Datasheet

OPA684
21
SBOS219D
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nominally compensated to operate with a 2pF parasitic
load. If a long trace is required, and the 6dB signal loss
intrinsic to a doubly-terminated transmission line is ac-
ceptable, implement a matched impedance transmis-
sion line using microstrip or stripline techniques (consult
an ECL design handbook for microstrip and stripline
layout techniques). A 50Ω 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 OPA684 is used, as well as
a terminating shunt resistor at the input of the destina-
tion device. Remember also that the terminating imped-
ance 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 OPA684 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
R
S
vs C
LOAD
. 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 imped-
ance.
e) Socketing a high-speed part like the OPA684 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 fre-
quency response. Best results are obtained by soldering
the OPA684 onto the board.
External
Pin
+V
CC
–V
CC
Internal
Circuitry
FIGURE 15. Internal ESD Protection.
INPUT AND ESD PROTECTION
The OPA684 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 where an absolute maximum 13V across the
supply pins is reported. All device pins have limited ESD
protection using internal diodes to the power supplies, as
shown in Figure 15.
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 OPA684), 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.