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6.1.1 CDS mode Minimum Clamp Capacitor Calculation:

The following equation takes the maximum leakage current into

the OS input, the maximum allowable droop, the number of pixels

the minimum clamp capacitor value:

For example, if the OS input leakage current is 25nA worst-case,

the sensor has 2700 active pixels, the conversion rate is 2MHz

clamp capacitor value is:

6.1.2 CIS mode Minimum Clamp Capacitor Calculation:

If CDS is disabled, then the maximum LM9822 OS input leakage

current can be calculated from:

capacitor (2pF). Inserting this into Equation 6 results in:

maximum input signal), then Equation 9 reduces to:

In CIS mode (CDS disabled), the max droop limit must be much

more carefully chosen, since any change in the clamp capacitor’s

DC value will affect the LM9822 conversion results. If a droop of

one 10 bit LSB across a line is considered acceptable, then the

allowed droop voltage is calculated as: 2V/1024, or approximately

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

6.1.3 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

occurs when CLMP and VSMP are both high on a rising edge of

MCLK. If SMPCL=0, the clamps are on immediately before the

sample reference time, if SMPCL=1, the clamps are on immedi-

ately after the sample reference time. If the LM9822 is operated

in Divide By 2 mode, then the clamp is on 50% of the time when

CLMP is high. In this case the available charge time per line can

be calculated using:

“Divide By” modes will have lower or higher clamp duty cycles

accordingly, depending on the SMPCL setting. See Diagram 8,

Clamp Timing With SMPCL = 0 and Diagram 9, Clamp Timing

With SMPCL = 1.

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:

LM9822 internal clamp switch, and accuracy is the ratio of the

worst-case initial capacitor voltage to the desired final capacitor

1500

Ω, the worst case voltage change required across the capac-

itor (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:

C

CLAMP MIN

i

dV

---------dt

=

leakage current (A)

max droop(V)

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

number of pixel

f

VS MP

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

=

C

CLAMP MIN

25n

0.1V

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

270

2MHz

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

=

340p

=

I

leakage

V

=

Equation 8: CIS mode Input Leakage Current Calculation

C

CLAMP MIN

i

dV

---------dt

=

V

SAT

t

SampCLK

---------------------------C

SAMP

t

SampCLK

max droop(V)

------------------------------------ num pixel

=

C

CLAMP MIN

4p(F)(V)

max droop(V)

------------------------------------num pixels

=

C

CLAMP MIN

4p(F)(V)

2mV

----------------------270

=

5.4uF

=

t

CLAMP

Number of optical black pixels

2f

VSMP

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

=

Equation 12: 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

15 5

Ω

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

1

ln(50)

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

=

72 8p

=

line

sR

CLAMP

C

CLAMP

t

CLAMP

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







Initial Error Voltag

Final Error Voltag

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





ln

=

Equation 15: Number of Lines Required for Clamping

lines

155

0.01

µF

4.5

µs

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





5V

0.1V

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





ln

13.5 lines

==

Equation 16: Clamping Lines Required Example