EL4089C
DC Restored Video Amplifier
Typical Application
The EL4089 can be used to DC-restore a video
waveform (see Fig 1) The above circuit forces
the cable driving video amplifier’s output to
ground when the HOLD pin is at a logic low
The ‘‘correction voltage’’ is stored on capacitor
CX1 an external ceramic capacitor The capaci-
tor value is chosen from the system require-
ments The typical input bias current to the vid-
eo amplifier is 1
mA
so for a 62
ms
hold time and
a 0 01
mF
capacitor the output voltage drift is
6 2 mV in one line
The S H amplifier can provide a typical current
of 300
mA
to charge capacitor CX1 so with a
1 2
ms
sampling time the output can be corrected
by 36 mV in each line
Using a smaller value of CX1 increases both the
voltage that can be corrected and the drift while
being held likewise using a larger value of CX1
reduces the voltages
The RX1 resistor is in the circuit purely to simu-
late some external source impedance and is not
needed as a real component Likewise for RX2
The 75X back terminating resistor RXT is rec-
ommended when driving 75X cables
The board layout should have a ground plane un-
derneath the EL4089 with the ground plane cut
away from the vicinity of the V
IN
b
pin (pin 1)
This helps to minimize the stray capacitance on
pin 1
Power supply bypassing is important and a
0 1
mF
ceramic capacitor from each power pin to
ground placed very close to the power pins to-
gether with a 4 7
mF
tantalum bead capacitor is
recommended
When both digital and Analog grounds are on the
same board the EL4089 should be on the Analog
ground The digital ground can be connected to
the Analog ground through a 100X –300X resis-
tor near the EL4089 This allows the digital sig-
nal a return path while preventing the digital
noise from corrupting the analog ground
4089 – 19
Figure 1
7
ELANTEC [ ELANTEC SEMICONDUCTOR ]
