OPA2652
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SBOS125A–JUNE 2000–REVISED MAY 2006
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 approximately 0.2pF in
shunt with the resistor. For resistor values >1.5kΩ,
this parasitic capacitance can add a pole and/or zero
below 500MHz that can effect circuit operation. Keep
resistor values as low as possible consistent with
load driving considerations. The 402Ω feedback
used in the typical performance specifications is a
good starting point for design. Note that a 25Ω
feedback resistor, rather than a direct short, is
suggested for the unity gain follower application. This
effectively isolates the inverting input capacitance
from the output pin that would otherwise cause
additional peaking in the gain of +1 frequency
response.
devices to be handled as separate transmission
lines, each with respective 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 RS vs Capacitive Load
(Figure 17). This configuration 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 OPA2652
is not recommended. The additional lead length
and pin-to-pin capacitance introduced by the socket
can create an extremely troublesome parasitic
network that can make it almost impossible to
achieve a smooth, stable frequency response. Best
results are obtained by soldering the OPA2652
directly onto the board.
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 RS from the plot of Recommended RS vs
Capacitive Load (Figure 17). Low parasitic capacitive
loads (< 5pF) may not need an RS since the
OPA2652 is nominally compensated to operate with
a 2pF parasitic load. Higher parasitic capacitive
loads without an RS 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
Input and ESD Protection
The OPA2652 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 37.
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 OPA2652), 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.
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
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 OPA2652 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
capability of the OPA2652 allows multiple destination
+VCC
External
Pin
Internal
Circuitry
-VCC
Figure 37. Internal ESD Protection
17
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