OPA3682 Datasheet, PDF (16/19 Pages) Burr-Brown (TI) – Triple, Wideband, Fixed Gain BUFFER AMPLIFIER With Disable

OPERATING SUGGESTIONS
GAIN SETTING
Setting the gain with the OPA3682 is very easy. For a gain
of +2, ground the –IN pin and drive the +IN pin with the
signal. For a gain of +1, leave the –IN pin open and drive the
+IN pin with the signal. For a gain of –1, ground the +IN pin
and drive the –IN pin with the signal. Since the internal
resistor values (but not their ratios) change significantly over
temperature and process, external resistors should not be
used to modify the gain.
OUTPUT CURRENT AND VOLTAGE
The OPA3682 provides output voltage and current capabili-
ties that are unsurpassed in a low cost monolithic op amp.
Under no-load conditions at 25°C, the output voltage typi-
cally swings closer than 1V to either supply rail; the guaran-
teed swing limit is within 1.2V of either rail. Into a 15Ω load
(the minimum tested load), it is guaranteed to deliver more
than ±135mA.
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,
which is more relevant to circuit operation. Refer to the
“Output Voltage and Current Limitations” plot in the Typical
Performance Curves. 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 give a
more detailed view of the OPA3682’s output drive capabili-
ties, noting that the graph is bounded by a “Safe Operating
Area” of 1W maximum internal power dissipation. Superim-
posing resistor load lines onto the plot shows that the
OPA3682 can drive ±2.5V into 25Ω or ±3.5V into 50Ω
without exceeding the output capabilities or the 1W dissipa-
tion limit. A 100Ω load line (the standard test circuit load)
shows the full ±3.9V output swing capability, as shown in
the Specifications Table.
The minimum specified output voltage and current over-
temperature are set by worst-case simulations at the cold
temperature extreme. Only at cold start-up will the output
current and voltage decrease to the numbers shown in the
guaranteed tables. As the output transistors deliver power,
their junction temperatures will increase, decreasing their
VBEs (increasing the available output voltage swing) and
increasing their current gains (increasing the available out-
put current). In steady-state operation, the available output
voltage and current will always be greater than that shown
in the over-temperature specifications since the output stage
junction temperatures will be higher than the minimum
specified operating ambient.
In order to maintain maximum output stage linearity, no
output short-circuit protection is provided. This will not
normally be a problem since most applications include a
series matching resistor at the output that will limit 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 will, in
most cases, destroy the amplifier. If additional short-
circuit protection is required, consider a small series resistor
in the power supply leads. This will, under heavy output
loads, reduce the available output voltage swing. A 5Ω series
resistor in each power supply lead will limit the internal
power dissipation to less than 1W for an output short circuit
while decreasing the available output voltage swing only
0.5V for up to 100mA 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, but yet very common load
conditions for an op amp is capacitive loading. Often, the
capacitive load is the input of an A/D converter—including
additional external capacitance which may be recommended
to improve A/D linearity. A high-speed amplifier like the
OPA3682 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’s open-loop
output resistance is considered, this capacitive load intro-
duces 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 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 Performance Curves show the recommended R_S
verses capacitive load and the resulting frequency response at
the load. Parasitic capacitive loads greater than 2pF can begin
to degrade the performance of the OPA3682. Long PC board
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 OPA3682 output pin (see Board
Layout Guidelines).
DISTORTION PERFORMANCE
The OPA3682 provides good distortion performance into a
100Ω load on ±5V supplies. Relative to alternative solutions,
it provides exceptional performance into lighter loads and/or
operating on a single +5V supply. Generally, until the funda-
mental 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 non-inverting configuration (Figure 1); this is the sum R_F
+ R_G, while in the inverting configuration, it is just R_F. Also,
providing an additional supply decoupling capacitor (0.1µF)
between the supply pins (for bipolar operation) improves the
2nd-order distortion slightly (3dB to 6dB).
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OPA3682
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