LM9801CCV NSC [National Semiconductor], LM9801CCV Datasheet - Page 24

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

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tion finds the optimum gain setting that places the output
Coefficients (Shading Calibration) describes how to calcu-
Applications Information
4 9 ADC
The ADC converts the normalized analog output signal to
an 8-bit digital code The EOC output goes from high to low
to indicate that a new conversion is ready ADC data can be
latched by external memory on the rising edge of EOC The
RD input takes the ADC’s output buffer in and out of
TRI-STATE RD may be tied to EOC in many applications
putting the data on the bus only when EOC is low and al-
lowing other data on the bus (such as CD0–CD7 correction
data) at other times In this way the output data and correc-
tion coefficient data can share the same databus (see Dia-
gram 12)
5 0 CALIBRATION
Calibration of a CCD scanner is done to normalize the pixels
of a linear CCD so that each pixel produces the same digital
output code at the output of the scanner when presented
with the same image light intensity This intensity ranges
from black (no light) to white (maximum light intensity) The
CCD’s analog output may have large pixel-to-pixel DC off-
sets (corresponding to errors on black signals) and pixel-to-
pixel variations in their output voltage given the same white
image (corresponding to errors on brighter signals) If these
offsets are subtracted from each pixel and each pixel is
given its own gain setting to correct for different efficiencies
then these errors can be eliminated
Ideally the digital output code for any pixel would be zero for
a black image and some code near fullscale for an image
with maximum brightness For an 8-bit system like the
LM9801 that code might be 250 This code will be called
the Target Code
The LM9801 eliminates these global and pixel-to-pixel off-
set and gain errors with its Correlated Double Sampling
(CDS) Offset DACs Variable Gain Amplifier and pixel-rate
Programmable Gain Amplifier This section describes how
to program the LM9801 and the coefficient RAM being used
with it to eliminate these errors
Calibration of a LM9801-based system requires 3 steps
The first described in Section 5 1 Offset Calibration takes
a black image and normalizes the digital output code for
each pixel to a code at or near 0
The second step Section 5 2 Coarse Gain (VGA) Calibra-
voltage of all the pixels within the 9 dB adjustment range of
the PGA
The final step described in Section 5 3 PGA Correction
late the gain required to normalize the output of each pixel
to the desired output code (the Target code)
5 1 Offset Calibration
This procedure corrects for static offsets generated by the
CCD and the LM9801 Because the LM9801 uses CDS to
eliminate the pixel-to-pixel offset errors of the CCD no pix-
el-rate offset correction is required
To use the Offset DAC and Offset Add bit for offset correc-
tion the offset errors (V
mined as shown in Figure 6 This is done be measuring the
voltage at the PGA output using the ADC with a black im-
age on the CCD (a black image can usually be created sim-
www national com
OS1
and V
OS2
) must first be deter-
(Continued)
24
ply by turning off the scanner’s illumination) If this voltage is
known with a PGA gain of 1 00V V (0 dB) and 2 95V V
(9 dB) then the offset errors (V
mined from the following two equations
V
V
Solving for V
These equations were used to produce this procedure for
cancelling the LM9801’s offset errors Please note that all
voltages and measurements are in units of ADC LSBs to
simplify calibration
10 Calculate V
11 Program the Offset DAC register using the formula
12 If 3V
13 The final value of the offset present at the ADC input
ADC1
ADC2
1 Set the VGA Gain to 1V V (VGA code
2 Set the Offset DAC (V
3 Set the Offset Add bit (V
4 Set the PGA Gain to 1V V (PGA code
5 Digitize a black line
6 Calculate the average (in ADC LSBs) of all the valid
7 Set the PGA Gain to 2 95V V (PGA code
8 Digitize a black line
9 Calculate the average (in ADC LSBs) of all the valid
V
V
OS1
OS2
V
Offset DAC code
(Note This calculation can be done as
15
(
therefore measurable by the ADC
pixels in the black line and store that number as V
pixels in the black line and store that number as V
e
e
for ease of programming in 8-bit microcontrollers)
If 3V
can be used for the shading calibration calculations
Calculate the final value of the ADC input offset
(V
V
V
a
OS1
OFFSET
OFFSET
OFFSET
e
e
(6 3
15
6 3 LSBs) to ensure the total offset is positive and
a
e
e
ADC1 l
ADC1 k
(PGA gain
(PGA gain
e
1(V
2 95(V
39(V
a
(V
(2 95V
a
OS1
ADC2
(V
1 22(V
) using
e
e
OS1
FIGURE 6 Offset Calibration
(V
ADC1
ADC2
OS1
(3V
(if the Offset Add bit is 0) or
(3V
(if the Offset Add bit is 1)
OS1
ADC1
and V
V
V
ADC1
a
– V
ADC2
ADC2
ADC1
ADC1
ADC1
–V
– V
V
ADC1
e
a
e
e
–V
DAC1
–V
ADC2
OS2
ADC1
1)
2 95)
V
-(V
ADC2
– V
then set the Offset Add bit to 0
set the Offset Add bit to 1
– V
– V
ADC2
DAC1
) 1 95– V
)
OS1
ADC2
ADC2
ADC2
) 32
) 1 95 – 6 3
DAC1
DAC2
a
) 1 95)(15 6 3)
OS1
) 1 95– V
)
)(15 6 3)
V
a
)
) 2
) 2
OS2
) to its maximum value
) to 0
and V
V
DAC1
OS2
a
a
DAC2
2
OS2
V
a
DAC2
e
e
) can be deter-
V
DAC2
0)
0 LSBs)
e
TL H 12814 – 33
255)
ADC1
ADC2