OPA2683 Datasheet, PDF (21/33 Pages) Texas Instruments – Very Low-Power, Dual, Current-Feedback Operational Amplifier

DISTORTION PERFORMANCE
The OPA2683 provides very low distortion in a low-power
part. The CFB_PLUS architecture also gives two significant
areas of distortion improvement. First, in operating regions
where the 2nd-harmonic distortion due to output stage
nonlinearities is very low (frequencies < 1MHz, low output
swings into light loads) the linearization at the inverting node
provided by the CFB_PLUS design gives 2nd-harmonic distor-
tions that extend into the –90dBc region. Previous current-
feedback amplifiers have been limited to approximately
–85dBc due to the nonlinearities at the inverting input. The
second area of distortion improvement comes in a distortion
performance that is largely gain independent. To the extent
that the distortion at a specific output power is output stage
dependent, 3rd-harmonics particularly, and to a lesser ex-
tend 2nd-harmonic distortion, remains constant as the gain
increases. This is due to the constant loop gain versus signal
gain provided by the CFB_PLUS design. As shown in the
Typical Characteristics, while the 3rd-harmonic is constant
with gain, the 2nd-harmonic degrades at higher gains. This
is largely due to board parasitic issues. Slightly imbalanced
load return currents will couple into the gain resistor to cause
a portion of the 2nd-harmonic distortion. At high gains, this
imbalance has more gain to the output giving increased
2nd-harmonic distortion.
Relative to alternative amplifiers with < 2mA supply current,
the OPA2683 holds much lower distortion at higher frequen-
cies (> 5MHz) and to higher gains. Generally, until the
fundamental signal reaches very high frequency or power
levels, the 2nd-harmonic will dominate the distortion with a
lower 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 1) this
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 opera-
tion) improves the 2nd-order distortion slightly (3dB to 6dB).
In most op amps, increasing the output voltage swing in-
creases harmonic distortion directly. A low-power part like
the OPA2683 includes quiescent boost circuits to provide the
full-power bandwidth shown in the Typical Characteristics.
These act to increase the bias in a very linear fashion only
when high slew rate or output power are required. This also
acts to actually reduce the distortion slightly at higher output
power levels. The Typical Characteristics show the 2nd-
harmonic holding constant from 500mV_PP to 5V_PP outputs
while the 3rd-harmonics actually decrease with increasing
output power.
The OPA2683 has an extremely low 3rd-order harmonic
distortion, particularly for light loads and at lower frequen-
cies. This also gives low 2-tone, 3rd-order intermodulation
distortion as shown in the Typical Characteristics. Since the
OPA2683 includes internal power boost circuits to retain
good full-power performance at high frequencies and out-
puts, it does not show a classical 2-tone, 3rd-order inter-
modulation intercept characteristic. Instead, it holds relatively
low and constant 3rd-order intermodulation spurious levels
over power. The Typical Characteristics show this spurious
level as a dBc below the carrier at fixed center frequencies
swept over single-tone power at a matched 50Ω load. These
spurious levels drop significantly (> 12dB) for lighter loads
than the 100Ω used in that plot. Converter inputs, for in-
stance, will see ≤ 82dBc 3rd-order spurious to 10MHz for full-
scale inputs. For even lower 3rd-order intermodulation distor-
tion to much higher frequencies, consider the OPA2691.
NOISE PERFORMANCE
Wideband current-feedback op amps generally have a higher
output noise than comparable voltage-feedback op amps.
The OPA2683 offers an excellent balance between voltage
and current noise terms to achieve low output noise in a low-
power amplifier. The inverting current noise (11.6pA/√Hz) is
lower than most other current-feedback op amps while the
input voltage noise (4.4nV/√Hz) is lower than any unity-gain
stable, comparable slew rate, < 5mA/ch voltage-feedback op
amp. This low input voltage noise was achieved at the price
of higher noninverting input current noise (5.1pA/√Hz). As
long as the AC source impedance looking out of the
noninverting node is less than 200Ω, this current noise will
not contribute significantly to the total output noise. The op
amp input voltage noise and the two input current noise
terms combine to give low output noise under a wide variety
of operating conditions. Figure 14 shows the op amp noise
analysis model with all the noise terms included. In this
model, all noise terms are taken to be noise voltage or
current density terms in either nV/√Hz or pA/√Hz.
E_NI
1/2
R_S
I_BN
OPA2683
E_O
E_RS
√4kTR_S
4kT
R_G
R_F
√4kTR_F
R_G
I_BI
4kT = 1.6E –20J
at 290°K
FIGURE 14. Op Amp Noise Analysis Model.
OPA2683
21
SBOS244H
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