LM9811 datasheet(25/36 Pages) NSC

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Applications Information (Continued)

While CIS devices do not usually have a reference level with

which to perform correlated double sampling, many have a

very repeatable reset level which can be used as a black ref-

erence allowing the LM9811 to perform pseudo CDS on the

signal. For more information on CIS applications, see Sec-

tion 9.0. When the Signal Polarity bit is set to a zero, the

LM9811 expects a positive going signal, typically from a CIS

device. When the Signal Polarity bit is set to a one, the

LM9811 expects a negative going signal, typically from a

CCD sensor.

4.6 Offset DAC

The 4 bit plus sign offset DAC is used to compensate for DC

offsets due to the correlated double sampling stage. The off-

set can be corrected in 31 steps of 1.7 ADC LSB size be-

tween −25.2 LSB and +25.2 LSB. Note that the DAC comes

before the PGA, so any offset errors at this stage are multi-

plied by the gain of the PGA. The calibration procedure de-

scribed in Section 5.0 demonstrates how to use the DAC to

eliminate offset errors before scanning begins.

Note that this DAC is programmed during LM9811

calibration/configuration and is not meant to compensate for

pixel-to-pixel CCD offset errors. CDS cancels the pixel-rate

offset errors.

4.7 Programmable Gain Amplifier (PGA)

The PGA provides 8 bits of pixel-to-pixel gain correction over

a 0 dB to 9 dB (x1 to x3) range. After the input signal is

sampled and held by the CDS stage, it is amplified by the

gain indicated by the data (“PGA Code”) on the CD0–CD7

databus using the formula:

4.8 Offset Add Bit

In addition to the Offset DAC, there is a bit in the configura-

tion register which, when set, adds a positive 8 LSB offset at

the output of the PGA. This offset ensures that any offset be-

tween the output of the PGA and the ADC is positive, so that

no dark level information is lost due to negative offsets. The

calibration procedure described in Section 5.0 demonstrates

how to set this bit.

4.9 ADC

The ADC converts the normalized analog output signal to a

10-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 allow-

ing 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

Figure

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 sig-

nals). 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 a 10-bit system like the

LM9811, that code might be 1000. This code will be called

the Target Code.

The LM9811 eliminates these global and pixel-to-pixel offset

and gain errors with its Correlated Double Sampling (CDS),

Offset DACs, Variable Gain Amplifier, and pixel-rate Pro-

grammable Gain Amplifier. This section describes how to

program the LM9811 and the coefficient RAM being used

with it to eliminate these errors.

Calibration of a LM9811-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-

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 5.3,

PGA Correction Co-

efficients (Shading Calibration), describes how to calculate

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 LM9811. Because the LM9811 uses CDS to

eliminate the pixel-to-pixel offset errors of the CCD, no

pixel-rate offset correction is required.

DS012813-32

FIGURE 24. CIS vs CCD Output Signals

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