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參數資料
| 型號： | DEM-VSP2232Y |
| 廠商： | Texas Instruments, Inc. |
| 英文描述： | CCD SIGNAL PROCESSOR FOR DIGITAL CAMERAS |
| 中文描述： | CCD信號處理器的數碼相機 |
| 文件頁數： | 6/19頁 |
| 文件大小： | 248K |
| 代理商： | DEM-VSP2232Y |
VSP2232
SLAS320
–
MAY 2001
6
www.ti.com
programmable gain amplifier (PGA) (continued)
–
10
0
10
20
30
40
50
0
100
200
Input Code for Gain Control (0 to 1023)
300
400
500
600
700
800
900
1000
G
–
Figure 2. Characteristics of PGA Gain
optical black (OB) level clamp loop
To extract the video information correctly, the CCD signal must be referenced to a well-established optical black
(OB) level. The VSP2232 has an autocalibration loop to establish the OB level using the optical black pixel
output from the CCD imager. The input signal level of the OB pixels is identified as the real OB level and the
loop should be closed during this period while CLPOB is active. During the effective pixel interval, the reference
level of the CCD output signal is clamped to the OB level by the OB level clamp loop. To determine the loop-time
constant, an off-chip capacitor is required, and should be connected to the COB (pin 28). The time constant T
is given in equation 1.
T
C
16384
I(min)
Where:
C is the capacitor value connected to COB, I
(min)
is the minimum current (0.15
μ
A) of the control DAC in the
OB level clamp loop, and 0.15
μ
A is equivalent to 1 LSB of the DAC output current. When C is 0.1
μ
F, then the
time constant T is 40.7
μ
s. Also, the slew rate (SR) is given in equation 2.
SR
I(max)
C
Where:
C is the capacitor value connected to COB. I
(max)
is the maximum current (153
μ
A) of the control DAC in the
OB level clamp loop, and 153
μ
A is equivalent to 1023 LSB of the DAC output current.
Generally, the OB level clamping at high-speed causes clamp noise (or white streak noise). However, the noise
will decrease by increasing the capacitor size. On the other hand, a larger capacitor requires a much longer time
to restore from the standby mode, or right after the power goes on. Therefore, we recommend a 0.1-
μ
F to
0.22-
μ
F capcitor. However, it depends on the application environment, and making careful adjustments using
the cut-and-try method is recommended.
(1)
(2)
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