OPA2846_14 Datasheet, PDF (22/30 Pages) Texas Instruments – Dual, Wideband, Low-Noise, Voltage-Feedback Operational Amplifier

OPA2846
SBOS274C −JUNE 2003 − REVISED AUGUST 2008
resistor directly under the package on the other side of the
board between the output and inverting input pins. Even
with a low parasitic capacitance shunting the external
resistors, excessively high resistor values can create
significant time constants that can degrade performance.
Good axial metal-film or surface-mount resistors have
approximately 0.2pF in shunt with the resistor. For resistor
values > 1.5kΩ, this parasitic capacitance can add a pole
and/or a zero below 500MHz that can affect circuit
operation. Keep resistor values as low as possible,
consistent with load driving considerations. It has been
suggested here that a good starting point for design would
be to set R_G to 50Ω. Doing this will automatically keep the
resistor noise terms low, and minimize the effect of their
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 (50mils to
100mils) should be used, preferably with ground and
power planes opened up around them. Estimate the total
capacitive load and set R_S from the plot of recommended
R_S vs Capacitive Load. Low parasitic capacitive loads
(< 5pF) may not need an R_S since the OPA2846 is
nominally compensated to operate with a 2pF parasitic
load. Higher parasitic capacitive loads without an R_S are
allowed as the signal gain increases (increasing the
unloaded phase margin). If a long trace is required, and the
6dB signal loss intrinsic to a doubly-terminated transmis-
sion 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 50Ω environment is
normally not necessary on board, 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 OPA2846 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. If the 6dB attenuation of a doubly-terminated
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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, R_S vs 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.
e) Socketing a high-speed part like the OPA2846 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
OPA2846 onto the board.
INPUT AND ESD PROTECTION
The OPA2846 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 13.
+V_CC
External
Pin
Internal
Circuitry
−V_CC
Figure 14. Internal ESD Protection
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
OPA2846), 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.
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