EL2250, EL2450
The output range of the EL2250/EL2450 is also quite large.
value. While driving a light load, such as 1kΩ, if the input
black level is kept above 1.25V, dG and dP are a respectable
0.03% and 0.03°.
It includes the negative rail, and extends to within 1V of the
top supply rail with a 1kΩ 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.
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
EL2250/EL2450 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 ±3V
Choice Of Feedback Resistor, R
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
with V = ±5V, making it an excellent choice for driving
S
peaking in the frequency domain. Therefore, R has some
F
isolation transformers in telecommunications applications.
maximum value which should not be exceeded for optimum
performance. If a large value of R must be used, a small
Driving Cables and Capacitive Loads
F
capacitor in the few picofarad range in parallel with R can
F
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 EL2250/EL2450 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
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 + R appear in parallel with R for gains other than +1.
F
G
L
As this combination gets smaller, the bandwidth falls off.
Consequently, R has a minimum value that should not be
F
exceeded for optimum performance.
output to eliminate most peaking. The gain resistor (R ) can
G
For A = +1, R = 0Ω is optimum. For A = -1 or +2 (noise
V
F
V
then be chosen to make up for any gain loss which may be
created by this additional resistor at the output.
gain of 2), optimum response is obtained with R between
F
500Ω and 1kΩ. For Av = -4 or +5 (noise gain of 5), keep R
between 2kΩ and 10kΩ.
F
Video Sync Pulse Remover Application
All CMOS Analog to Digital Converters (A/Ds) have a
parasitic latch-up problem when subjected to negative input
voltage levels. Since the sync tip contains no useful video
information and it is a negative going pulse, we can chop it
off.
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
150Ω, because of the change in output current with DC level.
Differential Gain and Differential Phase for the
Figure 1 shows a unity gain connected amplifier A of an
EL2250. Figure 2 shows the complete input video signal
applied at the input, as well as the output signal with the
negative going sync pulse removed.
EL2250/EL2450 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 for R =150Ω, if the output black level clamp is
L
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 EL2250/EL2450, 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
FIGURE 1.
FN7061.2
11
June 7, 2005
INTERSIL [ Intersil ]
