EL2150, EL2157
to the negative rail and split supplies are used, the output
range is extended to the negative rail.
For other biasing conditions see the Differential Gain and
Differential Phase vs Input Voltage curves.
Choice Of Feedback Res is tor, R
Output Drive Capability
F
The feedback resistor forms a pole with the input
capacitance. As this pole becomes larger, phase margin is
reduced. This increases ringing in the time domain and
In spite of their moderately low 5mA of supply current, the
EL2150 and EL2157 are capable of providing ±100mA of
output current into a 10Ω load, or ±60mA into 50Ω. With this
large output current capability, a 50Ω load can be driven to
peaking in the frequency domain. Therefore, R has some
F
maximum value which should not be exceeded for optimum
±3V with V = ±5V, making it an excellent choice for driving
S
performance. If a large value of R must be used, a small
isolation transformers in telecommunications applications.
F
capacitor in the few picofarad range in parallel with R can
help to reduce this ringing and peaking at the expense of
reducing the bandwidth.
F
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 resistor will de-
couple the EL2150 and EL2157 from the cable and allow
extensive capacitive drive. However, other applications may
have high capacitive loads without a back-termination
resistor. In these applications, a small series resistor (usually
between 5Ω and 50Ω) can be placed in series with the
As far as the output stage of the amplifier is concerned, R +
F
R
appear in parallel with R for gains other than +1. As this
G
L
combination gets smaller, the bandwidth falls off.
Consequently, R has a minimum value that should not be
F
exceeded for optimum performance.
For A = +1, R = 0Ω is optimum. For A = -1 or +2 (noise
V
F
V
output to eliminate most peaking. The gain resistor (R ) can
then be chosen to make up for any gain loss which may be
created by this additional resistor at the output.
G
gain of 2), optimum response is obtained with R between
F
500Ω and 1kΩ. For A = -4 or +5 (noise gain of 5), keep R
V
F
between 2kΩ and 10kΩ.
Dis able/Power-Down
Video Performance
The EL2157 amplifier can be disabled, placing its output in a
high-impedance state. The disable or enable action takes
only about 40ns. When disabled, the amplifier's supply
current is reduced to 0mA, thereby eliminating all power
consumption by the EL2157. The EL2157 amplifier's power
down can be controlled by standard CMOS signal levels at
the 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
For good video performance, an amplifier is required to
maintain the same output impedance and the same
frequency 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 EL2150
and EL2157 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 configuration. 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 acceptable, will compromise peak performance.
For example, looking at the single supply dG and dP curves
-3.0V. Letting the ENABLE pin float will disable the EL2157.
If the power-down feature is not desired, connect the
ENABLE pin to the V + pin. The guaranteed logic levels of
S
+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 pin.
for R =150Ω, if the output black level clamp is reduced from
L
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.
Output Voltage Clamp
The EL2157 amplifier has an output voltage clamp. This
clamping action is fast, being activated almost
If your application requires that the output goes to ground,
then the output stage of the EL2150 and EL2157, 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 1kΩ, if the input
black level is kept above 1.25V, dG and dP are a
instantaneously, and being deactivated in <7ns, and
prevents the output voltage from going above the preset
clamp voltage. This can be very helpful when the EL2157 is
used to drive an A/D converter, as some converters can
require long times to recover if overdriven. The output
voltage remains at the clamp voltage level as long as the
product of the input voltage and the gain setting exceeds the
clamp voltage. If the EL2157 is connected in a gain of 2, for
example, and +3V DC is applied to the CLAMP pin, any
voltage higher than +1.5V at the inputs will be clamped and
+3V will be seen at the output.
respectable 0.03% and 0.03°.
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