
17
OPA631, OPA632
nated 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 Recommended R
S
vs Capacitive
Load. This will not preserve signal integrity as well as a
doubly terminated line. If the input impedance of the desti-
nation 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 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 OPA631 and OPA632 onto the board.
INPUT AND ESD PROTECTION
The OPA631 and OPA632 are 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 are pro-
tected with internal ESD protection diodes to the power
supplies as shown in Figure 8.
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 OPA631 and OPA632), current-limit-
ing 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.
External
Pin
+V
CC
–V
CC
Internal
Circuitry
FIGURE 8. Internal ESD Protection.