LM9810 NSC [National Semiconductor], LM9810 Datasheet - Page 18

• Manufacturer Part Number: LM9810
• Description: LM9810/20 10/12-Bit Image Sensor Processor Analog Front End
• Manufacturer: NSC [National Semiconductor]

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
is on when
SampCLK
per line can be calculated using:
For example, if a sensor has 18 black reference pixels and f
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:
Where t
on, R
LM9810/20’s internal clamp switch, and accuracy is the ratio of
the worst-case initial capacitor voltage to the desired final capaci-
tor voltage. If t
sor is 1500 , 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:
The final value for C
C
In some cases, depending primarily on the choice of sensor,
C
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 C
and the final clamp voltage is forced across the capacitor over
multiple lines. This equation calculates how many lines are
CLAMP MAX
CLAMP MAX
Equation 12: CIS mode C
Equation 14: C
is 2MHz with a 50% duty cycle, then t
CLAMP
CLAMP
Equation 13: Clamp Time Per Line Calculation
C
is low for half its cycle, then the available charge time
CLAMP MIN
C CLAMP MAX
t
CLAMP
is the output impedance of the CCD plus 50
, but no less than C
NewLine
may actually be less than C
Equation 15: C
CLAMP
is the amount of time (per line) that the clamp is
C
CLAMP MAX
CLAMP MAX
=
is high and
is 4.5µs, the output impedance of the sen-
CLAMP
Number of optical black pixels
-------------------------------------------------------------------------------
=
=
5.4uF
4p(F)(V)
--------------------- - 2700
=
=
2mV
CLAMP MIN
----- -
--------------------------
R
R
CLAMP MAX
should be less than or equal to
for a single line of charge time
t
t
CLAMP MIN
CLAMP
CLAMP
------------------------------- -
ln(accuracy)
2f
=
=
CLAMP MIN
SampCLK
SampCLK
----------------- -
1550
728pF
4.5 s
1
------------------------------- -
ln(accuracy)
Calculation Example
CLAMP MIN
Example
.
CLAMP
-------------- -
ln(50)
is low. If the applied
1
requirement is met),
1
is 4.5µs.
, meaning that
for the
Samp-
18
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
C
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
output impedance of CCD sensors, and R
nated by the 50
With a smaller R
faster.
3.0 Performance Considerations
3.1 Power Supply
The LM9810/20 should be powered by a single +5V source. The
analog supplies (
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
placed near the
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
and outputs,
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
offset DAC and programmable gain amplifier. To allow for opti-
mum input signal sampling times,
chronously to
the its AFE (including the sampler, the offset DAC and the PGA)
by
The LM9810/20’s internal ADC clock is created through a combi-
CLAMP MIN
MCLK
Equation 16: Number of Lines Required for Clamping
lines
Equation 17: Clamping Lines Required Example
lines
.
is used to time the stages of the LM9810/20’s sampler,
=
value and can virtually always be used.
VA
=
MCLK
R CLAMP
,
AGND
VA
1550
CLAMP
VA
from the LM9810/20’s internal clamp switch.
. The LM9810/20’s ADC is synchronized with
) and the digital supply (
supply pin to provide low frequency bypass-
) on an analog ground plane, separate from
0.01 F
------------------ -
4.5 s
C
------------------------ -
value, the clamp capacitors will charge
t
CLAMP
CLAMP
ln
SampCLK
----------- -
0.1V
ln
5V
--------------------------------------------------- -
Initial Error Voltage
Final Error Voltage
=
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may be applied asyn-
13.5 lines
CLAMP
VD
) are brought out
will be domi-