Datasheet

4kT
R
G
R
G
R
F
R
S
1/2
OPA2890
I
BI
E
O
I
BN
4kT=1.6E 20J-
at290 K°
E
RS
E
NI
4kTR
S
Ö
4kTRFÖ
E =
O
[E +(I R ) +4kTR ]NG +(I R ) +4kTR NG
NIBN S S BI F F
2 2 2
2
E =
N
E +(I R ) +4kTR +
NIBN S S
2
2
I R
BI F
NG
( )
2
+
4kTR
F
NG
OPA2890
SBOS364C –DECEMBER 2007–REVISED DECEMBER 2009 www.ti.com
signal reaches very high frequency or power levels,
the 2nd harmonic dominates the distortion with a
negligible 3rd harmonic component. Focusing then on
the 2nd harmonic, increasing the load impedance
improves distortion directly. Remember that the total
load includes the feedback network; in the
noninverting configuration (see Figure 49), this value
is the sum of R
F
+ R
G
, while in the inverting
configuration it is only R
F
. Also, providing an
additional supply-decoupling capacitor (0.1μF)
between the supply pins (for bipolar operation)
improves the 2nd-order distortion slightly (3dB to
6dB). Operating differentially also lowers
2nd-harmonic distortion terms (see the plot on the
front page).
Figure 60. Op Amp Noise Analysis Model
In most op amps, increasing the output voltage swing
increases harmonic distortion directly. The output
The total output spot noise voltage can be computed
stage used in the OPA2890 holds the difference
as the square root of the sum of all squared output
between fundamental power and the 2nd- and
noise voltage contributors. Equation 5 shows the
3rd-harmonic powers relatively constant with
general form for the output noise voltage using the
increasing output power until very large output swings
terms shown in Figure 60.
are required ( > 4V
PP
). This also shows up in the
two-tone, 3rd-order intermodulation spurious (IM3)
response curves. The 3rd-order spurious levels are
(5)
extremely low at low output power levels. The output
stage continues to hold them low even as the
Dividing this expression by the noise gain (NG = (1 +
fundamental power reaches very high levels. As the
R
F
/R
G
)) gives the equivalent input-referred spot noise
Typical Characteristics show, the spurious
voltage at the noninverting input, as shown in
intermodulation powers do not increase as predicted
Equation 6.
by a traditional intercept model. As the fundamental
power level increases, the dynamic range does not
decrease significantly. For two tones centered at
10MHz, with 4dBm/tone into a matched 50Ω load
(6)
(that is, 1V
PP
for each tone at the load, which requires
4V
PP
for the overall two-tone envelope at the output
Evaluating these two equations for the OPA2890
pin), the Typical Characteristics show a 38dBc
circuit and component values (see Figure 49) gives a
difference between the test tone powers and the
total output spot noise voltage of 17.5nV/√Hz and a
3rd-order intermodulation spurious powers. This
total equivalent input spot noise voltage of 8.7nV/√Hz.
exceptional performance for all 22.5mW internal
This result includes the noise added by the bias
power dissipation parts improves further when
current cancellation resistor (350Ω) on the
operating at lower frequencies or powers.
noninverting input. This total input-referred spot noise
voltage is only slightly higher than the 8nV/√Hz
NOISE PERFORMANCE
specification for the op amp voltage noise alone. This
result is the case as long as the impedances
High slew rate, unity-gain stable, voltage-feedback op
appearing at each op amp input are limited to the
amps usually achieve the slew rate at the expense of
previously recommend maximum value of 400Ω.
a higher input noise voltage. However, the 8nV/√Hz
Keeping both (R
F
|| R
G
) and the noninverting input
input voltage noise for the OPA2890 is much lower
source impedance less than 400Ω satisfies both
than that of comparable amplifiers. The input-referred
noise and frequency response flatness
voltage noise, and the two input-referred current
considerations. Because the resistor-induced noise is
noise terms, combine to give low output noise under
relatively negligible, additional capacitive decoupling
a wide variety of operating conditions. Figure 60
across the bias current cancellation resistor (R
B
) for
shows the op amp noise analysis model with all the
the inverting op amp configuration of Figure 59 is not
noise terms included. In this model, all noise terms
required.
are taken to be noise voltage or current density terms
in either nV/√Hz or pA/√Hz.
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