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參數(shù)資料
| 型號： | LM9801CCVF |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 模擬信號調理 |
| 英文描述： | Greyscale/24-Bit Color Linear CCD Sensor Processor |
| 中文描述： | SPECIALTY ANALOG CIRCUIT, PQFP52 |
| 封裝： | TQFP-52 |
| 文件頁數(shù)： | 22/34頁 |
| 文件大?。?/td> | 520K |
| 代理商： | LM9801CCVF |
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Applications Information
(Continued)
of the CCD is AC coupled to the LM9801 through a DC
blocking capacitor, C
CLAMP
(the CCD’s DOS output is not
used). The value of this capacitor is determined by the leak-
age current of the LM9801’s OS input and the output imped-
ance of the CCD. The leakage through the OS input deter-
mines how quickly the capacitor value will drift from the
clamp value of REF OUT
MID
, which then determines how
many pixels can be processed before the droop causes er-
rors in the conversion (
g
0.1V is the recommended limit).
The output impedance of the CCD determines how quickly
the capacitor can be charged to the clamp value during the
black reference period at the beginning of every line.
The minimum clamp capacitor value is determined by the
maximum droop the LM9801 can tolerate while converting
one CCD line. The following equation takes the maximum
leakage current into the OS input, the maximum allowable
droop (100 mV), the number of pixels on the CCD, and the
pixel conversion rate (f
MCLK
/8) and provides the minimum
clamp capacitor value:
C
CLAMP MIN
e
i
e
leakage current (A)
max droop (V)
number of pixels
conversion rate (Hz)
For example, if the OS input leakage current is 20 nA worst-
case, the CCD has 2700 active pixels, the conversion rate is
2.5 MHz (f
MCLK
e
20 MHz), and the max droop desired is
0.1V, the minimum clamp capacitor value is:
C
CLAMP MIN
e
20 nA
0.1V
2700
2.5 MHz
e
216 pF
The maximum size of the clamp capacitor is determined by
the amount of time available to charge it to the desired val-
ue during the optical black portion of the CCD output. The
internal clamp is on for each pixel from the rising edge of
the S/H ref pulse to the falling edge of the S/H signal pulse
(see Diagrams 7 and 8). This time can be calculated using
the values stored in the Sample Signal and Sample Refer-
ence configuration registers and the MCLK frequency. For
normal CCDs:
t
DARK
(s)
e
2
a
SS–SR
2f
MCLK
(Hz)
And for even/odd CCDs:
t
DARK
(s)
e
18
a
SS–SR
2f
MCLK
(Hz)
Where SS is the value in the Sample Signal Position register
(0–15), SR is the value in the Sample Reference Position
register (0–14), f
MCLK
is the MCLK frequency, and t
DARK
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 CCD’s output impedance, and the desired accu-
racy 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:
C
CLAMP MAX
e
t
R
1
In(accuracy)
e
n
R
OUT
(
X
)
t
DARK
(s)
In(accuracy)
Where n
e
the number of optical black pixels, t
DARK
is the
amount of time (per pixel) that the clamp is on, R
OUT
is the
output impedance of the CCD, and accuracy is the ratio of
the worst-case initial capacitor voltage to the desired final
capacitor voltage. For example, if a CCD has 18 black refer-
ence pixels, the output impedance of the CCD is 1500
X
, the
LM9801 is configured to clamp for 300 ns, the worst case
initial voltage across the capacitor is 10V, and the desired
voltage after clamping is 0.1V (accuracy
e
10/0.1
e
100),
then:
C
CLAMP MAX
e
18
1500
X
e
514 pF
300 ns
In(100)
The final value for C
CLAMP
should be less than or equal to
C
CLAMP MAX
, but no less than C
CLAMP MIN
. A value of
470 pF will work in this example.
In some cases, depending primarily on the choice of CCD,
C
CLAMP
MAX
may
actually
C
CLAMP MIN
, meaning that the capacitor cannot be charged
to its final voltage during the black pixels at the beginning of
a line and hold its voltage without drooping for the duration
of that line. This is usually not a problem because in most
applications the CCD 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 ca-
pacitor over multiple lines. This equation calculates how
many lines are required before the capacitor settles to the
desired accuracy:
lines
e
18
be
less
than
the
lines
e
R
OUT
n
C
CLAMP
t
DARK
In
Initial Voltage
Final Voltage
Using the values shown before and a clamp capacitor value
of 0.01
m
F, this works out to be:
1500
X
0.01
m
F
300 ns
J
In
#
10V
0.1V
J
e
12.8 lines
At a 2.5 MHz conversion rate, this is about 14 ms.
In this example a 0.01
m
F capacitor takes 14 ms after pow-
er-up to charge to its final value, but its droop across all
subsequent lines is now less than 2 mV (using the previous
example’s values). This wide margin is the reason a C
CLAMP
value of 0.01
m
F will work in most applications.
4.3 VGA
The LM9801 has a VGA (Variable Gain Amplifier) that can
be used to increase the amplitude of the CCD signal prior to
sampling, correction, and digitization. The gain of the VGA is
0 dB to 9 dB and is determined by the codes in the 4-bit
VGA Gain register, as given by the equation:
Gain
VGA
(dB)
e
VGA code
16
9.55
This gain may be changed at the line rate (not the pixel rate)
by writing to the configuration register. You can write to the
configuration register to change the gain at any time, but if
you write during a line, the remaining pixels of that line may
be corrupted. It is best to change the gain after all active
pixels have been read out or while SYNC is low.
http://www.national.com
22
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