Datasheet

E =
O
E +(I R ) +4kTR
NI BN S S
xNG +(I R ) +4kTR NG
BI F F
2 2 2
4kT
R
G
R
G
R
F
R
S
1/2
OPA2673
I
BI
E
O
I
BN
4kT=1.6E 20J-
at290 K°
E
RS
E
NI
Ö4kTR
S
Ö4kTR
F
OPA2673
SBOS382F –JUNE 2008–REVISED MAY 2010 www.ti.com
frequency response at the load. Parasitic capacitive Typical Characteristics show 69dBc difference
loads greater than 2pF can begin to degrade the between the test-tone power and the third-order
performance of the OPA2673. Long PCB traces, intermodulation spurious levels. This exceptional
unmatched cables, and connections to multiple performance improves further when operating at
devices can easily cause this value to be exceeded. lower frequencies.
Always consider this effect carefully, and add the
recommended series resistor as close as possible to Noise Performance
the OPA2673 output pin (see the Board Layout
Wideband current-feedback op amps generally have
Guidelines section).
a higher output noise than comparable
voltage-feedback op amps. The OPA2673 offers an
Distortion Performance
excellent balance between voltage and current noise
The OPA2673 provides good distortion performance terms to achieve low output noise. The inverting
into a 100Ω load on ±6V supplies. Generally, until the current noise (35pA/√Hz) is lower than earlier
fundamental signal reaches very high frequency or solutions, whereas the input voltage noise
power levels, the second harmonic dominates the (2.4nV/√Hz) is lower than most unity-gain stable,
distortion with a negligible third harmonic component. wideband voltage-feedback op amps. This low input
Focusing then on the second harmonic, increasing voltage noise is achieved at the price of higher
the load impedance improves distortion directly. noninverting input current noise (5.2pA/√Hz). As long
Remember that the total load includes the feedback as the ac source impedance from the noninverting
network—in the noninverting configuration (see node is less than 100Ω, this current noise does not
Figure 76), this network is the sum of R
F
+ R
G
; in the contribute significantly to the total output noise. The
inverting configuration, it is R
F
. Also, providing an op amp input voltage noise and the two input current
additional supply decoupling capacitor (0.01mF) noise terms combine to give low output noise under a
between the supply pins (for bipolar operation) wide variety of operating conditions. Figure 84 shows
improves the second-order distortion slightly (3dB to the op amp noise analysis model with all noise terms
6dB). included. In this model, all noise terms are taken to
be noise voltage or current density terms in either
In most op amps, increasing the output voltage swing
nV/√Hz or pA/√Hz.
directly increases harmonic distortion. The Typical
Characteristics show the second harmonic increasing The total output spot noise voltage can be computed
at a little less than the expected 2x rate, whereas the as the square root of the sum of all squared output
third harmonic increases at a little less than the noise voltage contributors. Equation 16 shows the
expected 3x rate. Where the test power doubles, the general form for the output noise voltage using the
difference between it and the second harmonic terms given in Figure 84.
decreases less than the expected 6dB, while the
difference between it and the third harmonic
decreases by less than the expected 12dB. This
(16)
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
Figure 84. Op Amp Noise Analysis Model
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