OPA2690 Datasheet, PDF (22/30 Pages) Burr-Brown (TI) – Dual, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2690
SBOS238D − JUNE 2002 − REVISED DECEMBER 2004
DISTORTION PERFORMANCE
The OPA2690 provides good distortion performance into
a 100Ω 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 1), this is 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 new output
stage used in the OPA2690 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 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 2 tones centered at 20MHz, with
10dBm/tone into a matched 50Ω load (i.e., 2V_PP for each
tone at the load, which requires 8V_PP for the overall 2-tone
envelope at the output pin), the Typical Characteristics
show 46dBc difference between the test tone powers and
the 3rd-order intermodulation spurious powers. This
exceptional performance improves further when operating
at lower frequencies or powers.
NOISE PERFORMANCE
High slew rate, unity-gain stable, voltage-feedback op
amps usually achieve their slew rate at the expense of a
higher input noise voltage. The 5.5nV/√Hz input voltage
noise for the OPA2690 is, however, much lower than
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 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.
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R_S
I_BN
E_RS
√4kTR_S
4kT
R_G
E_NI
1/2
OPA2690
E_O
R_G
I_BI
R_F
√4kTR_F
4kT = 1.6E − 20J
at 290_K
Figure 15. 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 6 shows the general form for the
output noise voltage using the terms shown in Figure 15.
Ǹǒ Ǔ E_O +
ǒ Ǔ _{ENI2 )}
2
I_BNR_S ) 4kTR_S
NG² ) ^ǒI_BIR_F^Ǔ2 ) 4kTR_FNG
(6)
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 7.
Ǹ ǒ Ǔ E_N +
2
E_NI2
)
ǒ_IBNR
Ǔ²
S
)
4kTR_S
)
I_BIR_F
NG
)
4kTR_F
NG
(7)
Evaluating these two equations for the OPA2690 circuit
and component values (see Figure 1) gives a total output
spot noise voltage of 12.3nV/√Hz and a total equivalent
input spot noise voltage of 6.1nV/√Hz. This is including the
noise added by the bias current cancellation resistor
(175Ω) on the noninverting input. This total input-referred
spot noise voltage is only slightly higher than the
5.5nV/√Hz specification for the op amp voltage noise
alone. This will be the case as long as the impedances
appearing at each op amp input are limited to the
previously recommend maximum value of 300Ω. Keeping
both (R_F R_G) and the noninverting input source
impedance less than 300Ω will satisfy both noise and
frequency response flatness considerations. As 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 12 is not required.
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