LM9801CCV NSC [National Semiconductor], LM9801CCV Datasheet - Page 25

• Manufacturer Part Number: LM9801CCV
• Description: Greyscale/24-Bit Color Linear CCD Sensor Processor
• Manufacturer: NSC [National Semiconductor]

Applications Information
5 2 Coarse Gain Calibration
The LM9801’s PGA corrects for up to 9 dB of varIation in
the CCD output signal’s white level intensity That 9 dB
range has to be centered inside the 9 dB window of correc-
tion as shown in Figure 7 The window’s upper limit is deter-
mined by the Target code and the lower limit by the Target
code divided by 2 8 (this corresponds to the minimum gain
range of the PGA) To allow proper calibration the ampli-
tude of all the pixels in the CCD should be inside this range
when those pixels are scanning an image corresponding to
the Target code The placement of the pixels inside the 9
dB window can be controlled by any of three ways chang-
ing the gain of the VGA changing the integration time of the
CCD or changing the intensity of the light source
In most designs the output waveform of the CCD can be
brought into the 9 dB correction range of the PGA by adjust-
ing the gain of the VGA This is the next step in system
calibration
(Continued)
FIGURE 8 VGA Calibration Flowchart
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Figure 8 is a flowchart of one technique to find the optimum
VGA gain setting during calibration Calibration begins with
a VGA gain setting of 1V V and increments the VGA gain
until one of the four possible results occur Result 1 is the
desired outcome where the signal falls into the range
shown in Figure 7 and the VGA calibration has been suc-
cessful
FIGURE 7 CCD Input Signal In Range
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