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approximately 2mV. If there are 2700 active pixels on a line then:

2.1.3 Maximum Clamp Capacitor Calculation:

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

SampCLK

is low for half its cycle, then the available charge time

per line can be calculated using:

CLK

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:

LM9810/20’s internal clamp switch, and accuracy is the ratio of

the worst-case initial capacitor voltage to the desired final capaci-

sor is 1500

Ω, the worst case voltage change required across the

capacitor (before the first line) is 5V, and the desired accuracy

after clamping is to within 0.1V (accuracy = 5/0.1 = 50), then:

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

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

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:

In this example, a 0.01µF capacitor takes 14 lines after power-up

to charge to its final value. On subsequent lines, the only error will

be the droop across a single line which should be significantly

less than the initial error. If the LM9810/20 is operating in CDS

mode and multiple lines are used to charge up the clamping

capacitors after power-up, then a clamp capacitor value of

0.01µF should be significantly greater than the calculated

If the LM9810/20 is operating in CIS mode, then significantly

larger clamp capacitors must be used. Fortunately, the output

impedance of most CIS sensors is significantly smaller than the

nated by the 50

Ω from the LM9810/20’s internal clamp switch.

faster.

3.0 Performance Considerations

3.1 Power Supply

The LM9810/20 should be powered by a single +5V source. The

individually to allow separate bypassing for each supply input.

They should

not be powered by two or more different supplies.

In systems with separate analog and digital +5V supplies, all the

supply pins of the LM9810/20 should be powered by the analog

+5V supply. Each supply input should be bypassed to its respec-

tive ground with a 0.1µF capacitor located as close as possible to

the supply input pin. A single 10µF tantalum capacitor should be

ing.

To minimize noise, keep the LM9810/20 and all analog compo-

nents as far as possible from noise generators, such as switching

power supplies and high frequency digital busses. If possible, iso-

late all the analog components and signals (OS, reference inputs

the digital ground plane. The two ground planes should be tied

together at a single point, preferably the point where the power

supply enters the PCB.

3.2 SampCLK Timing

SampCLK

is used to time the stages of the LM9810/20’s sampler,

offset DAC and programmable gain amplifier. To allow for opti-

the its AFE (including the sampler, the offset DAC and the PGA)

The LM9810/20’s internal ADC clock is created through a combi-

C

CLAMP MIN

4p(F)(V)

2mV

---------------------- 2700

=

5.4uF

=

t

CLA MP

Number of optical black pixels

2f

SampCLK

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

=

Equation 13: Clamp Time Per Line Calculation

C

CLAMP MAX

t

R

------

1

ln(accuracy)

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

=

t

CLAMP

R

CLAMP

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

1

ln(accuracy)

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

=

C

CLAMP MAX

4.5

µs

1550

Ω

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

1

ln(50)

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

=

728pF

=

lines

R

CLAMP

C

CLAMP

t

CLAMP

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







Initial Error Voltage

Final Error Voltage

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





ln

=

Equation 16: Number of Lines Required for Clamping

lines

1550

0.01

µF

4.5

µs

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





5V

0.1V

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





ln

13.5 lines

==

Equation 17: Clamping Lines Required Example