VSP2230

SLAS319 – MAY 2001

6

www.ti.com

programmable gain amplifier (PGA) (continued)

–10

0

10

20

30

40

50

0

100

200

300

400

500

600

700

800

900

1000

Input Code for Gain Control (0 to 1023)

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 VSP2230 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:

OB level clamp loop, and 0.15

µA is equivalent to 1 LSB of the DAC output current. When C is 0.1 µF, then the

time constant T is 40.7

µs. Also, the slew rate (SR) is given in equation 2.

SR

+

I

(max)

C

Where:

OB level clamp loop, and 153

µA 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-

µF to

0.22-

µF capcitor. However, it depends on the application environment, and making careful adjustments using

the cut-and-try method is recommended.

(1)

(2)