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參數(shù)資料
| 型號： | LM9800 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 8-Bit Greyscale/24-Bit Color Linear CCD Sensor Processor |
| 中文描述： | 8位Greyscale/24-Bit彩色線陣CCD傳感器處理器 |
| 文件頁數(shù)： | 29/34頁 |
| 文件大?。?/td> | 619K |
| 代理商： | LM9800 |
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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 PGAalone. 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.
1.
Set the PGA gain to 0dB.
2.
If using the VGA, turn it on and set the gain voltage to
provide the approximate gain required.
3.
If not using the VGA, turn it off.
4.
Scan a reference line corresponding to the maximum
white input the scanner should ever have to digitize.
5.
Find the minimum and the maximum (in ADC LSBs) of
the valid pixels in the reference line.
6.
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.
7.
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.
8.
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.
9.
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.
1.
Set the PGA gain to 0dB.
2.
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).
3.
If not using the VGA, turn it off (set the VGA On/Off bit to
“0”).
4.
Scan a reference line corresponding to all white or light
grey and store it in memory.
5.
Calculate the required gain correction coefficients for
each pixel using the formula:
Where
Uncorrected Code
is the ADC output code for pixel
nwith the PGAgain=0dB,
Desired Code
is the number that
corresponds to the desired output from the ADC with the
given reference line input, and
Correction Coefficient
n
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
29
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相關(guān)代理商/技術(shù)參數(shù) |
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| LM9800CCV | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:8-Bit Greyscale/24-Bit Color Linear CCD Sensor Processor |
| LM9800CCVF | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Special Function Video Processor |
| LM9801 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Greyscale/24-Bit Color Linear CCD Sensor Processor |
| LM9801CCV | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Greyscale/24-Bit Color Linear CCD Sensor Processor |
| LM9801CCVF | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Greyscale/24-Bit Color Linear CCD Sensor Processor |
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