OPA2684 Datasheet, PDF (21/33 Pages) Texas Instruments – MINIMAL BANDWIDTH CHANGE VERSUS GAIN, 170MHz BANDWIDTH AT G = +2

the 1.6kΩ total feedback network impedance. This 94Ω load
will require no more than 40mA output current to support the
±3.8V minimum output voltage swing specified for 100Ω
loads. This is well under the specified minimum +120/–90mA
specifications over the full temperature range.
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
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 give a more
detailed view of the OPA2684’s 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 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
Electrical Characteristic tables. As the output transistors
deliver power, their junction temperatures will increase, de-
creasing their V_BE’s (increasing the available output voltage
swing) and increasing their current gains (increasing the
available output current). In steady-state operation, the avail-
able 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 matching
resistor at the output that will limit the internal power dissipa-
tion 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) can 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 0.25V for up to 50mA
desired load currents. Always place the 0.1µF power-supply
decoupling capacitors after these supply current limiting
resistors directly on the supply pins.
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 which may be recommended to im-
prove ADC linearity. A high-speed, high open-loop gain
amplifier like the OPA2684 can be very susceptible to de-
creased 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 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 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 vs
C_LOAD and the resulting frequency response at the load. The
1kΩ resistor shown in parallel with the load capacitor is a
measurement path and may be omitted. The required series
resistor value may be reduced by increasing the feedback
resistor value from its nominal recommended value. This will
increase the phase margin for the loop gain, allowing a lower
series resistor to be effective in reducing the peaking due
capacitive load. SPICE simulation can be effectively used to
optimize this approach. Parasitic capacitive loads greater
than 5pF can begin to degrade the performance of the
OPA2684. Long PCB traces, unmatched cables, and con-
nections 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
OPA2684 output pin (see Board Layout Guidelines).
DISTORTION PERFORMANCE
The OPA2684 provides very low distortion in a low-power
part. The CFB_PLUS architecture also gives two significant
areas of distortion improvement. First, in operating regions
where the 2nd-harmonic distortion due to output stage
nonlinearities is very low (frequencies < 1MHz, low output
swings into light loads) the linearization at the inverting node
provided by the CFB_PLUS design gives 2nd-harmonic distor-
tions that extend into the –90dBc region. Previous current-
feedback amplifiers have been limited to approximately
–85dBc due to the nonlinearities at the inverting input. The
second area of distortion improvement comes in a distortion
performance that is largely gain independent. To the extent
that the distortion at a particular output power is output stage
dependent, 3rd-harmonics particularly, and to a lesser ex-
tend 2nd-harmonic distortion, is constant as the gain is
increased. This is due to the constant loop gain versus signal
gain provided by the CFB_PLUS design. As shown in the
Typical Characteristics, while the 3rd-harmonic is constant
with gain, the 2nd-harmonic degrades at higher gains. This
is largely due to board parasitic issues. Slightly imbalanced
load return currents will couple into the gain resistor to cause
a portion of the 2nd-harmonic distortion. At high gains, this
imbalance has more gain to the output giving increased
2nd-harmonic distortion.
Relative to alternative amplifiers with < 2mA supply current,
the OPA2684 holds much lower distortion at higher frequen-
cies (> 5MHz) and to higher gains. Generally, until the
fundamental signal reaches very high frequency or power
levels, the 2nd-harmonic will dominate the distortion with a
OPA2684
21
SBOS239D
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