OPA2673 Datasheet, PDF (28/40 Pages) Texas Instruments – Dual, Wideband, High Output Current Operational Amplifier with Active Off-Line Control

OPA2673
SBOS382A – JUNE 2008 – REVISED OCTOBER 2008..................................................................................................................................................... www.ti.com
loads greater than 2pF can begin to degrade the
performance of the OPA2673. Long PCB traces,
unmatched cables, and connections to multiple
devices can easily cause this value to be exceeded.
Always consider this effect carefully, and add the
recommended series resistor as close as possible to
the OPA2673 output pin (see the Board Layout
Guidelines section).
Distortion Performance
The OPA2673 provides good distortion performance
into a 100Ω load on ±6V supplies. Generally, until the
fundamental signal reaches very high frequency or
power levels, the second harmonic dominates the
distortion with a negligible third harmonic component.
Focusing then on the second harmonic, increasing
the load impedance improves distortion directly.
Remember that the total load includes the feedback
network—in the noninverting configuration (see
Figure 76), this network is the sum of R_F + R_G; in the
inverting configuration, it is R_F. Also, providing an
additional supply decoupling capacitor (0.01µF)
between the supply pins (for bipolar operation)
improves the second-order distortion slightly (3dB to
6dB).
In most op amps, increasing the output voltage swing
directly increases harmonic distortion. The Typical
Characteristics show the second harmonic increasing
at a little less than the expected 2x rate, whereas the
third harmonic increases at a little less than the
expected 3x rate. Where the test power doubles, the
difference between it and the second harmonic
decreases less than the expected 6dB, while the
difference between it and the third harmonic
decreases by less than the expected 12dB. This
factor also shows up in the two-tone, third-order
intermodulation spurious (IM3) response curves. The
third-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 40MHz, 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 two-tone envelope at the output pin), the
Typical Characteristics show 69dBc difference
between the test-tone power and the third-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 OPA2673 offers an
excellent balance between voltage and current noise
terms to achieve low output noise. The inverting
current noise (35pA/√Hz) is lower than earlier
solutions, whereas the input voltage noise
(2.4nV/√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 (5.2pA/√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 84 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.
The total output spot noise voltage can be computed
as the square root of the sum of all squared output
noise voltage contributors. Equation 16 shows the
general form for the output noise voltage using the
terms given in Figure 84.
E_O =
E_NI² + (I_BNR_S)² + 4kTR_S + (I_BIR_F)² + 4kTR_FNG
(16)
E_NI
1/2
OPA2673
E_O
R_S
I_BN
E_RS
Ö4kTR_S
4kT
R_G
R_F
Ö4kTR_F
R_G
I_BI
4kT = 1.6E -20J
at 290°K
Figure 84. 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 17.
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