OPA2889 Datasheet, PDF (24/35 Pages) Burr-Brown (TI) – Dual, Low-Power, Wideband, Voltage Feedback OPERATIONAL AMPLIFIER with Disable

OPA2889
SBOS373 – JUNE 2007
DISTORTION PERFORMANCE
The OPA2889 provides good distortion performance
into a 200Ω load on ±5V supplies. Relative to
alternative solutions, it provides exceptional
performance into lighter loads and/or operating on a
single +5V supply. Generally, until the fundamental
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 50), this total
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 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 OPA2889 actually 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 result also shows
up in the 2-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
1MHz, 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 2-tone envelope at the output
pin), the Typical Characteristics show –73dBc
difference between the test tone powers and the
3rd-order intermodulation spurious powers. This
performance is exceptional for an amplifier with only
4.6mW of internal power dissipation.
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 8.4nV/√Hz input voltage noise for the OPA2889
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
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conditions. Figure 61 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
1/2
OPA2889
E_O
E_RS
Ö4kTR_S
4kT
R_G
R_G
I_BI
R_F
Ö4kTRF
4kT = 1.6E - 20J
at +290°K
Figure 61. 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 61.
E_O =
[
2
E _NI
+
2
(I_BNR_S)
+
4kTR_S
]NG²
+
2
(I_BIR_F)
+
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.
E_N =
( ) ²
2
E _NI + (I_BNR_S) + 4kTR_S +
I_BIR_F
NG
2
+
4kTR_F
NG
(6)
Evaluating these two equations for the OPA2889
circuit and component values (see Figure 50) gives a
total output spot noise voltage of 18.2nV/√Hz and a
total equivalent input spot noise voltage of
9.1nV/√Hz. This total includes the noise added by
the bias current cancellation resistor (350Ω) on the
noninverting input. This total input-referred spot
noise voltage is 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
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