OPA2889 Datasheet, PDF (24/38 Pages) Burr-Brown (TI) – Dual, Low-Power, Wideband, Voltage Feedback OPERATIONAL AMPLIFIER with Disable

OPA2889
SBOS373B – JUNE 2007 – REVISED AUGUST 2008 ....................................................................................................................................................... www.ti.com
the op amp input noise voltage. As a minimum, the
OPA2889 requires an R_B value of 50Ω to damp out
parasitic-induced peaking—a direct short to ground
on the noninverting input runs the risk of a very
high-frequency instability in the input stage.
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 ADC linearity. A
high-speed, high open-loop gain amplifier such as the
OPA2889 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 open-loop output resistance of the amplifier
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 solution 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 ±5 Typical Chararacteristics show the
recommended R_S versus capacitive load (see
Figure 15 and Figure 16) and the resulting frequency
response at the load. Parasitic capacitive loads
greater than 2pF can begin to degrade the
performance of the OPA2889. Long PCB traces,
unmatched cables, and connections to multiple
devices can easily exceed this value. Always
consider this effect carefully, and add the
recommended series resistor as close as possible to
the OPA2889 output pin (see the Board Layout
Guidelines section).
DISTORTION PERFORMANCE
The OPA2889 provides good distortion performance
into a 200Ω 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 fundamental
signal reaches very high frequency or power levels,
the 2nd-harmonic dominates the distortion with a
negligible 3rd-harmonic component. Focusing then on
the 2nd-harmonic, increasing the load impedance
improves distortion directly. Remember that the total
load includes the feedback network; in the
noninverting configuration (see Figure 50), this total 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). Operating differentially also lowers
2nd-harmonic distortion terms (see the plot on the
front page).
In most op amps, increasing the output voltage swing
increases harmonic distortion directly. The output
stage used in the OPA2889 actually holds the
difference between fundamental power and the 2nd-
and 3rd-harmonic powers relatively constant with
increasing output power until very large output swings
are required ( > 4V_PP). This result 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 hold them low even as the
fundamental power reaches very high levels. As the
Typical Characteristics show, the spurious
intermodulation powers do not increase as predicted
by a traditional intercept model. As the fundamental
power level increases, the dynamic range does not
decrease significantly. For two tones centered at
1MHz, with 4dBm/tone into a matched 50Ω load (that
is, 1V_PP for each tone at the load, which requires
4V_PP for the overall 2-tone envelope at the output
pin), the Typical Characteristics show –73dBc
difference between the test tone powers and the
3rd-order intermodulation spurious powers. This
performance is exceptional for an amplifier with only
4.6mW of internal power dissipation.
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