Datasheet

DRIVING CAPACITIVE LOADS
DISTORTION PERFORMANCE
OPA2695
SBOS354A – APRIL 2008 – REVISED AUGUST 2008 ...................................................................................................................................................... www.ti.com
total feedback network impedance. This 82 Ω load
requires no more than 45mA output current to
One of the most demanding, and yet very common,
support the ± 3.7V minimum output voltage swing
load conditions for an op amp is capacitive loading.
specified for 100 Ω loads. This minimal requirement is
Often, the capacitive load is the input of an
well below the minimum ± 90mA specifications.
ADC — including additional external capacitance that
The specifications described above, though familiar in may be recommended to improve analog-to-digital
the industry, consider voltage and current limits linearity. A high-speed, high open-loop gain amplifier
separately. In many applications, it is the voltage × such as the OPA2695 can be very susceptible to
current, or V-I, product that is more relevant to circuit decreased stability and closed-loop response peaking
operation. Refer to the Output Voltage and Current when a capacitive load is placed directly on the
Limitations plot (Figure 21 ) in the Typical output pin. When the amplifier open-loop output
Characteristics . The X- and Y-axes of this graph resistance is considered, this capacitive load
show the zero-voltage output current limit and the introduces an additional pole in the signal path that
zero-current output voltage limit, respectively. The can decrease the phase margin. Several external
four quadrants provide a more detailed view of the solutions to this problem have been suggested. When
OPA2695 output drive capabilities. Superimposing the primary considerations are frequency response
resistor load lines onto the plot shows the available flatness, pulse response fidelity, and/or distortion, the
output voltage and current for specific loads. simplest and most effective solution is to isolate the
capacitive load from the feedback loop by inserting a
The minimum specified output voltage and current
series isolation resistor between the amplifier output
overtemperature are set by worst-case simulations at
and the capacitive load. This isolation resistor does
the cold temperature extreme. Only at cold startup do
not eliminate the pole from the loop response, but
the output current and voltage decrease to the
rather shifts it and adds a zero at a higher frequency.
numbers shown in the specification tables. As the
The additional zero acts to cancel the phase lag from
output transistors deliver power, the junction
the capacitive load pole, thus increasing the phase
temperatures increase, decreasing the V
BE
s
margin and improving stability.
(increasing the available output voltage swing) and
increasing the current gains (increasing the available The Typical Characteristics show the recommended
output current). In steady-state operation, the R
S
versus capacitive load and the resulting frequency
available output voltage and current always are response at the load. Parasitic capacitive loads
greater than that shown in the over-temperature greater than 2pF can begin to degrade the
specifications, because the output stage junction performance of the OPA2695. Long PCB traces,
temperatures are greater than the minimum specified unmatched cables, and connections to multiple
operating ambient. devices can easily cause this value to be exceeded.
Always consider this effect carefully and add the
To maintain maximum output stage linearity, no
recommended series resistor as close as possible to
output shortcircuit protection is provided. This lack of
the OPA2695 output pin (see the Board Layout
protection is normally a problem, because most
Guidelines section).
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
The OPA2695 provides good distortion performance
adjacent positive power-supply pin does, in most
into a 100 Ω load on ± 5V supplies. Relative to
cases, destroy the amplifier. If additional short-circuit
alternative solutions, the OPA2695 holds much lower
protection is required, consider a small series resistor
distortion at higher frequencies (> 20MHz). Generally,
in the power-supply leads. Under heavy output loads,
until the fundamental signal reaches very high
this additional resistor reduces the available output
frequency or power levels, the 2nd harmonic will
voltage swing. A 5 Ω series resistor in each
dominate the distortion with a negligible 3rd-harmonic
power-supply lead limits the internal power
component. Focusing then on the 2nd harmonic,
dissipation to less than 1W for an output short circuit
increasing the load impedance improves distortion
while decreasing the available output voltage swing
directly. Remember that the total load includes the
only 0.25V for up to 50mA desired load currents.
feedback network. In the noninverting configuration
Always place the 0.1 µ F power-supply decoupling
(Figure 68 ), this value is the sum of R
F
+ R
G
, while in
capacitors directly on the supply pins after these
the inverting configuration, it is only R
F
. Also,
supply current-limiting resistors.
providing an additional supply decoupling capacitor
(0.01 µ F) between the supply pins (for bipolar
operation) improves the 2nd-order distortion slightly
(3dB to 6dB).
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