OPA653 Datasheet, PDF (12/19 Pages) Texas Instruments – Wideband, Fixed Gain, JFET-Input AMPLIFIER

OPA653
SBOS348 – DECEMBER 2008........................................................................................................................................................................................... www.ti.com
Other network components, such as noninverting
input termination resistors, should also be placed
close to the package. Even with a low parasitic
capacitance, excessively high resistor values can
create significant time constants that can degrade
device performance. Good axial metal film or
surface-mount resistors have approximately 0.2 pF in
shunt with the resistor. For resistor values greater
than 1.5 kΩ, this parasitic capacitance can add a pole
and/or zero below 500 MHz that can affect circuit
operation. Keep resistor values as low as possible.
Using values less than 500 Ω automatically holds the
resistor noise terms low, and minimizes the effects of
parasitic capacitance.
d) Connections to other wideband devices on the
board may be made with short direct traces or
through onboard transmission lines. For short
connections, consider the trace and the input to the
next device as a lumped capacitive load. Relatively
wide traces (50 mils to 100 mils, or 1,27 cm to 2,54
cm) should be used. Estimate the total capacitive
load and set R_ISO from the plot of Recommended
R_ISO vs Capacitive Load (Figure 17). Low parasitic
capacitive loads (less than 5 pF) may not need an
R_ISO because the OPA653 is nominally compensated
to operate with a 2-pF parasitic load.
Higher parasitic capacitive loads without an R_ISO are
allowed as the signal gain increases (increasing the
unloaded phase margin). If a long trace is required,
and the 6-dB 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-Ω environment is normally
not necessary onboard, and in fact a higher
impedance environment improves 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
OPA653 is used as well as a terminating shunt
resistor at the input of the destination device.
Remember also that the terminating impedance is 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. If the 6-dB 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_ISO vs Capacitive Load (Figure 17). This
configuration does 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 as a result of the voltage
divider formed by the series output into the
terminating impedance.
e) Socketing a high-speed part such as the
OPA653 is not recommended. The additional lead
length and pin-to-pin capacitance introduced by the
socket can create an extremely troublesome parasitic
network that can make it almost impossible to
achieve a smooth, stable frequency response. Best
results are obtained by soldering the OPA653 directly
onto the board.
Input and ESD Protection
The OPA653 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 Figure 25 shows.
+V_CC
External
Pin
Internal
Circuitry
-V_CC
Figure 25. Internal ESD Protection
These diodes provide moderate protection to input
overdrive voltages above the supplies as well. The
protection diodes can typically support 30-mA
continuous current. Where higher currents are
possible (for example, in systems with ±12-V supply
parts driving into the OPA653), current limiting series
resistors should be added into the two inputs. Keep
these resistor values as low as possible because high
values degrade both noise performance and
frequency response.
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