OPA2634 Datasheet, PDF (16/17 Pages) Burr-Brown (TI) – Dual, Wideband, Single-Supply OPERATIONAL AMPLIFIER

keep their leads and PC board traces as short as possible.
Never use wirewound type resistors in a high frequency
application. Since the output pin and inverting 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 non-inverting input termination resistors, should also be
placed close to the package. Where double-side component
mounting is allowed, place the feedback resistor directly
under the package on the other side of the board between the
output and inverting input pins. Even with a low parasitic
capacitance shunting the external resistors, excessively high
resistor values can create significant time constants that can
degrade performance. Good axial metal film or surface-
mount resistors have approximately 0.2pF in shunt with the
resistor. For resistor values > 1.5kΩ, this parasitic capaci-
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.
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
typical performance curve “Recommended R_S vs Capacitive
Load”. Low parasitic capacitive loads (< 5pF) may not need
an R_S since the OPA2634 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 transmission line using microstrip or
stripline techniques (consult an ECL design handbook for
microstrip and stripline layout techniques). A 50Ω environ-
ment 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 character-
istic board trace impedance defined (based on board material
and trace dimensions), a matching series resistor into the
trace from the output of the OPA2634 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 effec-
tive impedance should be set to match the trace impedance.
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
typical performance curve “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 OPA2634 onto the board.
INPUT AND ESD PROTECTION
The OPA2634 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 are protected with
internal ESD protection diodes to the power supplies, as
shown in Figure 9.
+V_CC
External
Pin
Internal
Circuitry
–V _CC
FIGURE 9. 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 OPA2634), 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.
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OPA2634
SBOS098A