Datasheet

INPUT AND ESD PROTECTION
External
Pin
+V
CC
-V
CC
Internal
Circuitry
OPA2695
SBOS354A – APRIL 2008 – REVISED AUGUST 2008 ...................................................................................................................................................... www.ti.com
wide traces (50mils to 100mils or 1.27mm to 2.54mm, e) Socketing a high-speed part such as the
respectively) should be used, preferably with ground OPA2695 is not recommended. The additional lead
and power planes opened up around them. Estimate length and pin-to-pin capacitance introduced by the
the total capacitive load and set R
S
from the plot of socket can create an extremely troublesome parasitic
Recommended R
S
vs Capacitive Load (Figure 17 , network that can make it almost impossible to
Figure 37 , Figure 61 , and Figure 51 ). Low parasitic achieve a smooth, stable frequency response. Best
capacitive loads ( < 5pF) may not need an R
S
since results are obtained by soldering the OPA2695
the OPA2695 is nominally compensated to operate directly onto the board.
with a 2pF parasitic load. If a long trace is required,
and the 6dB signal loss intrinsic to a
doubly-terminated transmission line is acceptable,
The OPA2695 is built using a very high-speed,
implement a matched impedance transmission line
complementary bipolar process. The internal junction
using microstrip or stripline techniques (consult an
breakdown voltages are relatively low for these very
ECL design handbook for microstrip and stripline
small geometry devices. These breakdowns are
layout techniques). A 50 Ω environment is usually not
reflected in the Absolute Maximum Ratings table
necessary on board. In fact, a higher impedance
where an absolute maximum ± 6.5V supply is
environment does improve distortion, as shown in the
reported. All device pins have limited ESD protection
distortion versus load plots. With a characteristic
using internal diodes to the power supplies, as shown
board trace impedance defined (based on board
in Figure 80 .
material and trace dimensions), a matching series
resistor into the trace from the output of the OPA2695
These diodes also provide moderate protection to
is used. A terminating shunt resistor at the input of
input overdrive voltages above the supplies. The
the destination device is used as well. Remember
protection diodes can typically support 30mA
also that the terminating impedance is the parallel
continuous current. Where higher currents are
combination of the shunt resistor and the input
possible (for example, in systems with ± 15V supply
impedance of the destination device; this total
parts driving into the OPA2695), current-limiting
effective impedance should be set to match the trace
series resistors should be added into the two inputs.
impedance. The high output voltage and current
Keep these resistor values as low as possible
capability of the OPA2695 allows multiple destination
because high values degrade both noise performance
devices to be handled as separate transmission lines,
and frequency response.
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 R
S
versus capacitive load. If the input impedance of the
destination device is low, there will be some signal
attenuation because of the voltage divider formed by
the series output into the terminating impedance.
Figure 80. Internal ESD Protection
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