Datasheet

OPA684
18
SBOS219D
www.ti.com
The Typical Characteristics show the recommended R
S
vs
C
LOAD
and the resulting frequency response at the load. To
reduce the required value of R
S
, those curves show a slight
increase in the feedback resistor value and an added load of
250Ω to ground. 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 performance of the OPA684. Long PC board
traces, unmatched cables, and connections to multiple de-
vices 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 OPA684 output pin
(see Board Layout Guidelines).
DISTORTION PERFORMANCE
The OPA684 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 distortions 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 2nd-area of distor-
tion 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, are 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 Characteristics,
while the 3rd-harmonic is constant with gain, the 2nd-har-
monic 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 OPA684 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 non-inverting 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 OPA684 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 OPA684 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
OPA684 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 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 the 2-tone 3rd-order intermodulation plot.
Converter inputs for instance 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 OPA685.
NOISE PERFORMANCE
Wideband current-feedback op amps generally have a higher
output noise than comparable voltage-feedback op amps.
The OPA684 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
lower than 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
non-inverting input current noise (9.4pA/√Hz). As long as the
AC source impedance looking out of the non-inverting 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 13 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.
4kT
R
G
R
G
R
F
R
S
OPA684
I
BI
E
O
I
BN
4kT = 1.6E –20J
at 290°K
E
RS
E
NI
4kTR
S
√
4kTR
F
√
FIGURE 13. Op Amp Noise Analysis Model.