EL2321C
EL2321C
Triple, Low Cost, Gain of 2, Video Op Amp
Gain = 2
1.8 V offset
Amp Out
Volts
+3.172
0.714 V
53 mV
0.0 V
-0.286 V
Average Picture Value Change — 555 mV
Average BL
+45 mV
0.686 V
+28 mV
+56 mV
+1.8
+1.172
Figure 3. Black Level Video
Capacitively coupling the input and output is worse than
a capacitor only on the input. Without any special com-
promises you can only take a gain of one. But if the
backmatch resistor is reduced to 36Ω, reducing the out-
put range requirement 25% and the output offset is
shifted to 2.1V you can take a gain of 1.5 and have a
standard NTSC 1Vat the 75Ω load.
A simple transistor, capacitor and resistor sync tip clamp
may be used when the input is already AC coupled to set
the sync tip to ground.This gives the input a fixed DC
level and can be used like a direct coupled input. The
clamp uses a PNP transistor with the collector at ground
and the base has a 200 KΩ resistor to 5V. The emitter
connects to the amplifier input and a capacitor from the
video input. The clamp functions as an inverted Beta
current source for input bias current with plus inputs and
a clamp to ground for minus inputs. The R
A
and R
B
resistors are removed for the clamp option (Figure
4
).
Printed Circuit Layout
The EL2321C is well behaved, and easy to apply in most
applications. However, a few simple techniques will
help assure rapid, high quality results. As with any high
frequency device, good PCB layout is necessary for
optimum performance. Ground-plane construction is
highly recommended, as is good power supply bypass-
ing. A 0.1 µF ceramic capacitor is recommended for
bypassing both supplies. Lead lengths should be as short
as possible, and bypass capacitors should be as close to
the device pins as possible. For good AC performance,
parasitic capacitances should be kept to a minimum at
both inputs and at the output. Resistor values should be
kept under 5 KΩ because of the RC time constants asso-
ciated with the parasitic capacitance. Metal-film and
carbon resistors are both acceptable, use of wire-wound
resistors is not recommended because of their parasitic
inductance. Similarly, capacitors should be low-induc-
tance for best performance.
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ELANTEC [ ELANTEC SEMICONDUCTOR ]
