LM9801 datasheet(25/34 Pages) NSC

25 / 34 page

Applications Information (Continued)

52 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 28 (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

TLH12814 – 34

FIGURE 7 CCD Input Signal In Range

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 1VV 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

TLH12814 – 35

FIGURE 8 VGA Calibration Flowchart

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LM9801 Datasheet(PDF) 25 Page - National Semiconductor (TI)