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OPA2690 Datasheet(PDF) 22 Page - Texas Instruments

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Part No. OPA2690
Description  Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable
Download  37 Pages
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI1 - Texas Instruments

OPA2690 Datasheet(HTML) 22 Page - Texas Instruments

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100
90
80
70
60
50
40
30
20
10
0
CapacitiveLoad(pF)
1
10
100
1000
NG=2
NG=3
NG=4
4kT
R
G
R
G
R
F
R
S
OPA690
I
BI
E
O
I
BN
4kT=1.6E
20J
-
at290 K
°
E
RS
E
NI
4kTR
F
Ö
4kTR
S
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OPA690
SBOS223F – DECEMBER 2001 – REVISED FEBRUARY 2010
www.ti.com
This gain of +2 circuit includes a noise gain tuning
In most op amps, increasing the output voltage swing
resistor across the two inputs to increase the noise
increases harmonic distortion directly. The new
gain, increasing the unloaded phase margin for the
output stage used in the OPA690 actually holds the
op amp. Although this technique will reduce the
difference between fundamental power and the 2nd-
required RS resistor for a given capacitive load, it
and 3rd-harmonic powers relatively constant with
does increase the noise at the output. It also will
increasing output power until very large output swings
decrease the loop gain, slightly decreasing the
are required ( > 4VPP). This also shows up in the
distortion performance. If, however, the dominant
2-tone,
3rd-order
intermodulation
spurious
(IM3)
distortion mechanism arises from a high RS value,
response curves. The 3rd-order spurious levels are
significant
dynamic
range
improvement
can
be
moderately low at low output power levels. The
achieved using this technique. Figure 45 shows the
output stage continues to hold them low even as the
required RS versus CLOAD parametric on noise gain
fundamental power reaches very high levels. As the
using this technique. This is the circuit of Figure 44
Typical
Characteristics
show,
the
spurious
with RNG adjusted to increase the noise gain
intermodulation powers do not increase as predicted
(increasing the phase margin) then sweeping CLOAD
by a traditional intercept model. As the fundamental
and finding the required RS to get a flat frequency
power level increases, the dynamic range does not
response. This plot also gives the required RS versus
decrease significantly. For two tones centered at
CLOAD for the OPA690 operated at higher signal
20MHz, with 10dBm/tone into a matched 50
Ω load
gains.
(that is, 2VPP for each tone at the load, which requires
8VPP for the overall two-tone envelope at the output
pin),
the
Typical
Characteristics
show
47dBc
difference between the test tone powers and the
3rd-order
intermodulation
spurious
powers.
This
performance improves further when operating at
lower frequencies.
NOISE PERFORMANCE
High slew rate, unity-gain stable, voltage-feedback op
amps usually achieve their slew rate at the expense
of a higher input noise voltage. The 5.5nV/
√Hz input
voltage noise for the OPA690 is, however, much
lower than comparable amplifiers. The input-referred
voltage noise, and the two input-referred current
noise terms, combine to give low output noise under
a wide variety of operating conditions. Figure 46
Figure 45. Required RS vs Noise Gain
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
DISTORTION PERFORMANCE
in either nV/
√Hz or pA/√Hz.
The OPA690 provides good distortion performance
into a 100
Ω load on ±5V supplies. Relative to
alternative
solutions,
it
provides
exceptional
performance into lighter loads and/or operating on a
single +5V supply. Generally, until the fundamental
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 36), this is sum
of RF + RG, while in the inverting configuration it is
just
RF.
Also,
providing
an
additional
supply-decoupling capacitor (0.1µF) between the
supply pins (for bipolar operation) improves the
Figure 46. Op Amp Noise Analysis Model
2nd-order distortion slightly (3dB to 6dB).
22
Copyright © 2001–2010, Texas Instruments Incorporated
Product Folder Link(s): OPA690


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