Datasheet

1/3
OPA3690
402W
175W
402W
+5V
50W
50W
C
L
R
NG
V
O
R
S
-5V
Power-supplydecoupling
notshown.
100
90
80
70
60
50
40
30
20
10
0
CapacitiveLoad(pF)
1 10 100 1000
RS (W)
NG=2
NG=3
NG=4
OPA3690
www.ti.com SBOS237G –MARCH 2002–REVISED MARCH 2010
DISTORTION PERFORMANCE
The OPA3690 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. The distortion plots show which
changes in operation will improve distortion.
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 (see Figure 46) it is just R
F
.
Also, providing an additional supply-decoupling
capacitor (0.1mF) between the supply pins (for bipolar
Figure 48. Capacitive Load Driving with Noise
operation) improves the 2nd-order distortion slightly
Gain Tuning
(3dB to 6dB).
In most op amps, increasing the output voltage swing
This gain of +2 circuit includes a noise gain tuning
increases intermodulation distortion directly. The new
resistor across the two inputs to increase the noise
output stage used in the OPA3690 actually holds the
gain, increasing the unloaded phase margin for the
difference between fundamental power and the
op amp. Although this technique will reduce the
3rd-order intermodulation powers relatively constant
required R
S
resistor for a given capacitive load, it
with increasing output power until very large output
does increase the noise at the output. It also will
swings are required ( > 4V
PP
). The 3rd-order spurious
decrease the loop gain, slightly decreasing the
levels are extremely low at low output power levels.
distortion performance. If, however, the dominant
The output stage continues to hold them low even as
distortion mechanism arises from a high R
S
value,
the fundamental power reaches very high levels. As
significant dynamic range improvement can be
the Typical Characteristics show, the spurious
achieved using this technique. Figure 49 shows the
intermodulation powers do not increase as predicted
required R
S
versus C
LOAD
parametric on noise gain
by a traditional intercept model. As the fundamental
using this technique. This is the circuit of Figure 48
power level increases, the dynamic range does not
with R
NG
adjusted to increase the noise gain
decrease significantly. For two tones centered at
(increasing the phase margin) then sweeping C
LOAD
20MHz, with 10dBm/tone into a matched 50Ω load
and finding the required R
S
to get a flat frequency
(that is, 2V
PP
for each tone at the load, which requires
response. This plot also gives the required R
S
versus
8V
PP
for the overall two-tone envelope at the output
C
LOAD
for the OPA3690 operated at higher signal
pin), the Typical Characteristics show 46dBc
gains without R
NG
.
difference between the test-tone powers and the
3rd-order intermodulation spurious powers. This
exceptional performance improves further when
operating at lower frequencies.
Figure 49. Required R
S
vs Noise Gain
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