OPA2686 Datasheet, PDF (18/18 Pages) Burr-Brown (TI) – Dual, Wideband, Low Noise, Voltage Feedback OPERATIONAL AMPLIFIER

c) Careful selection and placement of external compo-
nents will preserve the high frequency performance of
the OPA2686. Resistors should be a very low reactance
type. Surface-mount resistors work best and allow a tighter
overall layout. Metal-film and carbon composition, axially-
leaded resistors can also provide good high frequency per-
formance. Again, keep their leads and PC board trace length
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 capaci-
tance, always position the feedback and series output resis-
tor, if any, as close as possible to the output pin. Other
network components, such as non-inverting input termina-
tion 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 ap-
proximately 0.2pF in shunt with the resistor. For resistor
values > 1.5kΩ, this parasitic capacitance can add a pole
and/or a zero below 500MHz that can effect circuit opera-
tion. 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 to set R_G to 50Ω.
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_S from the plot of recommended R_S vs Capacitive
Load. Low parasitic capacitive loads (< 5pF) may not need
an R_S since the OPA2686 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 OPA2686 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 plot
of R_S vs Capacitive Load. This will not preserve signal
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 OPA2686 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 OPA2686
onto the board.
INPUT AND ESD PROTECTION
The OPA2686 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 13.
External
Pin
+V_CC
–V _CC
Internal
Circuitry
FIGURE 13. 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 OPA2686), 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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OPA2686
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