參數(shù)資料
型號(hào)： LM9800CCV
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 模擬信號(hào)調(diào)理
英文描述： 8-Bit Greyscale/24-Bit Color Linear CCD Sensor Processor
中文描述： SPECIALTY ANALOG CIRCUIT, PQCC52
封裝： PLASTIC, LCC-52
文件頁(yè)數(shù)： 25/34頁(yè)
文件大?。?/td> 619K
代理商： LM9800CCV
Applications Information
(Continued)
LM9800’s OS input and the output impedance of the CCD.
The leakage through the OS input determines how quickly
the capacitor value will drift from the clamp value of REF
OUT
, which then determines how many pixels can be
processed before the droop causes errors in the conversion
(
±
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 LM9800 can tolerate while converting
one CCD line. The following equation takes the maximum
leakage current into the OS input, the maximum allowable
droop (100mV), the number of pixels on the CCD, and the
pixel conversion rate (f
MCLK
/8) and provides the minimum
clamp capacitor value:
For example, if the OS input leakage current is 20nA worst-
case, the CCD has 2200 active pixels, the conversion rate is
2.5MHz (f
= 20MHz), and the max droop desired is
0.05V, the minimum clamp capacitor value is:
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 CCD output. The inter-
nal 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 Fig-
ures 7, 8 ). This time can be calculated using the values
stored in the Sample Signal and Sample Reference configu-
ration registers and the MCLK frequency. For normal CCDs:
And for even/odd CCDs:
Where SS is the value in the Sample Signal Position register
(0–15), SR is the value in the Sample Reference Position
register (0–15), f
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 pix-
els, the amount of time (per pixel) that the clamp is closed,
the CCD’s output impedance, and the desired accuracy of
the final clamp voltage and provides the maximum clamp ca-
pacitor value:
Where n = the number of optical black pixels, t
DARK
is the
amount of time (per pixel) that the clamp is on, R
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 1000
, the
LM9800 is configured to clamp for 300ns, the worst case ini-
tial voltage across the capaoitor is 10V, and the desired volt-
age after clamping is 0.05V (accuracy = 10/0.05 = 200),
then:
The final value for C
should be equal to or slightly less
than C
CLAMP MAX
, but no less than C
CLAMP MIN
.
The LM9800 has been designed to work with most of the
single output CCDs in use, but it is possible that some CCDs
could have a combination of high output impedance and low
optical black pixel count that would cause C
to be
greater than C
. In this case the LM9800 can be
“short cycled” as shown in Figure 30 by bringing the sync
pin low and then high again shortly after clamping the black
reference pixels. This starts the line over, effectively increas-
ing the number of black reference pixels, giving the capacitor
more time to charge up through the output impedance of the
CCD. This “short cycling” can be repeated as many times as
necessary in order to guarantee that the capacitor is charged
up to the required accuracy. Short cycling, if needed at all, is
only necessary on power up and at the beginning of new
scans where the OS coupling capacitor may have drooped
since the previous scan. If the SYNC input is continuously
being cycled at the line rate, short cycling is usually not
necessary.
DS012498-53
FIGURE 29. OS Clamp Capacitor and Internal Clamp
DS012498-35
FIGURE 30. Example of Short Cycling
25
www.national.com
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