OPA2674_17 Datasheet, PDF (23/36 Pages) Texas Instruments – Dual Wideband, High Output Current Operational Amplifier with Current Limit

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2nd-harmonic decreases less than the expected 6dB,
whereas the difference between it and the 3rd-harmonic
decreases by less than the expected 12dB. This factor
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 funda-
mental power reaches very high levels. As the Typical
Characteristics show, the spurious intermodulation pow-
ers do not increase as predicted by a traditional intercept
model. As the fundamental power level increases, the dy-
namic range does not decrease significantly. For two tones
centered at 20MHz, with 10dBm/tone into a matched 50Ω
load (that is, 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 67dBc difference between
the test-tone power and the 3rd-order intermodulation spu-
rious levels. This exceptional performance improves fur-
ther when operating at lower frequencies.
NOISE PERFORMANCE
Wideband current-feedback op amps generally have a
higher output noise than comparable voltage-feedback op
amps. The OPA2674 offers an excellent balance between
voltage and current noise terms to achieve low output
noise. The inverting current noise (24pA/√Hz) is lower than
earlier solutions whereas the input voltage noise (2.0nV/√
Hz) is lower than most unity-gain stable, wideband volt-
age-feedback op amps. This low input voltage noise is
achieved at the price of higher noninverting input current
noise (16pA/√Hz). As long as the AC source impedance
from the noninverting node is less than 100Ω, this current
noise does 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 un-
der a wide variety of operating conditions. Figure 13
shows the op amp noise analysis model with all 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.
OPA2674
SBOS270C − AUGUST 2003 − REVISED AUGUST 2008
The total output spot noise voltage can be computed as the
square root of the sum of all squared output noise voltage
contributors. Equation 17 shows the general form for the
output noise voltage using the terms given in Figure 13.
Ǹǒ E_O +
ǒ _{ENI2 ) IBN}
R_S^Ǔ2 ) 4kTR_S ) ^ǒI_BI
Ǔ RFǓ2 ) 4kTRFNG
(17)
E_NI
1/2
R_S
I_BN
OPA2674
E_O
E_RS
√4 kTR_S
4kT
R_G
R_F
√4kTR_F
R_G
I_BI
4kT = 1.6E −20J
at 290_K
Figure 13. Op Amp Noise Analysis Model
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 18.
Ǹ^{ǒ EN +}
ǒ ENI2 ) IBN
ǒ Ǔ Ǔ ²
Ǔ²
RS ) 4kTRS )
IBI RF
NG
) ^4kTRF
NG
(18)
Evaluating these two equations for the OPA2674 circuit
and component values of Figure 1 gives a total output spot
noise voltage of 14.3nV/√Hz and a total equivalent input
spot noise voltage of 3.6nV/√Hz. This total input-referred
spot noise voltage is higher than the 2.0nV/√Hz specifica-
tion for the op amp voltage noise alone. This reflects the
noise added to the output by the inverting current noise
times the feedback resistor. If the feedback resistor is re-
duced in high-gain configurations (as suggested previous-
ly), the total input-referred voltage noise given by Equation
18 approaches just the 2.0nV/√Hz of the op amp. For ex-
ample, going to a gain of +10 using R_F = 298Ω gives a total
input-referred noise of 2.3nV/√Hz.
DIFFERENTIAL NOISE PERFORMANCE
As the OPA2674 is used as a differential driver in xDSL ap-
plications, it is important to analyze the noise in such a con-
figuration. See Figure 14 for the op amp noise model for
the differential configuration.
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