OPA2680 Datasheet, PDF (21/21 Pages) Burr-Brown (TI) – Dual Wideband, Voltage Feedback OPERATIONAL AMPLIFIER With Disable

OPA2680 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 OPA2680
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 OPA2680 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 integ-
rity as well as a doubly terminated line. If the input imped-
ance 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 OPA2680 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 OPA2680
onto the board.
INPUT AND ESD PROTECTION
The OPA2680 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 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 (e.g., in systems with ±15V supply
parts driving into the OPA2680), 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.
+V_CC
External
Pin
–V _CC
FIGURE 16. Internal ESD Protection.
Internal
Circuitry
®
21
OPA2680