Datasheet
100
90
80
70
60
50
40
30
20
10
0
CapacitiveLoad(pF)
1 10 100 1000
RS (W)
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
Ö
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 R
S
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 ( > 4V
PP
). 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 R
S
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 R
S
versus C
LOAD
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 R
NG
adjusted to increase the noise gain intermodulation powers do not increase as predicted
(increasing the phase margin) then sweeping C
LOAD
by a traditional intercept model. As the fundamental
and finding the required R
S
to get a flat frequency power level increases, the dynamic range does not
response. This plot also gives the required R
S
versus decrease significantly. For two tones centered at
C
LOAD
for the OPA690 operated at higher signal 20MHz, with 10dBm/tone into a matched 50Ω load
gains. (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 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 R
S
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 100Ω 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 R
F
+ R
G
, while in the inverting configuration it is
just R
F
. 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