EL4390CM ELANTEC [Elantec Semiconductor], EL4390CM Datasheet - Page 9

• Manufacturer Part Number: EL4390CM
• Description: Triple 80 MHz Video Amplifier with DC Restore
• Manufacturer: ELANTEC [Elantec Semiconductor]

Applications Information
Circuit Operation
Each channel of the EL4390 contains a current
feedback amplifier and a TTL CMOS compatible
clamp circuit The current that the clamp can
source or sink into the non-inverting input is ap-
proximately
So when the non-inverting input is at the same
voltage as the clamp reference no current will
flow and hence no charge is added to the capaci-
tor When there is a difference in voltage current
will flow in an attempt to cancel the error AT
THE NON-INVERTING input The amplifier’s
offset voltage and (I
cancelled with this loop It is purely a method of
adding a controlled DC offset to the signal
As well as the offset voltage error which goes up
with gain and the I
with gain there is also the I
the amplifier is capacitively coupled this small
current is slowly integrated and shows up as a
very slow ramp voltage Table below shows the
output voltage drift in 60 S for various values of
coupling capacitor all assuming the very worst
I
B
Cap Value
a
Table 1 Charge Storage Capacitor Value vs
(nF)
100
220
current
10
22
47
I
Droop and Charging Rates
e
(V
60 S (mV)
Droop in
CLAMP
13 6
1 36
6 4
3 0
30
B
B
b
b
c
b
c
Triple 80 MHz Video Amplifier with DC Restore
Charge in
2 S (mV)
V
R
R
B
IN
F
F
a
400
182
) DC errors are not
85
40
18
error which drops
a
error term Since
) 120
Charge in
4 S (mV)
800
364
170
80
36
9
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 eg from
two different surveillance cameras it is recom-
mended to synchronize the switching with the
vertical blanking period and to drive the HOLD
pin (pin 6) low during these lines This will en-
sure that the system has been completely re-
stored regardless of the average intensity of the
two pictures
Application Hints
Figures 1
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 cir-
cuit can be used for all three channels A 560K
or similar value resistor helps to isolate each sig-
nal See Figure 2 The advantage of compensat-
ing for the droop voltage is that a smaller capaci-
tor can be used which allows a larger level resto-
ration within one line See Table 1 for values of
capacitor and charge droop rates
2 shows a three channel DC-restoring
EL4390C