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SBOS319C − SEPTEMBER 2004 − REVISED NOVEMBER 2004
Characteristic tables at a gain of +2 on 5V supplies is a
good starting point for design. Note that a 430Ω feedback
resistor, rather than a direct short, is recommended for the
unity-gain follower application. A current-feedback op amp
requires a feedback resistor even in the unity-gain follower
configuration to control stability.
BOARD LAYOUT GUIDELINES
Achieving optimum performance with a high-frequency
amplifier like the OPA694 requires careful attention to
board layout parasitics and external component types.
Recommendations that will optimize performance 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
noninverting input, it can react with the source impedance
to cause unintentional bandlimiting. To reduce unwanted
capacitance, a window around the signal I/O pins should
be opened in all of the ground and power planes around
those pins. Otherwise, ground and power planes should
be unbroken elsewhere on the board.
d) Connections to other wideband devices on the board
may be made with short, direct traces or through onboard
transmission lines. For short connections, consider the
trace and the input to the next device as a lumped
capacitive load. Relatively wide traces (50mils to 100mils)
should be used, preferably with ground and power planes
opened up around them. Estimate the total capacitive load
and set RS from the plot of Recommended RS vs
Capacitive Load. Low parasitic capacitive loads (< 5pF)
may not need an RS, since the OPA694 is nominally
compensated to operate with a 2pF parasitic load. If a long
trace is required, and the 6dB signal loss intrinsic to a
doubly-terminated transmission line is acceptable,
implement a matched impedance transmission line using
microstrip or stripline techniques (consult an ECL design
handbook for microstrip and stripline layout techniques). A
50Ω environment is normally not necessary onboard, and
in fact, a higher impedance environment will improve
distortion, as shown in the Distortion versus Load plots.
With a characteristic board trace impedance defined
based on board material and trace dimensions, a matching
series resistor into the trace from the output of the OPA694
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: this total effective impedance should
be set to match the trace impedance. The high output
voltage and current capability of the OPA694 allows
multiple destination devices to be handled as separate
transmission lines, each with their own series and shunt
terminations. If the 6dB attenuation of a doubly-terminated
transmission line is unacceptable, a long trace can be
series-terminated at the source end only. Treat the trace as
a capacitive load in this case and set the series resistor
value as shown in the plot of Recommended RS vs
Capacitive Load. This will not preserve signal integrity as
well as a doubly-terminated line. If the input impedance of
the destination device is low, there will be some signal
attenuation due to the voltage divider formed by the series
output into the terminating impedance.
b) Minimize the distance (< 0.25”) from the power-supply
pins to high-frequency 0.1µF decoupling capacitors. At the
device 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. The power-supply
connections (on pins 4 and 7) should always be decoupled
with these capacitors. An optional supply decoupling
capacitor across the two power supplies (for bipolar
operation) will improve 2nd-harmonic distortion
performance. Larger (2.2µF to 6.8µF) decoupling
capacitors, effective at lower frequencies, should also be
used on the main supply pins. 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
components will preserve the high-frequency
performance of the OPA694. Resistors should be a very
low reactance type. Surface-mount resistors work best
and allow a tighter overall layout. Metal-film and carbon
composition, axially-leaded resistors can also provide
good high-frequency performance. Again, keep their leads
and PC-board trace length as short as possible. Never use
wirewound type resistors in a high-frequency application.
Since the output pin and inverting input pin are the most
sensitive to parasitic capacitance, always position the
feedback and series output resistor, if any, as close as
possible to the output pin. Other network components,
such as noninverting input termination resistors, should
also be placed close to the package. Where double-side
component mounting is allowed, place the feedback
resistor directly under the package on the other side of the
board between the output and inverting input pins. The
frequency response is primarily determined by the
feedback resistor value, as described previously.
Increasing its value will reduce the bandwidth, while
decreasing it will give a more peaked frequency response.
The 402Ω feedback resistor used in the Electrical
e) Socketing a high-speed part like the OPA694 is not
recommended. The additional lead length and pin-to-pin
capacitance introduced by the socket can create an
extremely troublesome parasitic network which can make
it almost impossible to achieve a smooth, stable frequency
response. Best results are obtained by soldering the
OPA694 onto the board.
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