OPA2691 Datasheet, PDF (20/30 Pages) Texas Instruments – Dual Wideband, Current-Feedback OPERATIONAL AMPLIFIER

short as possible. Never use wirewound type resistors in a
high-frequency application. Since the output pin and invert-
ing 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 resis-
tors, should also be placed close to the package. Where
double-side component mounting is allowed, place the feed-
back 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 characteristics 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 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. Low parasitic
capacitive loads (< 5pF) may not need an R_S since the
OPA2691 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 OPA2691 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
OPA2691 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 attenuation due to the
voltage divider formed by the series output into the terminat-
ing impedance.
e) Socketing a high-speed part like the OPA2691 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 OPA2691
onto the board.
INPUT AND ESD PROTECTION
The OPA2691 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 13.
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 OPA2691), current-limiting se-
ries 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.
+V_CC
External
Pin
–V _CC
FIGURE 12. Internal ESD Protection.
Internal
Circuitry
20
OPA2691
www.ti.com
SBOS224D