OPA2674 Datasheet, PDF (23/33 Pages) Burr-Brown (TI) – Dual Wideband, High Output Current Operational Amplifier with Current Limit

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In most op amps, increasing the output voltage swing
directly increases harmonic distortion. The Typical
Characteristics show the 2nd-harmonic increasing at a
little less than the expected 2x rate, whereas the
3rd-harmonic increases at a little less than the expected 3x
rate. Where the test power doubles, the difference
between it and the 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 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 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
spurious levels. This exceptional performance improves
further 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 voltage-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 under 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
SBOS270A − AUGUST 2003 − REVISED MAY 2006
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 ǒ Ǔ Ǔ EN+}
2
ǒ Ǔ ENI2 ) IBN
2
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
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. If the feedback
resistor is reduced in high-gain configurations (as
suggested previously), the total input-referred voltage
noise given by Equation 18 approaches just the 2.0nV/√Hz
of the op amp. For example, 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
applications, it is important to analyze the noise in such a
configuration. See Figure 14 for the op amp noise model
for the differential configuration.
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