LM9800 datasheet(29/34 Pages) NSC

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Applications Information (Continued)

In this case it is necessary to decrease the maximum ampli-

tude of the CCD output. Typically this is done by reducing the

light source intensity, or decreasing the light integration time

of the CCD.

If the minimum white level output voltage (the voltage from

the weakest pixels) is less than 0.6V, then a similar situation

occurs, as shown in

Figure 40.

In this case the amplitude for some of the weak pixels is too

low, and cannot be corrected for by the 6 dB PGA alone. The

amplitude of the CCD’s output signal must be increased.

This can be accomplished by increasing the intensity of the

light source, increasing the integration time of the CCD, or

using the LM9800’s VGA feature to add gain to the signal

prior to the PGA.

If the minimum white level is less than 0.6V and the maxi-

mum white level is greater than 1.2V, then the LM9800 can-

not correct for this amount of pixel-to-pixel variation — the

variation is greater than 6dB

(Figure 41). In this case the sys-

tem should be examined and steps taken to bring the varia-

tion within a 6dB window. Typically this can be done by using

more even illumination, higher quality CCDs, or better optics.

The coarse gain calibration procedure follows. In scanner

systems with low unit-to-unit variations in light intensity and

CCD efficiency, this calibration can be done once and fixed

for all the systems. Other systems, where the light intensity

varies from unit-to-unit or the pixel-to-pixel variation is very

close to 6dB, should do this procedure for every system, ei-

ther once during manufacture or as part of the calibration

routine the scanner does on power-up or before a scan.

Set the PGA gain to 0dB.

If using the VGA, turn it on and set the gain voltage to

provide the approximate gain required.

If not using the VGA, turn it off.

Scan a reference line corresponding to the maximum

white input the scanner should ever have to digitize.

Find the minimum and the maximum (in ADC LSBs) of

the valid pixels in the reference line.

The reference line should correspond to a desired out-

put code. For example, if the reference is 100% white,

the desired output code might be 240 LSBs. If the refer-

ence was 80% white (light grey), then the desired output

code might be (240)(0.8) = 192. If the maximum code

obtained after scanning the reference line is less than

the desired output code, and the minimum output code

is greater than half the desired output code, the signal is

in range. Go to section 6.3.

If the maximum code obtained after scanning the refer-

ence line is greater than the desired output code, and

the minimum output code is less than half the desired

output code, then there is more than 6dB of variation on

the input signal and the LM9800 will not be able to cor-

rect for the variations. The pixel-to-pixel variation must

be reduced to less than 6dB before the LM9800 can cor-

rect for variations.

If the maximum code obtained after scanning the refer-

ence line is greater than the desired output code for that

reference, the CCD output is too large. Reduce the light

intensity, shorten the integration time, or (if using the

VGA) reduce the voltage on the GAIN input. Go back to

Step 4.

If the minimum code obtained after scanning the refer-

ence line is smaller than half the desired output code for

that reference, the CCD output is too weak. Increase the

light intensity, lengthen the integration time, or (if using

the VGA) increase the voltage on the GAIN input. Go

back to Step 4.

6.3 PGA Correction Coefficients (Shading Calibration)

Once the input signal has been centered inside the range the

LM9800 can correct for, correction coefficients must be gen-

erated for each pixel to compensate for the gain error of that

pixel.

Set the PGA gain to 0dB.

If using the VGA, turn it on and set the gain voltage to

provide the required gain (determined by the procedure

in Section 6.2).

If not using the VGA, turn it off (set the VGA On/Off bit to

“0”).

Scan a reference line corresponding to all white or light

grey and store it in memory.

Calculate the required gain correction coefficients for

each pixel using the formula:

Where Uncorrected Coden is the ADC output code for pixel

n with the PGA gain = 0dB, Desired Code is the number that

corresponds to the desired output from the ADC with the

given reference line input, and Correction Coefficientn is

DS012498-45

FIGURE 39. CCD Input Signal Too Strong

DS012498-46

FIGURE 40. CCD Input Signal Too Weak

DS012498-47

FIGURE 41. CCD Input Signal Range Too Wide

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