Inverting feedback optimization is somewhat complicated by
the impedance matching requirement at the input, as shown
in Figure 2. The resistor values shown in Table III should be
used in this case.
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 A/D converter—including
additional external capacitance which may be recommended
to improve A/D linearity. A high-speed, high open-loop gain
amplifier like the OPA695 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.
OUTPUT CURRENT AND VOLTAGE
The OPA695 provides output voltage and current capabilities
that are consistent with driving doubly-terminated 50Ω lines.
For a 100Ω load at a gain of +8 (see Figure 1), the total load
is the parallel combination of the 100Ω load and the 456Ω
total feedback network impedance. This 82Ω load will require
no more than 45mA output current to support the ±3.7V
minimum output voltage swing specified for 100Ω loads. This
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
which is more relevant to circuit operation. Refer to the
Output Voltage and Current Limitations plot in the Typical
Characteristic 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 provide
a more detailed view of the OPA695 output drive capabilities.
Superimposing resistor load lines onto the plot shows the
available output voltage and current for specific loads.
The Typical Characteristics show the recommended RS ver-
sus capacitive load and the resulting frequency response at
the load. Parasitic capacitive loads greater than 2pF can
begin to degrade the performance of the OPA695. 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 OPA695 output pin
(see Board Layout Guidelines).
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
specification tables. As the output transistors deliver power,
the junction temperatures will increase, decreasing the VBEs
(increasing the available output voltage swing) and increas-
ing the current gains (increasing the available output cur-
rent). 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 junc-
tion temperatures will be higher than the minimum specified
operating ambient.
DISTORTION PERFORMANCE
The OPA695 provides good distortion performance into a
100Ω load on ±5V supplies. Relative to alternative solutions,
the OPA695 holds much lower distortion at higher frequen-
cies (> 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, the total load includes the feedback network. In
the non-inverting configuration (Figure 1), this is the sum of
RF + RG, while in the inverting configuration, it is just RF. 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).
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 will, 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 will 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 directly on the supply pins after these
supply current-limiting resistors.
In most op amps, increasing the output voltage swing in-
creases harmonic distortion directly. The Typical Perfor-
mance 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.
OPA695
SBOS293B
25
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