OPA3875 Datasheet, PDF (17/22 Pages) Burr-Brown (TI) – Triple 2:1 High-Speed Video Multiplexer

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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 OPA3875 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 OPA3875 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
Figure 5. 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 OPA3875
is not recommended. The additional lead length
and pin-to-pin capacitance introduced by the socket
can create an extremely troublesome parasitic
network which can make it almost impossible to
achieve a smooth, stable frequency response. Best
results are obtained by soldering the OPA3875 onto
the board.
OPA3875
SBOS341B – DECEMBER 2006 – REVISED DECEMBER 2006
INPUT AND ESD PROTECTION
The OPA3875 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 have limited ESD protection using
internal diodes to the power supplies as shown in
Figure 31.
+V_CC
External
Pin
Internal
Circuitry
-V_CC
Figure 31. 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 (for example, in systems with ±15V supply
parts driving into the OPA3875), current-limiting
series resistors should be added into the two inputs.
Keep these resistor values as low as possible
because high values degrade both noise
performance and frequency response.
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