OPA4684 Datasheet, PDF (20/33 Pages) Burr-Brown (TI) – Quad, Low-Power, Current-Feedback OPERATIONAL AMPLIFIER

The Typical Characteristics show the recommended R_S vs
C_LOAD and the resulting frequency response at the load. The
1kΩ resistor shown in parallel with the load capacitor is a
measurement path and may be omitted. Parasitic capacitive
loads greater than 5pF can begin to degrade the perfor-
mance of the OPA4684. Long PCB traces, unmatched cables,
and 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 pos-
sible to the OPA4684 output pin (see the Board Layout
Guidelines section).
DISTORTION PERFORMANCE
The OPA4684 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 particular output power is output stage
dependent, 3rd-harmonics particularly, and to a lesser extent
2nd-harmonic distortion, is constant as the gain is increased.
This is due to the constant loop gain versus signal gain
provided by the CFB_PLUS design. As shown in the Typical
Characteristic curves, 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 reduced 2nd-
harmonic distortion. Differential stages using two of the
channels together can reduce this 2nd-harmonic issue enor-
mously getting back to an essentially gain independent
distortion.
Relative to alternative amplifiers with < 2mA/ch supply cur-
rent, the OPA4684 holds much lower distortion at higher
frequencies (> 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 OPA4684 includes quiescent boost circuits to provide the
large-signal bandwidth in the Electrical 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 Characteristic curves show the 2nd-
harmonic holding constant from 500mV_PP to 5V_PP outputs
while the 3rd-harmonics actually decrease with increasing
output power.
The OPA4684 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 Characteristic curves.
Since the OPA4684 includes internal power boost circuits to
retain good full-power performance at high frequencies and
outputs, it does not show a classical 2-tone, 3rd-order
intermodulation intercept characteristic. Instead, it holds rela-
tively low and constant 3rd-order intermodulation spurious
levels over power. The Typical Characteristic curves 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
loads lighter than the 100Ω used in the 2-Tone, 3rd-Order
Intermodulation Distortion curve. Converter inputs, for in-
stance, will see < –82dBc 3rd-order spurious to 10MHz for
full-scale inputs. For even lower 3rd-order intermodulation
distortion to much higher frequencies, consider the OPA2691
dual or OPA691 and OPA695 single-channel current-feed-
back amplifiers.
NOISE PERFORMANCE
Wideband current-feedback op amps generally have a higher
output noise than comparable voltage-feedback op amps.
The OPA4684 offers an excellent balance between voltage
and current noise terms to achieve low output noise in a low-
power amplifier. The inverting current noise (17pA/√Hz) is
comparable to most other current-feedback op amps while
the input voltage noise (3.7nV/√Hz) is lower than any unity-
gain stable, comparable slew rate, voltage-feedback op amp.
This low input voltage noise was achieved at the price of
higher noninverting input current noise (9.4pA/√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 15 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
1/4
R_S
OPA4684
I_BN
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 15. Op Amp Noise Analysis Model.
20
OPA4684
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