OPA2695 Datasheet, PDF (28/40 Pages) Texas Instruments – Dual, Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2695
SBOS354 – APRIL 2008..................................................................................................................................................................................................... www.ti.com
total feedback network impedance. This 82Ω load
requires no more than 45mA output current to
support the ±3.7V minimum output voltage swing
specified for 100Ω loads. This minimal requirement is
well below the minimum ±90mA specifications.
The specifications described above, though familiar in
the industry, consider voltage and current limits
separately. In many applications, it is the voltage ×
current, or V-I, product that is more relevant to circuit
operation. Refer to the Output Voltage and Current
Limitations plot (Figure 21) in the Typical
Characteristics. The X- and Y-axes of this graph
show the zero-voltage output current limit and the
zero-current output voltage limit, respectively. The
four quadrants provide a more detailed view of the
OPA2695 output drive capabilities. Superimposing
resistor load lines onto the plot shows the available
output voltage and current for specific loads.
The minimum specified output voltage and current
overtemperature are set by worst-case simulations at
the cold temperature extreme. Only at cold startup do
the output current and voltage decrease to the
numbers shown in the specification tables. As the
output transistors deliver power, the junction
temperatures increase, decreasing the V_BEs
(increasing the available output voltage swing) and
increasing the current gains (increasing the available
output current). In steady-state operation, the
available output voltage and current always are
greater than that shown in the over-temperature
specifications, because the output stage junction
temperatures are greater than the minimum specified
operating ambient.
To maintain maximum output stage linearity, no
output shortcircuit protection is provided. This lack of
protection is normally a problem, because most
applications include a series-matching resistor at the
output that limits the internal power dissipation if the
output side of this resistor is shorted to ground.
However, shorting the output pin directly to the
adjacent positive power-supply pin does, in most
cases, destroy the amplifier. If additional short-circuit
protection is required, consider a small series resistor
in the power-supply leads. Under heavy output loads,
this additional resistor reduces the available output
voltage swing. A 5Ω series resistor in each
power-supply lead limits the internal power
dissipation to less than 1W for an output short circuit
while decreasing the available output voltage swing
only 0.25V for up to 50mA desired load currents.
Always place the 0.1µF power-supply decoupling
capacitors directly on the supply pins after these
supply current-limiting resistors.
DRIVING CAPACITIVE LOADS
One of the most demanding, and yet very common,
load conditions for an op amp is capacitive loading.
Often, the capacitive load is the input of an
ADC—including additional external capacitance that
may be recommended to improve analog-to-digital
linearity. A high-speed, high open-loop gain amplifier
such as the OPA2695 can be very susceptible to
decreased stability and closed-loop response peaking
when a capacitive load is placed directly on the
output pin. When the amplifier open-loop output
resistance is considered, this capacitive load
introduces an additional pole in the signal path that
can decrease the phase margin. Several external
solutions to this problem have been suggested. When
the primary considerations are frequency response
flatness, pulse response fidelity, and/or distortion, the
simplest and most effective solution is to isolate the
capacitive load from the feedback loop by inserting a
series isolation resistor between the amplifier output
and the capacitive load. This isolation resistor does
not eliminate the pole from the loop response, but
rather shifts it and adds a zero at a higher frequency.
The additional zero acts to cancel the phase lag from
the capacitive load pole, thus increasing the phase
margin and improving stability.
The Typical Characteristics show the recommended
R_S versus capacitive load and the resulting frequency
response at the load. Parasitic capacitive loads
greater than 2pF can begin to degrade the
performance of the OPA2695. Long PCB traces,
unmatched cables, and connections to multiple
devices can easily cause this value to be exceeded.
Always consider this effect carefully and add the
recommended series resistor as close as possible to
the OPA2695 output pin (see the Board Layout
Guidelines section).
DISTORTION PERFORMANCE
The OPA2695 provides good distortion performance
into a 100Ω load on ±5V supplies. Relative to
alternative solutions, the OPA2695 holds much lower
distortion at higher frequencies (> 20MHz). Generally,
until the fundamental signal reaches very high
frequency or power levels, the 2nd harmonic will
dominate the distortion with a negligible 3rd-harmonic
component. Focusing then on the 2nd harmonic,
increasing the load impedance improves distortion
directly. Remember that the total load includes the
feedback network. In the noninverting configuration
(Figure 68), this value is the sum of R_F + R_G, while in
the inverting configuration, it is only R_F. Also,
providing an additional supply decoupling capacitor
(0.01µF) between the supply pins (for bipolar
operation) improves the 2nd-order distortion slightly
(3dB to 6dB).
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