Datasheet

DISTORTION PERFORMANCE
DRIVING CAPACITIVE LOADS
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 50 Ω to damp out
The OPA2889 provides good distortion performance
parasitic-induced peaking — a direct short to ground
into a 200 Ω load on ± 5V supplies. Relative to
on the noninverting input runs the risk of a very
alternative solutions, it provides exceptional
high-frequency instability in the input stage.
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
One of the most demanding and yet very common
negligible 3rd-harmonic component. Focusing then on
load conditions for an op amp is capacitive loading.
the 2nd-harmonic, increasing the load impedance
Often, the capacitive load is the input of an
improves distortion directly. Remember that the total
ADC — including additional external capacitance that
load includes the feedback network; in the
may be recommended to improve ADC linearity. A
noninverting configuration (see Figure 50 ), this total is
high-speed, high open-loop gain amplifier such as the
the sum of R
F
+ R
G
, while in the inverting
OPA2889 can be very susceptible to decreased
configuration it is just R
F
. Also, providing an
stability and closed-loop response peaking when a
additional supply-decoupling capacitor (0.1 µ F)
capacitive load is placed directly on the output pin.
between the supply pins (for bipolar operation)
When the open-loop output resistance of the amplifier
improves the 2nd-order distortion slightly (3dB to
is considered, this capacitive load introduces an
6dB). Operating differentially also lowers
additional pole in the signal path that can decrease
2nd-harmonic distortion terms (see the plot on the
the phase margin. Several external solutions to this
front page).
problem have been suggested. When the primary
considerations are frequency response flatness,
In most op amps, increasing the output voltage swing
pulse response fidelity, and/or distortion, the simplest
increases harmonic distortion directly. The output
and most effective solution is to isolate the capacitive
stage used in the OPA2889 actually holds the
load from the feedback loop by inserting a
difference between fundamental power and the 2nd-
series-isolation resistor between the amplifier output
and 3rd-harmonic powers relatively constant with
and the capacitive load. This solution does not
increasing output power until very large output swings
eliminate the pole from the loop response, but rather
are required ( > 4V
PP
). This result also shows up in
shifts it and adds a zero at a higher frequency. The
the 2-tone, 3rd-order intermodulation spurious (IM3)
additional zero acts to cancel the phase lag from the
response curves. The 3rd-order spurious levels are
capacitive load pole, thus increasing the phase
extremely low at low output power levels. The output
margin and improving stability.
stage continues to hold them low even as the
fundamental power reaches very high levels. As the
The ± 5 Typical Chararacteristics show the
Typical Characteristics show, the spurious
recommended R
S
versus capacitive load (see
intermodulation powers do not increase as predicted
Figure 15 and Figure 16 ) and the resulting frequency
by a traditional intercept model. As the fundamental
response at the load. Parasitic capacitive loads
power level increases, the dynamic range does not
greater than 2pF can begin to degrade the
decrease significantly. For two tones centered at
performance of the OPA2889. Long PCB traces,
1MHz, with 4dBm/tone into a matched 50 Ω load (that
unmatched cables, and connections to multiple
is, 1V
PP
for each tone at the load, which requires
devices can easily exceed this value. Always
4V
PP
for the overall 2-tone envelope at the output
consider this effect carefully, and add the
pin), the Typical Characteristics show – 73dBc
recommended series resistor as close as possible to
difference between the test tone powers and the
the OPA2889 output pin (see the Board Layout
3rd-order intermodulation spurious powers. This
Guidelines section).
performance is exceptional for an amplifier with only
4.6mW of internal power dissipation.
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