OPA2631 Datasheet, PDF (17/17 Pages) Burr-Brown (TI) – Dual, Low Power, Single-Supply OPERATIONAL AMPLIFIER

tance can add a pole and/or zero below 500MHz that can
effect circuit operation. Keep resistor values as low as
possible consistent with load driving considerations. The
750Ω feedback used in the typical performance specifica-
tions is a good starting point for design. See Figure 6 for the
unity gain follower application.
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
OPA2631 is nominally compensated to operate with a 2pF
parasitic load. Higher parasitic capacitive loads without an
R_S are allowed as the signal gain increases (increasing the
unloaded phase margin) If a long trace is required, and the
6dB signal loss intrinsic to a doubly terminated transmission
line is acceptable, implement a matched impedance trans-
mission line using microstrip or stripline techniques (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 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 OPA2631
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. If the 6dB attenuation of a doubly termi-
nated 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 Recommended 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 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 OPA2631 onto the board.
INPUT AND ESD PROTECTION
The OPA2631 is built using a very high speed complemen-
tary bipolar process. The internal junction breakdown volt-
ages are relatively low for this very small geometry device.
This breakdown is reflected in the Absolute Maximum
Ratings table. All device pins are protected with internal
ESD protection 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 OPA2631), 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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OPA2631