Datasheet

-V
CC
+V
CC
External
Pin
Internal
Circuitry
OPA694
SBOS319G –SEPTEMBER 2004–REVISED JANUARY 2010 www.ti.com
resistor used in the Electrical Characteristic tables at trace as a capacitive load in this case and set the
a gain of +2 on ±5V supplies is a good starting point series resistor value as shown in the plot of
for design. Note that a 430Ω feedback resistor, rather Recommended R
S
vs Capacitive Load. This will not
than a direct short, is recommended for the unity-gain preserve signal integrity as well as a
follower application. A current-feedback op amp doubly-terminated line. If the input impedance of the
requires a feedback resistor even in the unity-gain destination device is low, there will be some signal
follower configuration to control stability. attenuation due to the voltage divider formed by the
series output into the terminating impedance.
d) Connections to other wideband devices on the
board may be made with short, direct traces or e) Socketing a high-speed part like the OPA694 is
through onboard transmission lines. For short not recommended. The additional lead length and
connections, consider the trace and the input to the pin-to-pin capacitance introduced by the socket can
next device as a lumped capacitive load. Relatively create an extremely troublesome parasitic network
wide traces (50mils to 100mils, or 1,270mm to which can make it almost impossible to achieve a
2,540mm) should be used, preferably with ground smooth, stable frequency response. Best results are
and power planes opened up around them. Estimate obtained by soldering the OPA694 onto the board..
the total capacitive load and set R
S
from the plot of The additional lead length and pin-to-pin capacitance
Recommended R
S
vs Capacitive Load (Figure 15). introduced by the socket can create an extremely
Low parasitic capacitive loads (< 5pF) may not need troublesome parasitic network which can make it
an R
S
, since the OPA694 is nominally compensated almost impossible to achieve a smooth, stable
to operate with a 2pF parasitic load. If a long trace is frequency response. Best results are obtained by
required, and the 6dB signal loss intrinsic to a soldering the OPA694 onto the board.
doubly-terminated transmission line is acceptable,
implement a matched impedance transmission line
INPUT AND ESD PROTECTION
using microstrip or stripline techniques (consult an
The OPA694 is built using a very high speed
ECL design handbook for microstrip and stripline
complementary bipolar process. The internal junction
layout techniques). A 50Ω environment is normally
breakdown voltages are relatively low for these very
not necessary onboard, and in fact, a higher
small geometry devices. These breakdowns are
impedance environment will improve distortion, as
reflected in the Absolute Maximum Ratings table. All
shown in the Distortion versus Load plots. With a
device pins have limited ESD protection using internal
characteristic board trace impedance defined based
diodes to the power supplies, as shown in Figure 42.
on board material and trace dimensions, a matching
series resistor into the trace from the output of the
These diodes provide moderate protection to input
OPA694 is used as well as a terminating shunt
overdrive voltages above the supplies as well. The
resistor at the input of the destination device.
protection diodes can typically support 30mA
Remember also that the terminating impedance will
continuous current. Where higher currents are
be the parallel combination of the shunt resistor and
possible (for example, in systems with ±15V supply
the input impedance of the destination device: this
parts driving into the OPA694), current-limiting series
total effective impedance should be set to match the
resistors should be added into the two inputs. Keep
trace impedance. The high output voltage and current
these resistor values as low as possible, since high
capability of the OPA694 allows multiple destination
values degrade both noise performance and
devices to be handled as separate transmission lines,
frequency response.
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
Figure 42. Internal ESD Protection
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Product Folder Link(s): OPA694