Datasheet

OPA695
28
SBOS293G
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BOARD LAYOUT GUIDELINES
Achieving optimum performance with a high-frequency am-
plifier like the OPA695 requires careful attention to board
layout parasitics and external component types. Recommen-
dations 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 non-inverting input, it can react with the source imped-
ance to cause unintentional bandlimiting. To reduce un-
wanted 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.
b) Minimize the distance (< 0.25") from the power supply
pins to high frequency 0.1
µF 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 induc-
tance between the pins and the decoupling capacitors.
The power-supply connections should always be decoupled
with these capacitors. An optional supply-decoupling ca-
pacitor across the two power supplies (for bipolar opera-
tion) will improve 2nd-harmonic distortion performance.
Larger (2.2µF to 6.8µF) decoupling capacitors, effective at
a lower frequency, 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 compo-
nents will preserve the high frequency performance
of the OPA695. 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 fre-
quency 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 resis-
tor 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 feed-
back 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 typical performance speci-
fications at a gain of +8 on ±5V supplies) is a good starting
point for design. Note that a 523Ω feedback resistor,
rather than a direct short, is required for the unity gain
follower application. A current-feedback op amp requires
a feedback resistor—even in the unity gain follower con-
figuration—to control stability.
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, con-
sider 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 R
S
from the plot of Recommended
R
S
vs Capacitive Load . Low parasitic capacitive loads
(< 5pF) may not need an R
S
since the OPA695 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 ac-
ceptable, 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 usually not necessary
on board. In fact, a higher impedance environment will
improve distortion, as shown in the distortion versus load
plots. With a characteristic board trace impedance de-
fined (based on board material and trace dimensions), a
matching series resistor into the trace from the output of
the OPA695 is used. A terminating shunt resistor at the
input of the destination device is used as well. 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 OPA695
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
R
S
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 attenu-
ation due to the voltage divider formed by the series
output into the terminating impedance.
e) Socketing a high-speed part like the OPA695 is not
recommended. The additional lead length and pin-to-pin
capacitance introduced by the socket can create an ex-
tremely troublesome parasitic network which can make it
almost impossible to achieve a smooth, stable frequency
response. Best results are obtained by soldering the
OPA695 directly onto the board.