OPA4650 Datasheet, PDF (8/14 Pages) Burr-Brown (TI) – Wideband, Low Power, Quad Voltage Feedback OPERATIONAL AMPLIFIER

DISCUSSION OF
PERFORMANCE
the package pins. Surface mount feedback resistors directly
adjacent to the output and inverting input pins work well for
the quad pinout. Other network components, such as non-
The OPA4650 is a quad low power, wideband voltage
feedback operational amplifier. Each channel is internally
inverting input termination resistors, should also be placed
close to the package.
compensated to provide unity gain stability. The OPA4650’s Even with a low parasitic capacitance shunting the resistor,
voltage feedback architecture features true differential and excessively high resistor values can create significant time
fully symmetrical inputs. This minimizes offset errors, mak- constants and degrade performance. Good metal film or
ing the OPA4650 well suited for implementing filter and
instrumentation designs. As a quad operational amplifier,
surface mount resistors have approximately 0.2pF in shunt
with the resistor. For resistor values > 1.5kΩ, this adds a
OPA4650 is an ideal choice for designs requiring multiple pole and/or zero below 500MHz that can affect circuit
channels where reduction of board space, power dissipation operation. Keep resistor values as low as possible consistent
and cost are critical. Its ac performance is optimized to with output loading considerations. The 402Ω feedback
provide a gain bandwidth product of 160MHz and a fast used for the Typical Performance Plots is a good starting
0.1% settling time of 10.3ns, which is an important consid- point for design. Note that a 25Ω feedback resistor, rather
eration in high speed data conversion applications. Along than a direct short, is suggested for a unity gain follower.
with its excellent settling characteristics, the low dc input This effectively reduces the Q of what would otherwise be
offset of ±1mV and drift of ±3µV/°C support high accuracy a parasitic inductance (the feedback wire) into the parasitic
requirements. In applications requiring a higher slew rate capacitance at the inverting input.
and wider bandwidth, such as video and high bit rate digital
communications, consider the quad current feedback
OPA4658.
d) Connections to other wideband devices on the board
may be made with short direct traces or through on-board
transmission lines. For short connections, consider the trace
and the input to the next device as a lumped capacitive load.
CIRCUIT LAYOUT AND BASIC OPERATION
Relatively wide traces (50 to 100 mils) should be used,
Achieving optimum performance with a high frequency am-
plifier like the OPA4650 requires careful attention to layout
parasitics and selection of external components. Recommen-
dations for PC board layout and component selection include:
a) Minimize parasitic capacitance to any ac ground for all
of the signal I/O pins. Parasitic capacitance on the output
and inverting input pins can cause instability; on the non-
inverting input it can react with the source impedance to
cause unintentional bandlimiting. To reduce unwanted ca-
pacitance, a window around the signal I/O pins should be
opened in all of the ground and power planes. Otherwise,
ground and power planes should be unbroken elsewhere on
the board.
b) Minimize the distance (< 0.25") from the two power pins
to high frequency 0.1µF decoupling capacitors. At the pins,
the ground and power plane layout should not be in close
proximity to the signal I/O pins. Avoid narrow power and
ground traces to minimize inductance between the pins and
the decoupling capacitors. Larger (2.2µF to 6.8µF) decoupling
capacitors, effective at lower frequencies, should also be
used. These may be placed somewhat farther from the
device and may be shared among several devices in the same
area of the PC board.
c) Careful selection and placement of external compo-
nents will preserve the high frequency performance of the
OPA4650. Resistors should be a very low reactance type.
Surface mount resistors work best and allow a tighter overall
layout. Metal film or carbon composition axially-leaded
resistors can also provide good high frequency performance.
Again, keep their leads as short as possible. Never use
wirewound type resistors in a high frequency application.
Since the output pin and the inverting input pin are most
sensitive to parasitic capacitance, always position the feed-
back and series output resistor, if any, as close as possible to
®
preferably with ground and power planes opened up around
them. Estimate the total capacitive load and set R_ISO from
the plot of recommended R_ISO vs capacitive load. Low
parasitic loads may not need an R_ISO since the OPA4650 is
nominally compensated to operate with a 2pF parasitic load.
If a long trace is required and the 6dB signal loss intrinsic to
doubly terminated transmission lines is acceptable, imple-
ment a matched impedance transmission line using microstrip
or stripline techniques (consult an ECL design handbook for
microstrip and stripline layout techniques). A 50Ω environ-
ment is not necessary on board, and in fact a higher imped-
ance environment will improve distortion as shown in the
distortion vs load plot. With a characteristic impedance
defined based on board material and desired trace dimen-
sions, a matching series resistor into the trace from the
output of the amplifier is used as well as a terminating shunt
resistor at the input of the destination device. Remember
also that the terminating impedance will be the parallel
combination of the shunt resistor and the input impedance of
the destination device; the total effective impedance should
match the trace impedance. Multiple destination devices are
best handled as separate transmission lines, each with their
own series and shunt terminations.
If the 6dB attenuation loss of a doubly terminated line is
unacceptable, a long trace can be series-terminated at the
source end only. This will help isolate the line capacitance
from the op amp output, but will not preserve signal integrity
as well as a doubly terminated line. If the shunt impedance
at the destination end is finite, there will be some signal
attenuation due to the voltage divider formed by the series
and shunt impedances.
e) Socketing a high speed part like the OPA4650 is not
recommended. The additional lead length and pin-to-pin
capacitance introduced by the socket creates an extremely
troublesome parasitic network which can make it almost
OPA4650
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