Datasheet

INPUT AND ESD PROTECTION
External
Pin
+V
CC
-VCC
Internal
Circuitry
OPA3832
SBOS370A – DECEMBER 2006 – REVISED AUGUST 2008 ............................................................................................................................................ www.ti.com
d) Connections to other wideband devices on the e) Socketing a high-speed part is not
board may be made with short direct traces or recommended. The additional lead length and
through onboard transmission lines. For short pin-to-pin capacitance introduced by the socket can
connections, consider the trace and the input to the create an extremely troublesome parasitic network
next device as a lumped capacitive load. Relatively that can make it almost impossible to achieve a
wide traces (50mils to 100mils) should be used, smooth, stable frequency response. Best results are
preferably with ground and power planes opened up obtained by soldering the OPA3832 directly onto the
around them. Estimate the total capacitive load and board.
set R
S
from the typical characteristic curve, Figure 5 .
Low parasitic capacitive loads ( < 5pF) may not need
an R
S
since the OPA3832 is nominally compensated
The OPA3832 is built using a very high-speed,
to operate with a 2pF parasitic load. Higher parasitic
complementary bipolar process. The internal junction
capacitive loads without an R
S
are allowed as the
breakdown voltages are relatively low for these very
signal gain increases (increasing the unloaded phase
small geometry devices. These breakdowns are
margin). If a long trace is required, and the 6dB
reflected in the Absolute Maximum Ratings table. All
signal loss intrinsic to a doubly-terminated
device pins are protected with internal ESD protection
transmission line is acceptable, implement a matched
diodes to the power supplies, as shown in Figure 56 .
impedance transmission line using microstrip or
stripline techniques (consult an ECL design handbook
for microstrip and stripline layout techniques). A 50 Ω
environment is normally not necessary onboard, 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 OPA3832 is used as well as a terminating
shunt resistor at the input of the destination device.
Figure 56. Internal ESD Protection
Remember also that the terminating impedance is the
parallel combination of the shunt resistor and the
input impedance of the destination device; this total
These diodes provide moderate protection to input
effective impedance should be set to match the trace
overdrive voltages above the supplies as well. The
impedance. If the 6dB attenuation of a
protection diodes can typically support 30mA
doubly-terminated transmission line is unacceptable,
continuous current. Where higher currents are
a long trace can be series-terminated at the source
possible (that is, in systems with ± 15V supply parts
end only. Treat the trace as a capacitive load in this
driving into the OPA3832), current-limiting series
case and set the series resistor value as shown in the
resistors should be added into the two inputs. Keep
typical characteristic curve, Figure 5 . This
these resistor values as low as possible, since high
configuration will not preserve signal integrity as well
values degrade both noise performance and
as a doubly-terminated line. If the input impedance of
frequency response.
the destination device is low, there will be some
signal attenuation as a result of the voltage divider
formed by the series output into the terminating
impedance.
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