EL4390CN INTERSIL [Intersil Corporation], EL4390CN Datasheet - Page 8

• Manufacturer Part Number: EL4390CN
• Description: Triple 80MHz Video Amplifier with DC Restore
• Manufacturer: INTERSIL [Intersil Corporation]

voltage drift in 60µS for various values of coupling capacitor,
all assuming the very worst I
In normal circuit operation, the picture content will also
cause a slow change in voltage across the capacitor, so at
every back porch time period, these error terms can be
corrected.
When a signal source is being switched, e.g., from two
different surveillance cameras, it is recommended to
synchronize the switching with the vertical blanking period,
and to drive the HOLD pin (pin 6) low, during these lines.
This will ensure that the system has been completely
restored, regardless of the average intensity of the two
pictures.
Application Hints
Figures 1 & 2 shows a three channel DC-restoring system,
suitable for R-G-B or Y-U-V component video, or three
synchronous composite signals.
Figure 1 shows the amplifiers configured for non-inverting
gain, and Figure 2 shows the amplifiers configured for
inverting gains. Note that since the DC-restoring function is
accomplished by clamping the amplifier’s non-inverting
input, during the back porch period, any signal on the non-
inverting input will be distorted. For this reason, it is
recommended to use the inverting configuration for
composite video, since this avoids the color burst being
altered during the clamp time period.
Since all three amplifiers are monolithic, they run at the
same temperature, and will have very similar input bias
currents. This can be used to advantage, in situations where
the droop voltage needs to be compensated, since a single
trim circuit can be used for all three channels. A 560kΩ or
similar value resistor helps to isolate each signal. See Figure
2. The advantage of compensating for the droop voltage, is
that a smaller capacitor can be used, which allows a larger
level restoration within one line. See Table 1 for values of
capacitor and charge/droop rates.
CAP VALUE
TABLE 2. CHARGE STORAGE CAPACITOR VALUE VS.
(NF)
100
220
10
22
47
DROOP AND CHARGING RATES
DROOP IN
60µS (MV)
13.6
1.36
6.4
3.0
30
B
+ current.
8
CHARGE IN
2µS (MV)
400
182
85
40
18
CHARGE IN
4µS (MV)
800
364
170
80
36
EL4390