el5244 Intersil Corporation, el5244 Datasheet - Page 16

• Manufacturer Part Number: el5244
• Description: 100mhz Single-supply Rail-to-rail Amplifiers
• Manufacturer: Intersil Corporation

Figure 1 shows the output of the EL5144 series amplifier
swinging rail to rail with R
Figure 2 is with R
Choice of Feedback Resistor, R
These amplifiers are optimized for applications that require a
gain of +1. Hence, no feedback resistor is required.
However, for gains greater than +1, the feedback resistor
forms a pole with the input capacitance. As this pole
becomes larger, phase margin is reduced. This causes
ringing in the time domain and peaking in the frequency
domain. Therefore, R
should not be exceeded for optimum performance. If a large
value of R
picofarad range in parallel with R
ringing and peaking at the expense of reducing the
bandwidth.
As far as the output stage of the amplifier is concerned, R
R
combination gets smaller, the bandwidth falls off.
Consequently, R
be exceeded for optimum performance.
For A
gain of 2), optimum response is obtained with R
300Ω and 1kΩ. For A
between 300Ω and 15kΩ.
G
appear in parallel with R
V
= +1, R
5V
0V
5V
0V
F
must be used, a small capacitor in the few
F
F
= 0Ω is optimum. For A
L
also has a minimum value that should not
= 150Ω.
V
F
= -4 or +5 (noise gain of 5), keep R
has some maximum value that
F
FIGURE 1.
FIGURE 2.
= 1kΩ, A
L
16
for gains other than +1. As this
F
can help to reduce this
EL5144, EL5146, EL5244, EL5246, EL5444
V
F
= +2 and R
V
= -1 or +2 (noise
F
L
between
= 1MΩ.
F
F
+
Video Performance
For good video signal integrity, 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.
A look at the Differential Gain and Differential Phase curves
for various supply and loading conditions will help you obtain
optimal performance. Curves are provided for A
+2, and R
2.5V. As with all video amplifiers, there is a common mode
sweet spot for optimum differential gain/differential phase.
For example, with A
the output common mode voltage kept between 0.8V and
3.2V, dG/dP is a very low 0.1%/0.1°. This condition
corresponds to driving an AC-coupled, double terminated
75Ω coaxial cable. With A
and the video level kept between 0.85V and 2.95V, these
amplifiers provide dG/dP performance of 0.05%/0.20°. This
condition is representative of using the EL5144 series
amplifier as a buffer driving a DC coupled, double
terminated, 75Ω coaxial cable. Driving high impedance
loads, such as signals on computer video cards, gives
similar or better dG/dP performance as driving cables.
Driving Cables and Capacitive Loads
The EL5144 series amplifiers can drive 50pF loads in
parallel with 150Ω with 4dB of peaking and 100pF with 7dB
of peaking. If less peaking is desired in these applications, a
small series resistor (usually between 5Ω and 50Ω) can be
placed in series with the output to eliminate most peaking.
However, this will obviously reduce the gain slightly. If your
gain is greater than 1, the gain resistor (R
chosen to make up for any gain loss which may be created
by this additional resistor at the output. Another method of
reducing peaking is to add a “snubber” circuit at the output. A
snubber is a resistor in a series with a capacitor, 150Ω and
100pF being typical values. The advantage of a snubber is
that it does not draw DC load current.
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 EL5144 series amplifier from the cable and allow
extensive capacitive drive. However, other applications may
have high capacitive loads without a back-termination
resistor. Again, a small series resistor at the output can
reduce peaking.
Disable/Power-Down
The EL5146 and EL5246 amplifiers can be disabled, placing
its output in a high-impedance state. Turn off time is only
10ns and turn on time is around 500ns. When disabled, the
amplifier’s supply current is reduced to 2.6µA typically,
thereby effectively eliminating power consumption. The
amplifier’s power down can be controlled by standard TTL or
CMOS signal levels at the CE pin. The applied logic signal is
L
= 150Ω and 10kΩ tied both to ground as well as
V
= +2 and R
V
= +1, R
L
= 150Ω tied to 2.5V, and
L
= 150Ω tied to ground,
G
) can then be
V
= +1 and