el2074 Intersil Corporation, el2074 Datasheet - Page 8

• Manufacturer Part Number: el2074
• Description: 400mhz Gbwp Gain-of-2 Stable Operational Amplifier
• Manufacturer: Intersil Corporation

Parasitic Capacitances and Stability
When used in positive-gain configurations, the EL2074 can
be quite sensitive to parasitic capacitances at the inverting
input, especially with values 250 for the gain resistor. The
problem stems from the feedback and gain resistance in con-
junction with the approximately 3pF of board-related
parasitic capacitance from the inverting input to ground.
Assuming a gain-of-2 configuration with R
feedback pole occurs at 424MHz, which is equivalent to a
zero in the forward path at the same frequency. This zero
reduces stability by reducing the effective phase-margin from
about 50° to about 30°.
A common solution to this problem is to add an additional
capacitor from the inverting input to the output. This capaci-
tor, in conjunction with the parasitic capacitance, maintains a
constant voltage-divider between the output and the invert-
ing input. This technique is used for AC testing of the
EL2074. A 3pF capacitor is placed in parallel with the feed-
back resistor for all AC tests. When this capacitor is used, it
is also possible to increase the resistance values of the feed-
back and gain resistors without loss of stability, resulting in
less loading of the EL2074 from the feedback network.
Video Performance
An industry-standard method of measuring the video distor-
tion of a component such as the EL2074 is to measure the
amount of differential gain (dG) and differential phase (dP)
that it introduces. To make these measurements, a
0.286V
offset (0IRE) at either 3.58MHz for NTSC, 4.43MHz for PAL,
or 30MHz for HDTV. A second measurement is then made at
0.714V DC offset (100IRE). Differential gain is a measure of
the change in amplitude of the sine wave, and is measured in
percent. Differential phase is a measure of the change in
phase, and is measured in degrees.
For signal transmission and distribution, a back-terminated
cable (75
the receiving end) is preferred since the impedance match at
both ends will absorb any reflections. However, when double
termination is used, the received signal is halved; therefore a
gain of 2 configuration is typically used to compensate for the
attenuation.
The EL2074 has been designed to be among the best video
amplifiers in the marketplace today. It has been thoroughly
characterized for video performance in the topology
described above, and the results have been included as min-
imum dG and dP specifications and as typical performance
curves. In a gain of +2, driving 150 , with standard video test
levels at the input, the EL2074 exhibits dG and dP of only
0.01% and 0.015° at NTSC and PAL. Because dG and dP
vary with different DC offsets, the superior video perfor-
mance of the EL2074 has been characterized over the entire
DC offset range from -0.714V to +0.714V. For more informa-
tion, refer to the curves of dG and dP vs DC Input Offset.
PP
(40IRE) signal is applied to the device with 0V DC
in series at the drive end, and 75
8
F
= R
to ground at
G
= 250 , a
EL2074
The excellent output drive capability of the EL2074 allows it
to drive up to 4 back-terminated loads with excellent video
performance. With 4, 150 loads, dG and dP are only 0.15%
and 0.08° at NTSC and PAL. For more information, refer to
the curves for Video Performance vs Number of 150
Loads.
Output Drive Capability
The EL2074 has been optimized to drive 50
loads. It can easily drive 6V
put drive capability makes the EL2074 an ideal choice for
RF, IF and video applications. Furthermore, the current drive
of the EL2074 remains a minimum of 50mA at low tempera-
tures. The EL2074 is current-limited at the output, allowing it
to withstand momentary shorts to ground. However, power
dissipation with the output shorted can be in excess of the
power-dissipation capabilities of the package.
Capacitive Loads
Although the EL2074 has been optimized to drive resistive
loads as low as 50 , capacitive loads will decrease the
amplifier's phase margin which may result in peaking, over-
shoot, and possible oscillation. For optimum AC
performance, capacitive loads should be reduced as much
as possible or isolated via a series output resistor. Coax lines
can be driven, as long as they are terminated with their char-
acteristic impedance. When properly terminated, the
capacitance of coaxial cable will not add to the capacitive
load seen by the amplifier. Capacitive loads greater than
10pF should be buffered with a series resistor (R
the load capacitance from the amplifier output. A curve of
recommended R
Values of R
without peaking.
Printed-Circuit Layout
As with any high-frequency device, good PCB layout is nec-
essary for optimum performance. Ground-plane construction
is highly recommended, as is good power supply bypassing.
A 1µF–10µF tantalum capacitor is recommended in parallel
with a 0.01µF ceramic capacitor. All pin lengths should be as
short as possible, and all bypass capacitors should be as
close to the device pins as possible. Parasitic capacitances
should be kept to an absolute minimum at both inputs and at
the output. Resistor values should be kept under 1000
2000 because of the RC time constants associated with the
parasitic capacitance. Metal-film and carbon resistors are
both acceptable, use of wire-wound resistors is not recom-
mended because of parasitic inductance. Similarly,
capacitors should be low-inductance for best performance. If
possible, solder the EL2074 directly to the PC board without
a socket. Even high quality sockets add parasitic capaci-
tance and inductance which can potentially degrade
performance. Because of the degradation of AC perfor-
mance due to parasitics, the use of surface-mount
components (resistors, capacitors, etc.) is also
recommended.
S
were chosen to maximize resulting bandwidth
S
vs Cload has been included for reference.
PP
into a 50 load. This high out-
and 75
S
) to isolate
to