OPA643 Burr-Brown, OPA643 Datasheet - Page 16

• Manufacturer Part Number: OPA643
• Description: Wideband Low Distortion / High Gain OPERATIONAL AMPLIFIER
• Manufacturer: Burr-Brown

e) Socketing a high speed part like the OPA643 is not
INPUT AND ESD PROTECTION
The OPA643 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 protected with internal
ESD protection diodes to the power supplies as shown in
Figure 11
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 OPA643), 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.
capacitive loads without an R
gain increases (increasing the unloaded phase margin). If
a long trace is required, and the 6dB signal loss intrinsic
to a doubly terminated transmission line is acceptable,
implement a matched impedance transmission line using
microstrip or stripline techniques (consult an ECL design
handbook for microstrip and stripline layout techniques).
A 50 environment is normally not necessary on board,
and in fact a higher impedance environment will improve
distortion as shown in the distortion vs 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 OPA643 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. Multiple destination devices are best
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
Capacitive 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 impedance.
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
OPA643 onto the board. If socketing for the DIP package
is desired, high frequency flush mount pins (e.g.,
McKenzie Technology #710C) can give good results.
®
OPA643
S
are allowed as the signal
S
vs
16
High input overdrive signals can also cause significant
differential voltage between the + and – inputs. Where this
voltage can exceed the maximum rated voltage of 1.2V,
external Schottky protection diodes should be added across
the two inputs. Again, the capacitance added by these diodes
can degrade the noise and AC performance and should be
used only where necessary. Figure 12 shows a fully featured
input protection circuit for the OPA643. This is the circuit of
Figure 1 with additional limiting resistors into the inputs and
Schottky clamp diodes across the inputs. These resistor
values have been selected to limit the degradation in noise
and frequency response, achieve DC bias current cancellation,
and limit the current that will flow under overdrive conditions.
FIGURE 11. Internal ESD Protection.
FIGURE 12. OPA643 Gain of +5 with Input Protection.
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
Several PC boards are available in the initial evaluation of
circuit performance using the OPA643 in its three package
styles. Two partially assembled boards are available for sale
to support the DIP (P suffix) and SO-8 (U-suffix) packages.
These boards come partially assembled with power supply
and I/O connectors but do not have the amplifier or resistor
networks loaded. Both boards are configured for low
50 Source
External
Pin
50
125
D1
50
126
D2
+V
–V
CC
CC
D1, D2
OPA643
+5V
–5V
505
Decoupling Not Shown
IN5911 (or equivalent)
Power Supply
Internal
Circuitry
50