EL2357CS 데이터 시트보기 (PDF) - Elantec -> Intersil
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EL2357CS Datasheet PDF : 21 Pages
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EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
As far as the output stage of the amplifier is concerned,
RF + RG appear in parallel with RL for gains other than
+1. As this combination gets smaller, the bandwidth
falls off. Consequently, RF has a minimum value that
should not be exceeded for optimum performance.
For AV = +1, RF = 0Ω is optimum. For AV = -1 or +2
(noise gain of 2), optimum response is obtained with RF
between 500Ω and 1 kΩ. For AV = -4 or +5 (noise gain
of 5), keep RF between 2 kΩ and 10 kΩ.
Video Performance
For good video performance, an amplifier is required to
maintain the same output impedance and the same fre-
quency response as DC levels are changed at the output.
This can be difficult when driving a standard video load
of 150Ω, because of the change in output current with
DC level. Differential Gain and Differential Phase for
the EL2257C/EL2357C are specified with the black
level of the output video signal set to +1.2V. This allows
ample room for the sync pulse even in a gain of +2 con-
figuration. This results in dG and dP specifications of
0.05% and 0.05° while driving 150Ω at a gain of +2.
Setting the black level to other values, although accept-
able, will compromise peak performance. For example,
looking at the single supply dG and dP curves for
RL=150Ω, if the output black level clamp is reduced
from 1.2V to 0.6V dG/dP will increase from
0.05%/0.05° to 0.08%/0.25°. Note that in a gain of +2
configuration, this is the lowest black level allowed such
that the sync tip doesn’t go below 0V.
If your application requires that the output goes to
ground, then the output stage of the EL2257C/EL2357C,
like all other single supply op amps, requires an external
pull down resistor tied to ground. As mentioned above,
the current flowing through this resistor becomes the DC
bias current for the output stage NPN transistor. As this
current approaches zero, the NPN turns off, and dG and
dP will increase. This becomes more critical as the load
resistor is increased in value. While driving a light load,
such as 1 kΩ, if the input black level is kept above
1.25V, dG and dP are a respectable 0.03% and 0.03°.
For other biasing conditions see the Differential Gain
and Differential Phase vs. Input Voltage curves.
Output Drive Capability
In spite of their moderately low 5 mA of supply current,
the EL2257C/EL2357C are capable of providing ±100
mA of output current into a 10Ω load, or ±60 mA into
50Ω. With this large output current capability, a 50Ω
load can be driven to ±3V with VS = ±5V, making it an
excellent choice for driving isolation transformers in
telecommunications applications.
Driving Cables and Capacitive Loads
When used as a cable driver, double termination is
always recommended for reflection-free performance.
For those applications, the back-termination series resis-
tor will de-couple the EL2257C/EL2357C from the
cable and allow extensive capacitive drive. However,
other applications may have high capacitive loads with-
out a back-termination resistor. In these applications, a
small series resistor (usually between 5Ω and 50Ω) can
be placed in series with the output to eliminate most
peaking. The gain resistor (RG) can then be chosen to
make up for any gain loss which may be created by this
additional resistor at the output.
Disable/Power-Down
Each amplifier in the EL2257C/EL2357C can be indi-
vidually disabled, placing each output in a high-
impedance state. The disable or enable action takes only
about 40 ns. When all amplifiers are disabled, the total
supply current is reduced to 0 mA, thereby eliminating
all power consumption by the EL2257C/EL2357C. The
EL2257C/EL2357C amplifier’s power down can be
controlled by standard CMOS signal levels at each
ENABLE pin. The applied CMOS signal is relative to
the GND pin. For example, if a single +5V supply is
used, the logic voltage levels will be +0.5V and +2.0V.
If using dual ±5V supplies, the logic levels will be -4.5V
and -3.0V. Letting all ENABLE pins float will disable
the EL2257C/EL2357C. If the power-down feature is
not desired, connect all ENABLE pins to the VS+ pin.
The guaranteed logic levels of +0.5V and +2.0V are not
standard TTL levels of +0.8V and +2.0V, so care must
be taken if standard TTL will be used to drive the
ENABLE pins.
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