LM9801 datasheet(24/34 Pages) NSC

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Applications Information (Continued)

49 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)

50 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 51

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 52

Coarse Gain (VGA) Calibra-

tion finds the optimum gain setting that places the output

voltage of all the pixels within the 9 dB adjustment range of

the PGA

The final step described in Section 53

PGA Correction

Coefficients (Shading Calibration) describes how to calcu-

late the gain required to normalize the output of each pixel

to the desired output code (the Target code)

51 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 (VOS1 and VOS2) must first be deter-

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-

TLH12814 – 33

FIGURE 6 Offset Calibration

ply by turning off the scanner’s illumination) If this voltage is

known with a PGA gain of 100VV (0 dB) and 295VV

(9 dB) then the offset errors (VOS1 and VOS2) can be deter-

mined from the following two equations

VADC1 e 1(VOS1 a VDAC1) a VOS2 a VDAC2

(PGA gain e 1)

VADC2 e 295(VOS1 a VDAC1) a VOS2 a VDAC2

(PGA gain e 295)

Solving for VOS1 and VOS2

VOS1 e (VADC2–VADC1)195–VDAC1

VOS2 e (295VADC1–VADC2)195–VDAC2

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

1 Set the VGA Gain to 1VV (VGA code e 0 LSBs)

2 Set the Offset DAC (VDAC1) to its maximum value

(a63 LSBs) to ensure the total offset is positive and

therefore measurable by the ADC

3 Set the Offset Add bit (VDAC2)to0

4 Set the PGA Gain to 1VV (PGA code e 0)

5 Digitize a black line

6 Calculate the average (in ADC LSBs) of all the valid

pixels in the black line and store that number as VADC1

7 Set the PGA Gain to 295VV (PGA code e 255)

8 Digitize a black line

9 Calculate the average (in ADC LSBs) of all the valid

pixels in the black line and store that number as VADC2

10 Calculate VOS1

VOS1 e (VADC2–VADC1)195–63

11 Program the Offset DAC register using the formula

Offset DAC code e -(VOS1)(1563)

(63 a (VADC1–VADC2)195)(1563)

15 a 122(VADC1–VADC2)

(Note This calculation can be done as

15 a 39(VADC1–VADC2)32

for ease of programming in 8-bit microcontrollers)

12 If 3VADC1 l VADC2 then set the Offset Add bit to 0

If 3VADC1 k VADC2 set the Offset Add bit to 1

13 The final value of the offset present at the ADC input

can be used for the shading calibration calculations

Calculate the final value of the ADC input offset

(VOFFSET) using

VOFFSET e (3VADC1–VADC2)2

(if the Offset Add bit is 0) or

VOFFSET e (3VADC1–VADC2)2 a 2

(if the Offset Add bit is 1)

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