OPA2674_17 Datasheet, PDF (26/36 Pages) Texas Instruments – Dual Wideband, High Output Current Operational Amplifier with Current Limit

OPA2674
SBOS270C − AUGUST 2003 − REVISED AUGUST 2008
age on the other side of the board between the output and
inverting input pins. The frequency response is primarily
determined by the feedback resistor value as described
previously. Increasing the value reduces the bandwidth,
whereas decreasing it gives a more peaked frequency re-
sponse. The 402Ω feedback resistor used in the Typical
Characteristics at a gain of +4 on ±6V supplies is a good
starting point for design. Note that a 511Ω feedback resis-
tor, rather than a direct short, is recommended for the uni-
ty-gain follower application. A current-feedback op amp re-
quires a feedback resistor even in the unity-gain follower
configuration to control stability.
d) Connections to other wideband devices on the board
may be made with short direct traces or through onboard
transmission lines. For short connections, consider the
trace and the input to the next device as a lumped capaci-
tive 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
(see page 10). Low parasitic capacitive loads (< 5pF) may
not need an R_S because the OPA2674 is nominally com-
pensated 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, imple-
ment a matched impedance transmission line using micro-
strip or stripline techniques (consult an ECL design hand-
book for microstrip and stripline layout techniques). A 50Ω
environment is normally not necessary onboard. In fact, a
higher impedance environment improves distortion; see
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 OPA2674 is used, as well as a termi-
nating shunt resistor at the input of the destination device.
Remember also that the terminating impedance is the par-
allel combination of the shunt resistor and the input imped-
ance of the destination device.
This total effective impedance should be set to match the
trace impedance. The high output voltage and current ca-
pability of the OPA2674 allows multiple destination de-
vices to be handled as separate transmission lines, each
with their own series and shunt terminations. If the 6dB at-
tenuation of a doubly-terminated transmission line is unac-
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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 R_S vs Capacitive Load. However, this does not preserve
signal integrity as well as a doubly-terminated line. If the
input impedance of the destination device is low, there is
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 OPA2674 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
OPA2674 onto the board.
INPUT AND ESD PROTECTION
The OPA2674 is built using a high-speed complementary
bipolar process. The internal junction breakdown voltages
are relatively low for these very small geometry devices
and are reflected in the absolute maximum ratings table.
All device pins have limited ESD protection using internal
diodes to the power supplies, as shown in Figure 16.
These diodes provide moderate protection to input over-
drive voltages above the supplies as well. The protection
diodes can typically support 30mA continuous current.
Where higher currents are possible (for example, in sys-
tems with ±15V supply parts driving into the OPA2674),
current-limiting series resistors should be added into the
two inputs. Keep these resistor values as low as possible,
because high values degrade both noise performance and
frequency response.
+V_CC
External
Pin
Internal
Circuitry
−V_CC
Figure 16. ESD Steering Diodes
26