OPA2682 Datasheet, PDF (19/19 Pages) Burr-Brown (TI) – Dual, Wideband, Fixed Gain BUFFER AMPLIFIER With Disable

c) Careful selection and placement of external compo-
nents will preserve the high frequency performance of
the OPA2682. Resistors should be a very low reactance
type. Surface-mount resistors work best and allow a tighter
overall layout. Metal-film and carbon composition, axially-
leaded resistors can also provide good high-frequency per-
formance. Keep the leads and PC board trace lengths as
short as possible. Never use wirewound type resistors in a
high frequency application. Other network components, such
as non-inverting input termination resistors, should also be
placed close to the package.
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 OPA2682 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 im-
pedance transmission line using microstrip or stripline tech-
niques (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 Distor-
tion vs Load plots. With a characteristic board trace imped-
ance defined based on board material and trace dimensions,
a matching series resistor into the trace from the output of
the OPA2682 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 destina-
tion device: this total effective impedance should be set to
match the trace impedance. The high output voltage and
current capability of the OPA2682 allows multiple destina-
tion 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 unac-
ceptable, 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 Recom-
mended 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 OPA2682 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 OPA2682
onto the board.
INPUT AND ESD PROTECTION
The OPA2682 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 9.
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 OPA2682), 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
–V _CC
FIGURE 9. Internal ESD Protection.
Internal
Circuitry
®
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OPA2682