el2257 Intersil Corporation, el2257 Datasheet - Page 12

• Manufacturer Part Number: el2257
• Description: 125mhz Single Supply, Clamping Op Amp
• Manufacturer: Intersil Corporation

supply rail with a 1k: load. On a +5V supply, the output is
therefore capable of swinging from 0V to +4V. On split
supplies, the output will swing ±4V. If the load resistor is tied
to the negative rail and split supplies are used, the output
range is extended to the negative rail.
Choice of Feedback Resistor, R
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
peaking in the frequency domain. Therefore, R
maximum value which should not be exceeded for optimum
performance. If a large value of R
capacitor in the few picofarad range in parallel with R
help to reduce this ringing and peaking at the expense of
reducing the bandwidth.
As far as the output stage of the amplifier is concerned,
R
As this combination gets smaller, the bandwidth falls off.
Consequently, R
exceeded for optimum performance.
For A
gain of 2), optimum response is obtained with R
500: and 1k:. For A
between 2k: and 10k:.
Video Performance
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
150:, because of the change in output current with DC level.
Differential Gain and Differential Phase for the EL2257 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° whi le driving 150: 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 for
R
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 EL2257, 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 1k:, if the input black level is
F
L
= 150:, if the output black level clamp is reduced from
+ R
V
G
= +1, R
appear in parallel with R
F
F
= 0: is optimum. For A
has a minimum value that should not be
V
= -4 or +5 (noise gain of 5), keep R
12
F
L
must be used, a small
for gains other than +1.
F
V
= -1 or +2 (noise
F
F
has some
between
F
can
F
EL2257
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 5mA of supply current, the
EL2257 is capable of providing ±100mA of output current
into a 10: load, or ±60mA into 50:. With this large output
current capability, a 50: load can be driven to ±3V with
V
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 resistor will de-
couple the EL2257 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 5: and
50:) can be placed in series with the output to eliminate
most peaking. The gain resistor (R
make up for any gain loss which may be created by this
additional resistor at the output.
Disable/Power-Down
Each amplifier in the EL2257 can be individually disabled,
placing each output in a high-impedance state. The disable
or enable action takes only about 40ns. When all amplifiers
are disabled, the total supply current is reduced to 0mA,
thereby eliminating all power consumption by the EL2257.
The EL2257 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 EL2257. If the power-down feature
is not desired, connect all ENABLE pins to the V
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.
Output Voltage Clamp
The EL2257 amplifier has an output voltage clamp. This
clamping action is fast, being activated almost
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 EL2257 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. For example, if the EL2257 is connected in a
gain of 2, and +3V DC is applied to the CLAMP pin, any
S
= ±5V, making it an excellent choice for driving isolation
G
) can then be chosen to
S
+ pin. The