參數(shù)資料
型號(hào)： LM9822
廠商： National Semiconductor Corporation
英文描述： LM9822 3 Channel 42-Bit Color Scanner Analog Front End
中文描述： LM9822 3通道42位彩色掃描儀模擬前端
文件頁(yè)數(shù)： 19/22頁(yè)
文件大?。?/td> 242K
代理商： LM9822
19
www.national.com
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
on the sensor, and the pixel conversion rate, f
VSMP
, and provides
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
(t
= 500ns), and the max droop desired is 0.1V, the minimum
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:
where V
SAT
is the peak pixel signal swing of the CIS OS output
and C
SAMP
is the capacitance of the LM9822 internal sampling
capacitor (2pF). Inserting this into Equation 6 results in:
V
SampCLK
Equation 9: CIS mode C
CLAMP MIN
Calculation
with C
SAMP
equal to 2pF and V
SAT
equal to 2V (the LM9822
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:
Number of2f
=
For example, if a sensor has 18 black reference pixels and f
is 2MHz with a 50% duty cycle, then t
CLAMP
is 4.5μs. Other
“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:
t
R
Equation 13: C
CLAMP MAX
for a single line of charge time
Where t
CLAMP
is the amount of time (per line) that the clamp is
on, R
CLAMP
is the output impedance of the CCD plus 50
for the
LM9822 internal clamp switch, and accuracy is the ratio of the
worst-case initial capacitor voltage to the desired final capacitor
voltage. If t
CLAMP
is 4.5μs, the output impedance of the sensor is
1500
, 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:
The final value for C
CLAMP
should be less than or equal to
C
CLAMP MAX
, but no less than C
CLAMP MIN
.
In some cases, depending primarily on the choice of sensor,
C
CLAMP MAX
may actually be lessthan C
CLAMP 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 C
CLAMP 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:
Equation 15: Number of Lines Required for Clamping
Using the values shown before and a clamp capacitor value of
0.01μF, this works out to be:
155-------------------
μ
F
μ
s
Equation 16: Clamping Lines Required Example
CCLAMP MIN
dV
=
leakage current (A)
-------max droop(V)
VSMP
-------------f
=
Equation 6: CDS mode C
CLAMP MIN
Calculation
CCLAMP MIN
25n
340p
0.1V
2MHz
=
=
Equation 7: CDS mode C
CLAMP MIN
Example
I
leakage
V
SAT
f
SampCLK
C
SAMP
=
Equation 8: CIS mode Input Leakage Current Calculation
CCLAMP MIN
dV
=
t
SAMP
t
max droop(V)
=
CCLAMP MIN
max droop(V)
=
Equation 10: CIS mode C
CLAMP MIN
Calculation
CCLAMP MIN
5.4uF
42mV
=
Equation 11: CIS mode C
CLAMP MIN
Calculation Example
=
t
CLAMP
VSMP
Equation 12: Clamp Time Per Line Calculation
CCLAMP MAX
-----
ln(ac 1
=
RCLAMP
ln(accuracy)
=
CCLAMP MAX
μ
s
728p
155
ln(50)
=
=
Equation 14: C
CLAMP MAX
Example
line
sRCLAMP
tCLAMP
Final Error Voltag
ln
=
lines
0.1V
ln
13.5 lines
=
=
Applications Information
(Continued)
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