OPA690 Datasheet, PDF (20/24 Pages) Texas Instruments – Wideband, Voltage Feedback OPERATIONAL AMPLIFIER With Disable

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 since the
OPA690 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 trans-
mission line using microstrip or stripline techniques (consult
an ECL design handbook for microstrip and stripline layout
techniques). A 50Ω environment 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 characteristic board trace impedance defined
(based on board material and trace dimensions), a matching
series resistor into the trace from the output of the OPA690
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 effective impedance should be set to match the
trace impedance. The high output voltage and current capa-
bility of the OPA690 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. Treat
the trace as a capacitive load in this case and set the series
resistor value as shown in the plot of “Recommended 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 attenua-
tion due to the voltage divider formed by the series output
into the terminating impedance.
e) Socketing a high-speed part like the OPA690 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 OPA690
onto the board.
INPUT AND ESD PROTECTION
The OPA690 is built using a very high-speed complementary
bipolar process. The internal junction breakdown voltages
are relatively low for these very small geometry devices.
These breakdowns are reflected in the Absolute Maximum
Ratings table. All device pins are protected with internal
ESD protection diodes to the power supplies as shown in
Figure 15.
External
Pin
+V_CC
–V _CC
Internal
Circuitry
FIGURE 15. 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 OPA690), 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.
20
OPA690
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