OPA2890 Datasheet, PDF (24/38 Pages) Texas Instruments – Low-Power, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2890
SBOS364C – DECEMBER 2007 – REVISED DECEMBER 2009
signal reaches very high frequency or power levels,
the 2nd harmonic dominates the distortion with a
negligible 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 49), this value
is the sum of R_F + R_G, while in the inverting
configuration it is only R_F. Also, providing an
additional supply-decoupling capacitor (0.1μF)
between the supply pins (for bipolar operation)
improves the 2nd-order distortion slightly (3dB to
6dB). Operating differentially also lowers
2nd-harmonic distortion terms (see the plot on the
front page).
In most op amps, increasing the output voltage swing
increases harmonic distortion directly. The output
stage used in the OPA2890 holds the difference
between fundamental power and the 2nd- and
3rd-harmonic powers relatively constant with
increasing output power until very large output swings
are required ( > 4V_PP). This also shows up in the
two-tone, 3rd-order intermodulation spurious (IM3)
response curves. The 3rd-order spurious levels are
extremely low at low output power levels. The output
stage continues to hold them low even as the
fundamental power reaches very high levels. As the
Typical Characteristics show, the spurious
intermodulation powers do not increase as predicted
by a traditional intercept model. As the fundamental
power level increases, the dynamic range does not
decrease significantly. For two tones centered at
10MHz, with 4dBm/tone into a matched 50Ω load
(that is, 1V_PP for each tone at the load, which requires
4V_PP for the overall two-tone envelope at the output
pin), the Typical Characteristics show a 38dBc
difference between the test tone powers and the
3rd-order intermodulation spurious powers. This
exceptional performance for all 22.5mW internal
power dissipation parts improves further when
operating at lower frequencies or powers.
NOISE PERFORMANCE
High slew rate, unity-gain stable, voltage-feedback op
amps usually achieve the slew rate at the expense of
a higher input noise voltage. However, the 8nV/√Hz
input voltage noise for the OPA2890 is much lower
than that of comparable amplifiers. The input-referred
voltage noise, and the two input-referred current
noise terms, combine to give low output noise under
a wide variety of operating conditions. Figure 60
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.
R_S
I_BN
E_RS
Ö4kTR_S
4kT
R_G
E_NI
1/2
OPA2890
www.ti.com
E_O
R_G
I_BI
R_F
Ö4kTRF
4kT = 1.6E - 20J
at 290°K
Figure 60. 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 5 shows the
general form for the output noise voltage using the
terms shown in Figure 60.
E_O =
[E_N² _I + (I_BNR_S)² + 4kTR_S]NG² + (I_BIR_F)² + 4kTR_FNG
(5)
Dividing this expression by the noise gain (NG = (1 +
R_F/R_G)) gives the equivalent input-referred spot noise
voltage at the noninverting input, as shown in
Equation 6.
( ) ^{EN =}
E_N² _I + (I_BNR_S)² + 4kTR_S +
I_BIR_F
2
+
4kTR_F
NG
NG
(6)
Evaluating these two equations for the OPA2890
circuit and component values (see Figure 49) gives a
total output spot noise voltage of 17.5nV/√Hz and a
total equivalent input spot noise voltage of 8.7nV/√Hz.
This result includes the noise added by the bias
current cancellation resistor (350Ω) on the
noninverting input. This total input-referred spot noise
voltage is only slightly higher than the 8nV/√Hz
specification for the op amp voltage noise alone. This
result is the case as long as the impedances
appearing at each op amp input are limited to the
previously recommend maximum value of 400Ω.
Keeping both (R_F || R_G) and the noninverting input
source impedance less than 400Ω satisfies both
noise and frequency response flatness
considerations. Because the resistor-induced noise is
relatively negligible, additional capacitive decoupling
across the bias current cancellation resistor (R_B) for
the inverting op amp configuration of Figure 59 is not
required.
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