LM9812 datasheet(33/37 Pages) NSC

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The maximum size of the clamp capacitor is determined by the

amount of time available to charge it to the desired value during

the optical black portion of the sensor output. The internal clamp

is on for each pixel for the time specified in Registers 19 and 20

(see Diagram 11). This time can be calculated using this equa-

tion:

Where Start is the Optical Black Clamp Start Position (Register

19), Stop is the Optical Black Clamp Stop Position (Register 20),

fMCLK is the MCLK frequency, and tDARK is the amount of time

(per pixel) that the clamp is on.

The following equation takes the number of optical black pixels,

the amount of time (per pixel) that the clamp is closed, the sen-

sor’s output impedance, and the desired accuracy of the final

clamp voltage and provides the maximum clamp capacitor value

that allows the clamp capacitor to settle to the desired accuracy

within a single line:

Where n = the number of optical black pixels, tDARK is the amount

of time (per pixel) that the clamp is on, ROUT is the output imped-

ance of the CCD, and accuracy is the ratio of the worst-case ini-

tial capacitor voltage to the desired final capacitor voltage. For

example, if a sensor has 18 black reference pixels, the output

impedance of the sensor is 1500

Ω, the LM9812 is configured to

clamp for 375ns, the worst case initial voltage across the capaci-

tor is 10V, and the desired voltage after clamping is 0.1V (accu-

racy = 10/0.1 = 100), then:

The final value for CCLAMP should be less than or equal to

CCLAMP MAX, but no less than CCLAMP MIN. A value of 470pF will

work in this example.

In some cases, depending primarily on the choice of sensor,

CCLAMP MAX may actually be less than CCLAMP MIN, meaning that

the capacitor can not be charged to its final voltage during the

black pixels at the beginning of a line and hold it’s voltage without

drooping for the duration of that line. This is usually not a problem

because in most applications the sensor is clocked continuously

as soon as power is applied. In this case, a larger capacitor can

be used (guaranteeing that the CCLAMP MIN requirement is met),

and the final clamp voltage is forced across the capacitor over

multiple lines. This equation calculates how many lines are

required before the capacitor settles to the desired accuracy:

Using the values shown before and a clamp capacitor value of

0.01µF, this works out to be:

At a 2MHz conversion rate, this is about 14ms.

In this example a 0.01µF capacitor takes 14ms after power-up to

charge to its final value, but its droop across all subsequent lines

is now less than 3mV (using the previous example’s values). This

wide margin is the reason a CCLAMP value of 0.01µF will work in

most applications.

4.0 CALIBRATION

System calibration is required to correct the many sources of

error in a scanner and optimize image quality. There are many dif-

ferent ways to calibrate a system, some take a long time but pro-

duce better results, others are faster with potentially lower quality

images. The method described below should produce very good

results, but it is by no means the only way to approach scanner

calibration. Some calibration steps could be eliminated to speed

up the calibration procedure. Other steps could be improved by

iteratively repeating them, verifying that the previous calculation

achieved the desired result. Every scanner system is different, so

every system may benefit from optimization of the calibration rou-

tine.

4.0.1 LM9812 Configuration Sequence After Power-On

The power-on reset circuit of the LM9812 may take several hun-

dred microseconds. Wait 1ms after power-on before writing to the

Configuration Registers.

Make sure the SYNC and RUN/STOP inputs are low before writ-

ing to the Configuration Registers. When SYNC is high, you can

not read the configuration register (but you can still write to it).

The LM9812 configuration sequence:

• Set bit 2 of Test Register 28 to 0 (to allow writing of

configuration data). This register is reset to 0 by the power-on

reset, but this step is still recommended to ensure that writes

will work even if this register was corrupted.

• Program bit 4 of register 24 to determine if the LM9812 will be

used in the SYNC OUT or SYNC IN mode.

• Cycle the Powerdown bit to reset the LM9812’s state

machines. (Take bit 7 of register 25 to a 1, then back to a 0).

This procedure will completely reset the part. At this time the

LM9812 is ready for data to be written to all 28 configuration reg-

isters. Data can also be read back from all 28 registers if write

confirmation is desired.

After all 28 configuration registers have been programmed, scan-

ning can begin by taking SYNC high (in the SYNC IN mode) or

RUN/STOP high (in the SYNC OUT mode).

4.1 Calibration Initialization

• Set the Offset DACs to their maximum positive value (Offset

DAC registers 0, 1, and 2 =31)

• Set the PGA gains to 1V/V (PGA gain registers 3, 4, and 5 =1)

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

bit 0=1)

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

1=1)

• Set the Internal Pixel Rate Offset Adder value to 0 (register

6=0)

• Set the Internal Pixel Rate Multiplier value for a gain of 1

(registers 7 and 8 = 0)

DARK

(s)

Start-Stop

2 f

MCLK

(Hz)

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

Equation 8: tDARK Calculation

CLAMP MAX

------

ln(accuracy)

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

OUT

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

DARK

(s)

ln(accuracy)

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

Equation 9: CCLAMP MAX Calculation

CLAMP MAX

1500

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

375ns

ln(100)

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

977pF

Equation 10: CCLAMP MAX Example

lines

OUT

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

CLAMP

DARK

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







Initial Voltage

Final Voltage

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





Equation 11: Line Settling Formula

lines

1500

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

0.01µF

375ns

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





10V

0.1V

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





10.2 lines

Equation 12: Line Settling Example

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