of the Voltage Buffer Amplifier. The accompanying Table 6B, Timing Symbol's Definition, is on
the next page.
Cp is the same clock that is shown in Figure 3A. As in the Figure 3A, it is the reference for
defining the video signal pulse times. Vp is the peak amplitude of the pixel when the image
sensor is under light exposure. Vd is the dark level of the pixel when the image sensor has no
light exposure. The Reset Level is during the time when image sensor is reset to ground with
an external shunting switch, SW. Refer to any of the simplified block diagrams in the section
under Output Circuits for Converting the Video Signal. The video line reset is active while Cp
is high. The video signal charges video line with the falling edge of Cp.
The shape of the video is a typical characteristic that is exhibited when the sensor current
charges the video line capacitance. It continues to rise until it becomes asymptotic to a
horizontal line. However for clock frequency >2.0MHz, the slope does not reach the
asymptotic condition. Because of this ever-charging slope, the output voltage changes with
the clock frequency and its duty cycle. Hence, there is no optimum point for the video pixel
sampling position. Using an edge triggered sampling A/D with a very narrow aperture, the
users of these CIS devices samples the signal as close to top of the waveform as possible.
Although the optimum way is adjust the sampling position in the application, the following
sampling time given in terms of clock-time ratio will provide a rule-of-thumb in setting the
sampling time. By using the relationship below, the user can place the sampling clock within
an acceptable range.
Tsmp » [to x (1.0 – Dty) + Damp]
where Dty is the clock duty cycle defined in above Table 6A.
Item
Symbol
to(1)
tsmp (2)
Damp (3)
Minimum
166
107
Mean
200
120
20
Maximum
10000
Units
ns
ns
Clock Pulse Period
Video Sample Time
Amplifier Group
Delay
15
%
Video fall time
tvf
20
30
ns
Table 6B. Supplement Timing Symbol's Definition
Notes:
1) to is the clock cycle period with minimum set with 6.0 MHz.
2) tsmp has been previously defined above, with Dty=0.5
3) Damp is group delay time associated with the amplifier design in Figure 4C, Video
Buffer Amplifier, EL2044 by Elantec.
PAGE 7 OF 10, PI3039, 12/2/02
AMI [ AMI SEMICONDUCTOR ]
