OPA2690 Datasheet, PDF (25/30 Pages) Burr-Brown (TI) – Dual, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2690
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the pins and the decoupling capacitors. The power-supply
connections should always be decoupled with these
capacitors. An optional supply decoupling capacitor
(0.1µF) 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 perfor-
mance of the OPA2690. 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
and PC board traces 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. Even with a low
parasitic capacitance shunting the external resistors,
excessively high resistor values can create significant time
constants that can degrade performance. Good axial
metal film or surface-mount resistors have approximately
0.2pF in shunt with the resistor. For resistor values >
1.5kΩ, this parasitic capacitance can add a pole and/or
zero below 500MHz that can effect circuit operation. Keep
resistor values as low as possible consistent with load
driving considerations. The 402Ω feedback used in the
Electrical Characteristics is a good starting point for
design. Note that a 25Ω feedback resistor, rather than a
direct short, is suggested for the unity-gain follower
application. This effectively isolates the inverting input
capacitance from the output pin that would otherwise
cause an additional peaking in the gain of +1 frequency
response.
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)
SBOS238D − JUNE 2002 − REVISED DECEMBER 2004
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 (< 3pF)
may not need an R_S because the OPA2690 is nominally
compensated to operate with a 2pF parasitic load. Higher
parasitic capacitive loads without an R_S are allowed as the
signal gain increases (increasing the unloaded phase
margin, see Figure 14). If a long trace is required, and the
6dB signal loss intrinsic to a doubly-terminated transmis-
sion 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 on board, 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 OPA2690 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 OPA2690 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 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 attenuation due to the voltage
divider formed by the series output into the terminating
impedance.
e) Socketing a high-speed part like the OPA2690 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
OPA2690 onto the board.
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