OPA2682 Datasheet, PDF (16/20 Pages) Burr-Brown (TI) – Dual, Wideband, Fixed Gain BUFFER AMPLIFIER With Disable

The specifications described above, though familiar in the
industry, consider voltage and current limits separately. In
many applications, it is the voltage x 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 OPA2682’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
OPA2682 can drive ±2.5V into 25Ω or ±3.5V into 50Ω
without exceeding the output capabilities or the 1W dissipa-
tion limit for a single channel. A 100Ω load line (the
standard test circuit load) shows the full ±3.9V output swing
capability, as shown in the Typical Specifications.
The minimum specified output voltage and current over
temperature are set by worst-case simulations at the cold
temperature extreme. Only at cold startup will the output
current and voltage decrease to the numbers shown in the
decrease the phase margin. Several external solutions to this
problem have been suggested. When the primary consider-
ations are frequency response flatness, pulse response fidel-
ity, 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
vs Capacitive Load and the resulting frequency response at
the load. Parasitic capacitive loads greater than 2pF can begin
to degrade the performance of the OPA2682. 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 OPA2682 output pin (see Board
Layout Guidelines).
guaranteed tables. As the output transistors deliver power,
their junction temperatures will increase, decreasing their
V_BE’s (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.
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 match-
ing 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 (8-pin packages) 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 after these
supply current limiting resistors, and directly at the supply pins.
DISTORTION PERFORMANCE
The OPA2682 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. Increasing the load impedance im-
proves 2nd harmonic distortion directly. Remember that the
total load includes the feedback network-in the non-inverting
configuration (see Figure 1); this is the sum of 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).
In most op amps, increasing the output voltage swing increases
harmonic distortion directly. The Typical Performance Curves
show the 2nd harmonic increasing at a little less than the
expected 2X rate while the 3rd harmonic increases at a little
less than the expected 3X rate. Where the test power doubles,
the difference between it and the 2nd harmonic decreases less
than the expected 6dB while the difference between it and the
3rd decreases by less than the expected 12dB. This also shows
up in the 2-tone, 3rd-order intermodulation spurious (IM3)
response curves. The 3rd-order spurious levels are extremely
low at low output power levels. The output stage continues to
DRIVING CAPACITIVE LOADS
hold them low even as the fundamental power reaches very
high levels. As the Typical Performance Curves show, the
One of the most demanding and yet very common load spurious intermodulation powers do not increase as predicted
conditions for an op amp is capacitive loading. Often, the by a traditional intercept model. As the fundamental power
capacitive load is the input of an A/D converter—including level increases, the dynamic range does not decrease signifi-
additional external capacitance which may be recommended cantly. For two tones centered at 20MHz, with 10dBm/tone
to improve A/D linearity. A high-speed amplifier like the into a matched 50Ω load (i.e., 2Vp-p for each tone at the load,
OPA2682 can be very susceptible to decreased stability and which requires 8Vp-p for the overall 2-tone envelope at the
closed-loop response peaking when a capacitive load is output pin), the Typical Performance Curves show 62dBc
placed directly on the output pin. When the amplifier’s open- difference between the test-tone power and the 3rd-order
loop output resistance is considered, this capacitive load intermodulation spurious levels. This exceptional performance
introduces an additional pole in the signal path that can improves further when operating at lower frequencies.
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OPA2682
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