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• If 1.5 < Range < 2.0, then set the Multiplier Range bits (register

9, bits 3 and 2) to 01 (50%).

• If 2.0 < Range < 3.0, then set the Multiplier Range bits (register

9, bits 3 and 2) to 00 (66%).

• If Range > 3.0, then set the range is too large and the system

can not be calibrated.

4.5 Calculating the Pixel Rate Offset Coefficients

• Scan one line of a pure black test pattern, or scan one line with

the light source turned off. (Caution: turning the light source on

and off may cause thermal drift in light intensity as the light

source warms up and cools down.) Scanning multiple black

lines and averaging each pixel from each line will reduce noise

and increase the accuracy of this step.

• The value of each pixel is the offset error for that pixel. Store

each pixel value into that pixel's offset correction RAM

location.

• Set the Pixel Rate Offset Adder Source to external (register 9,

bit 0 = 0)

4.6 Calculating the Pixel Rate Multiplier Coefficients

• Scan one line of a pure white test pattern. Scanning multiple

white lines and averaging each pixel from each line will reduce

noise and increase the accuracy of this step.

• The equation for the multiplier gain is:

where:

x is the 10 bit multiplier gain coefficient (0 to 1023),

N = 2048 (Multiplier range = 1:1.5),

N = 1024 (Multiplier range = 1:2),

N = 512 (Multiplier range = 1:3).

• Determine the multiplier gain for each pixel n so the result is

the Target Code:

• The multiplier coefficient required is therefore:

• Store the Multiplier Gain Coefficients for each pixel into that

pixel's offset correction RAM location.

• Set the Pixel Rate Multiplier Source to external (register 9, bit

1=0)

CALIBRATION COMPLETE

4.7 Additional Calibration Information

The procedure detailed in sections 4.1 through 4.6 is only one of

many approaches to scanner calibration. In some approaches it

may be useful to use a model for the analog front end and actu-

ally measure VOS1 and VOS2.

Figure 13 is a mathematical model of the analog front end of one

channel of the LM9812. The constant C (typically 2, given in the

Electrical Characteristics table) represents the total gain error

from Vin through DOUT with G = 1V/V (PGA register setting = 1).

To further simplify the model, the signal is always assumed to be

0V for black and increasing in the positive direction as pixel

brightness increases. In reality, black may be offset from ground,

and white may be negative (CCD) or positive (CIS) with respect to

black, but this is all taken care of by the CDS function and the cor-

rect setting of the Signal Polarity bit (Register 24, bit 7).

The analog front end’s transfer function is shown in Equation 18.

The typical values of C (2) and VREFIN (2V) cancel each other

out. It is also useful to convert all the voltages to ADC codes,

since that is the domain in which they will be seen by the calibra-

tion software. VIN (in ADC codes) = VIN (in V)*1024. Calibration

can be more easily accomplished using the simplified model of

Equation 19.

Measurement of the two offset voltages (VOS1 and VOS2) is done

by measuring the ADC output, with a black image on the sensor

(if a black reference image is not available, it may be created sim-

ply by turning off the scanner’s illumination). In this example, VIN

is considered to be 0V, and any offsets from the sensor are

lumped in with the LM9812’s VOS1. If the ADC output is measured

with a PGA gain of 1V/V and 3V/V, then the offset errors (VOS1

and VOS2) can be determined from the following two equations:

VADC1 = 1VOS1 + VOS2 + VDAC2 (PGA gain = 1V/V)

VADC2 = 3VOS1 + VOS2 + VDAC2 (PGA gain = 3V/V)

Solving for VOS1 and VOS2:

VOS1 = (VADC2 - VADC1)/2

VOS2 = (3VADC1 - VADC2)/2

5.0 POWER SUPPLY CONSIDERATIONS

5.1 General

The LM9812 should be powered by a single +5V source (unless

3V-compatible digital I/O is required-see Section

5.2). The analog

supplies (VA) and the digital supplies (VD and VDI/O) are brought

out individually to allow separate bypassing for each supply input.

They should

not be powered by two or more different supplies.

Multiplier Gain

----

Equation 15: Multiplier Gain Equation

Gain(n)

Target Code

pixel data

---------------------------------

Equation 16: Gain Calculation for Each Pixel

Multiplier Coefficent (n)

Integer(N(Gain(n) - 1))

Equation 17: Multiplier Coefficient Calculation for Each Pixel

Integer N

Target Code

pixel data

---------------------------------1

–









VOD

DAC

Offset

Figure 13: Simplified Model of One Channel of the Ana-

log Front End

10 Bit

ADC

VOS2

VOS1

VIN

DOUT

VREF IN

OUT

1024

OS1

()GV

OS2

REFIN

----------------------------------------------------------------------------------------

Equation 18: AFE Transfer Equation

OUT

OS1

()GV

OS2

Equation 19: AFE Transfer Equation (units of ADC LSBs)

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