OPA3690ID Datasheet, PDF (23/39 Pages) Texas Instruments – Triple, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA3690
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50W
50W
175W
R_NG
+5V
Power-supply decoupling
not shown.
1/3
R_S
OPA3690
V_O
402W
C_L
402W
-5V
Figure 48. Capacitive Load Driving with Noise
Gain Tuning
This gain of +2 circuit includes a noise gain tuning
resistor across the two inputs to increase the noise
gain, increasing the unloaded phase margin for the
op amp. Although this technique will reduce the
required R_S resistor for a given capacitive load, it
does increase the noise at the output. It also will
decrease the loop gain, slightly decreasing the
distortion performance. If, however, the dominant
distortion mechanism arises from a high R_S value,
significant dynamic range improvement can be
achieved using this technique. Figure 49 shows the
required R_S versus C_LOAD parametric on noise gain
using this technique. This is the circuit of Figure 48
with R_NG adjusted to increase the noise gain
(increasing the phase margin) then sweeping C_LOAD
and finding the required R_S to get a flat frequency
response. This plot also gives the required R_S versus
C_LOAD for the OPA3690 operated at higher signal
gains without R_NG.
100
90
80
70
60
50
40
30
20
10
0
1
NG = 2
NG = 3
NG = 4
10
100
Capacitive Load (pF)
1000
Figure 49. Required R_S vs Noise Gain
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
operation) improves the 2nd-order distortion slightly
(3dB to 6dB).
In most op amps, increasing the output voltage swing
increases intermodulation distortion directly. The new
output stage used in the OPA3690 actually holds the
difference between fundamental power and the
3rd-order intermodulation powers relatively constant
with increasing output power until very large output
swings are required ( > 4V_PP). The 3rd-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
20MHz, 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 Typical Characteristics show 46dBc
difference between the test-tone powers and the
3rd-order intermodulation spurious powers. This
exceptional performance improves further when
operating at lower frequencies.
Copyright © 2002–2010, Texas Instruments Incorporated
Product Folder Link(s): OPA3690
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