OPA2677 Datasheet, PDF (21/38 Pages) Burr-Brown (TI) – Dual, Wideband, High Output Current OPERATIONAL AMPLIFIER

In most op amps, increasing the output voltage swing in-
creases harmonic distortion directly. The Typical Character-
istics 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 differ-
ence between it and the 3rd-harmonic decreases by less
than the expected 12dB. 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 dy-
namic range does not decrease significantly. For 2-tone
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 63dBc 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 OPA2677 offers an excellent balance between voltage
and current noise terms to achieve low output noise. The
inverting current noise (24pA/√Hz) is significantly 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 looking out of 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 14 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
1/2
R_S
I_BN
OPA2677
E_O
E_RS
√4kTR_S
4kT
R_G
R_F
√4kTR_F
R_G
I_BI
4kT = 1.6E –20J
at 290°K
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 shown in Figure 13.
(17)
( ) ( ) E_O =


E_NI2
+
I_BN • R_S
2
+ 4kTR_S
+
I_BI • R_F
2
+ 4kTR_FNG^
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.
(18)
( ) E_N =
_ENI2

+
I_BN • R_S
2
+ 4kTR_S
+

I_BI • R_F
NG

2
+
4kTR_F
NG



Evaluating these two equations for the OPA2677 circuit and
component values (see Figure 1) gives a total output spot
noise voltage of 13.5nV/√Hz and a total equivalent input spot
noise voltage of 3.3nV/√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 con-
figurations (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 OPA2677 is used as a differential driver in xDSL
applications, it is important to analyze the noise in such a
configuration. Figure 15 shows the op amp noise model for
the differential configuration.
I_N
Driver
E_N
R_S
E_RS
√4kTR_S
I_N
R_F
√4kTR_F
R_G
E_O2
√4kTR_G
R_F
√4kTR_F
I_N
R_S
E_N
I_N
E_RS
√4kTR_S
FIGURE 14. Op Amp Noise Analysis Model.
OPA2677
SBOS126I
FIGURE 15. Differential Op Amp Noise Analysis Model.
21
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