OPA2670IRGVR Datasheet, PDF (16/25 Pages) Texas Instruments – Single Port, High Output Current VDSL2 Line Driver with Power Control

OPA2670
SBOS434 – AUGUST 2010
while decreasing the available output voltage swing
only 0.5V for up to 100mA desired load currents.
Always place the 0.1mF 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 that
may be recommended to improve the ADC linearity.
A high-speed, high open-loop gain amplifier such as
the OPA2670 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 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 series
resistor does not eliminate the pole from the loop
response, but 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 Capacitive Load (see Figure 23 and Figure 24)
and the resulting frequency response at the load.
Parasitic capacitive loads greater than 2pF can begin
to degrade device performance. Long printed circuit
board (PCB) traces, unmatched cables, and
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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 OPA2670 output pin (see
the Board Layout Guidelines section).
DISTORTION PERFORMANCE
The OPA2670 provides good distortion performance
into a 100Ω load on +12V supplies. Relative to
alternative solutions, the amplifier provides
exceptional performance into lighter loads. Generally,
until the fundamental signal reaches very high
frequency or power levels, the second harmonic
dominates the distortion with a negligible
third-harmonic component. Focusing then on the
second harmonic, increasing the load impedance
improves distortion directly. Remember that the total
load includes the feedback network—in the
noninverting, fully-differential configuration (see
Figure 38), this value is the sum of 2R_F + R_G.
In most op amps, increasing the output voltage swing
directly increases harmonic distortion. The Typical
Characteristics show the second harmonic increasing
at a little less than the expected 2x rate, whereas the
third harmonic increases at a little less than the
expected 3x rate. Where the test power doubles, the
difference between it and the second harmonic
decreases less than the expected 6dB, whereas the
difference between it and the third harmonic
decreases by less than the expected 12dB. This
difference also shows up in the two-tone, third-order
intermodulation spurious (IM3) response curves. The
third-order spurious levels are extremely low at
low-output power levels. The output stage continues
to hold them low even as the fundamental power
reaches very high levels.
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