OPA3684 Datasheet, PDF (19/32 Pages) Burr-Brown (TI) – Low-Power, Triple Current-Feedback OPERATIONAL AMPLIFIER With Disable

The OPA3684 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 OPA3684 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
lighter loads 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 OPA3691
triple or OPA691 and OPA685 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 OPA3684 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 12 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
R_S
I_BN
E_RS
√4kTR_S
4kT
R_G
1/3
OPA3684
E_O
R_F
√4kTR_F
R_G
I_BI
4kT = 1.6E –20J
at 290°K
FIGURE 12. Op Amp Noise Analysis Model.
The total output spot noise voltage can be computed as the
square root of the sum of all squared output noise voltage
contributors. Equation 3 shows the general form for the
output noise voltage using the terms presented in Figure 12.
(3)
( ) ( ) E_O =


E_NI2
+
I_BNR_S
2
+
4kTR_S


NG²
+
I_BIR_F
2
+ 4kTR_FNG
Dividing this expression by the noise gain (NG = (1+R_F/R_G))
will give the equivalent input referred spot noise voltage at
the noninverting input, as shown in Equation 4.
(4)
( ) E_N =
E_NI2
+
I_BNR_S
2
+ 4kTR_S
+

I_BIR_F
NG

2
+
4kTR_F
NG
Evaluating these two equations for the OPA3684 circuit and
component values presented in Figure 1 will give a total
output spot noise voltage of 16.3nV/√Hz and a total equiva-
lent input spot noise voltage of 8.1nV/√Hz. This total input
referred spot noise voltage is higher than the 3.7nV/√Hz
specification for the op amp voltage noise alone. This reflects
the noise added to the output by the inverting current noise
times the feedback resistor. As the gain is increased, this
fixed output noise power term contributes less to the total
output noise and the total input referred voltage noise given
by Equation 3 will approach just the 3.7nV/√Hz of the op amp
itself. For example, going to a gain of +20 in the circuit of
Figure 1, adjusting only the gain resistor to 42.1Ω, will give
a total input referred noise of 3.9nV/√Hz. A more complete
description of op amp noise analysis can be found in the
Texas Instruments application note, AB-103, Noise Analysis
for High-Speed Op Amps (SBOA066), located at www.ti.com.
DC ACCURACY AND OFFSET CONTROL
A current-feedback op amp like the OPA3684 provides
exceptional bandwidth in high gains, giving fast pulse settling
but only moderate DC accuracy. The Electrical Specifica-
tions show an input offset voltage comparable to high slew
rate voltage-feedback amplifiers. The two input bias currents,
however, are somewhat higher and are unmatched. Whereas
bias current cancellation techniques are very effective with
most voltage-feedback op amps, they do not generally re-
duce the output DC offset for wideband current-feedback op
amps. Since the two input bias currents are unrelated in both
magnitude and polarity, matching the source impedance
looking out of each input to reduce their error contribution to
the output is ineffective. Evaluating the configuration of
Figure 1, using worst-case +25°C input offset voltage and the
two input bias currents, gives a worst-case output offset
range equal to:
±(NG • V_OS(MAX)) + (I_BN • R_S/2 • NG) ± (I_BI • R_F)
where NG = noninverting signal gain
= ±(2 • 3.9mV) ± (12µA • 25Ω • 2) ± (800Ω • 17µA)
= ±7.8mV + 0.6mV ± 13.6mV
= ±22mV
OPA3684
19
SBOS241C
www.ti.com