OPA3681 Datasheet, PDF (21/21 Pages) Burr-Brown (TI) – Triple Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

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, 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
OPA3681 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 OPA3681 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 capa-
bility of the OPA3681 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 desti-
nation 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 OPA3681 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 OPA3681
onto the board.
INPUT AND ESD PROTECTION
The OPA3681 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
protection using internal diodes to the power supplies as
shown in Figure 15.
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 OPA3681), 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.
+V_CC
External
Pin
Internal
Circuitry
–V _CC
FIGURE 15. Internal ESD Protection.
DESIGN-IN TOOLS
APPLICATIONS SUPPORT
The Burr-Brown Applications Department is available for
design assistance at phone number 1-800-548-6132
(US/Canada only). The Burr-Brown Internet web page
(http://www.burr-brown.com) has the latest data sheets and
other design aids.
DEMONSTRATION BOARDS
A PC board will be available to assist in the initial evaluation
of circuit performance of the OPA3681. This is available as
an unpopulated PCB with descriptive documentation. See
the demonstration board literature for more information. The
summary information for this board is shown below:
PRODUCT
OPA3681E
OPA3681U
PACKAGE
SSOP-16
SO-16
DEMONSTRATION
BOARD
DEM-OPA368xE
DEM-OPA368xU
LITERATURE
REQUEST
NUMBER
MKT-354
MKT-364
Check the Burr-Brown web site for availability of these
boards.
SPICE MODELS
Computer simulation of circuit performance using SPICE is
often useful when analyzing the performance of analog
circuits and systems. This is particularly true for high speed
active devices, like the OPA3681, where parasitic capaci-
tance and inductance can have a major effect on frequency
response.
SPICE models will be available through the Burr-Brown
web page or on a disk (call our Applications Department).
These models do a good job of predicting small-signal AC
and transient performance under a wide variety of operating
conditions. They do not do as well in predicting the har-
monic distortion or differential gain and phase characteris-
tics. These models do not distinguish between the AC
performance of different package types.
®
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OPA3681