OPA2674 Datasheet, PDF (26/33 Pages) Burr-Brown (TI) – Dual Wideband, High Output Current Operational Amplifier with Current Limit

OPA2674
SBOS270A − AUGUST 2003 − REVISED MAY 2006
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 the value reduces the bandwidth, whereas
decreasing it gives a more peaked frequency response.
The 402Ω feedback resistor used in the Typical
Characteristics at a gain of +4 on ±6V supplies is a good
starting point for design. Note that a 511Ω 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.
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 R_S from the plot of Recommended R_S vs
Capacitive Load (see page 10). Low parasitic capacitive
loads (< 5pF) may not need an R_S because the OPA2674
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 transmis-
sion 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. In fact, a higher impedance environment
improves distortion; see 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
OPA2674 is used, as well as a terminating shunt resistor
at the input of the destination device. Remember also that
the terminating impedance is 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 OPA2674 allows multiple destination
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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. However, this does not
preserve signal integrity as well as a doubly-terminated
line. If the input impedance of the destination device is low,
there is 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 OPA2674 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
OPA2674 onto the board.
INPUT AND ESD PROTECTION
The OPA2674 is built using a high-speed complementary
bipolar process. The internal junction breakdown voltages
are relatively low for these very small geometry devices
and are reflected in the absolute maximum ratings table.
All device pins have limited ESD protection using internal
diodes to the power supplies, as shown in Figure 16.
These diodes provide moderate protection to input
overdrive voltages above the supplies as well. The
protection diodes can typically support 30mA continuous
current. Where higher currents are possible (for example,
in systems with ±15V supply parts driving into the
OPA2674), current-limiting series resistors should be
added into the two inputs. Keep these resistor values as
low as possible, because high values degrade both noise
performance and frequency response.
+V_CC
External
Pin
Internal
Circuitry
−V_CC
Figure 16. ESD Steering Diodes
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