OPA687 Burr-Brown, OPA687 Datasheet - Page 16

• Manufacturer Part Number: OPA687
• Description: Wideband / Ultra-Low Noise / Voltage Feedback OPERATIONAL AMPLIFIER With Power Down
• Manufacturer: Burr-Brown

back 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 resis-
tors, 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 > 2.0k ,
this parasitic capacitance can add a pole and/or a zero below
400MHz that can effect circuit operation. Keep resistor
values as low as possible, consistent with load driving
considerations. It has been suggested here that a good
starting point for design would be set the R
for non-inverting applications. Doing this will automatically
keep the resistor noise terms low, and minimize the effect of
their parasitic capacitance.
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
Load. Low parasitic capacitive loads (< 4pF) may not need
an R
operate with a 2pF parasitic load. Higher parasitic cap. loads
without an R
(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 50 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 OPA687 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
S
since the OPA687 is nominally compensated to
S
from the plot of recommended R
®
S
OPA687
are allowed as the signal gain increases
G
S
be set to 39.2
vs Capacitive
16
FIGURE 10. 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 OPA687), current-limiting series resis-
tors 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.
this case and set the series resistor value as shown in the plot
of R
integrity as well as a doubly-terminated line. If the input
impedance of the destination 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 like the OPA687 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 OPA687
onto the board.
INPUT AND ESD PROTECTION
The OPA687 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 10.
External
S
vs Capacitive Load. This will not preserve signal
Pin
+V
–V
CC
CC
Internal
Circuitry