OPA3691 Datasheet, PDF (21/25 Pages) Burr-Brown (TI) – Triple Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

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 typical performance specifications at a gain of +2
on ±5V supplies is a good starting point for design. Note that
a 453Ω feedback resistor, rather than a direct short, is
recommended for the unity-gain follower application. A cur-
rent-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”. Low parasitic
capacitive loads (< 5pF) may not need an R_S since the
OPA3691 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 tech-
niques). A 50Ω environment is normally not necessary on
board, and in fact, a higher impedance environment will
improve distortion as shown in the “Distortion vs 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 OPA3691 is used
as well as a terminating shunt resistor at the input of the
destination device. Remember also that the terminating im-
pedance 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 imped-
ance. The high output voltage and current capability of the
OPA3691 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 destina-
tion 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 OPA3691 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 OPA3691
onto the board.
INPUT AND ESD PROTECTION
The OPA3691 is built using a very high-speed complemen-
tary bipolar process. The internal junction breakdown volt-
ages are relatively low for these very small geometry de-
vices. These breakdowns are reflected in the Absolute Maxi-
mum Ratings table. All device pins have limited ESD protec-
tion using internal diodes to the power supplies as shown in
Figure 15.
+V_CC
External
Pin
Internal
Circuitry
–V _CC
FIGURE 15. Internal ESD Protection.
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 (e.g., in systems with ±15V supply parts
driving into the OPA3691), current limiting series resistors
should be added into the two inputs. Keep these resistor
values as low as possible since high values degrade both
noise performance and frequency response.
OPA3691
21
SBOS227A
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