d) Connections to other wideband devices on the board
may be made with short direct traces or through on-
board transmission lines. For short connections, con-
sider 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 recom-
mended Rs vs CLOAD. Low parasitic capacitive loads
(< 5pF) may not need an RS since the OPA2683 is
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 OPA2683 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 OPA2683 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 Rs vs CLOAD. 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 OPA2683 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 OPA2683 onto the board.
INPUT AND ESD PROTECTION
The OPA2683 is built using a very high-speed complemen-
tary bipolar process. The internal junction breakdown volt-
ages are relatively low for these very small geometry de-
vices. These breakdowns are reflected in the Absolute Maxi-
mum 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 (for example, in systems with ±15V
supply parts driving into the OPA2683), current-limiting se-
ries 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.
+VCC
External
Pin
Internal
Circuitry
–VCC
FIGURE 15. Internal ESD Protection.
OPA2683
SBOS244H
24
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