Datasheet

OPA2677
23
SBOS126I
www.ti.com
Again, keep leads and PCB trace length as short as possible.
Never use wire-wound type resistors in a high-frequency
application. Although the output pin and inverting input pin
are the most sensitive to parasitic capacitance, always posi-
tion the feedback and series output resistor, if any, as close
as possible to the output pin. Other network components,
such as noninverting input termination resistors, should also
be placed close to the package. Where double-side compo-
nent 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. The frequency response
is primarily determined by the feedback resistor value as
described previously. Increasing the value reduces the band-
width, whereas decreasing it gives a more peaked frequency
response. 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 resistor,
rather than a direct short, is recommended for the unity-gain
follower application. A current-feedback op amp requires a
feedback resistor even in the unity-gain follower configura-
tion 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 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
because the
OPA2677 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 impedance transmission
line using microstrip or stripline techniques (consult an ECL
design handbook for microstrip and stripline layout tech-
niques). A 50Ω environment is normally not necessary on
board; 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 OPA2677 is used, as well
as a terminating shunt resistor at the input of the destination
device. Remember also that the terminating impedance is
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. The high output voltage and current capa-
bility of the OPA2677 allows multiple destination 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 unacceptable, a
long trace can be series-terminated at the source end only.
FIGURE 16. Internal ESD Protection.
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 OPA2677 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 OPA2677
directly onto the board.
f) Use the –V
S
plane to conduct heat out of the
HSOP-8 PowerPAD package (OPA2677IDDA) or the
QFN-16 (OPA2677IRGV). These packages attach the die
directly to an exposed thermal pad on the bottom, which
should be soldered to the board. This pad must be connected
electrically to the same voltage plane as the most negative
supply applied to the OPA2677 (in Figure 6, this would be
ground), which must have a minimum area of 3.5" x 3.5"
(88.9mm x 88.9mm) to produce the
θ
JA
values in the Electri-
cal Characteristics
tables.
INPUT AND ESD PROTECTION
The OPA2677 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 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 overdrive
voltages above the supplies as well. The protection diodes
can typically support 30mA continuous current. Where higher
currents are possible (for example, in systems with ±15V
supply parts driving into the OPA2677), current-limiting se-
ries 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.
External
Pin
+V
S
–V
S
Internal
Circuitry