) to 10 (33%).

Data Pixels

000

Figure 8: Finding BlackMIN

BlackMIN

008

010

ADC

Output

Code

004

00C

014

018

Data Pixels

000

Figure 9: Setting the Static Offset DACs

BlackMIN = 13:

too high

008

010

ADC

Output

Code

004

00C

014

018

BlackMIN = 6:

correct

BlackMIN = 0:

too low

Data Pixels

Figure 10: Finding WhiteMAX

Target Code

ADC

Output

Code

WhiteMAX

Data Pixels

Figure 11: CCD Input Signal In Range

Target Code

ADC

Output

Code

gain too high

gain too low

gain correct

PGA code

Integer

Target Code

White

MAX

---------------------------------0.94

–



 32

2.137

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





Equation 13: PGA Code Calculation

PGA code

Integer

Target Code

White

MAX

---------------------------------2.82

–



 32

6.411

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





Equation 14: Blue PGA Code Calculation, x3 Bit Set

Data Pixels

Figure 12: Determining Correction Range Setting

Target Code

ADC

Output

Code

WhiteMAX

WhiteMIN

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