OPA1662
OPA1664
SBOS489 –DECEMBER 2011
www.ti.com
TOTAL HARMONIC DISTORTION
MEASUREMENTS
The validity of this technique can be verified by
duplicating measurements at high gain and/or high
frequency where the distortion is within the
measurement capability of the test equipment.
Measurements for this data sheet were made with an
Audio Precision System Two distortion/noise
analyzer, which greatly simplifies such repetitive
measurements. The measurement technique can,
however, be performed with manual distortion
measurement instruments.
The OPA166x series op amps have excellent
distortion characteristics. THD + noise is below
0.0006% (G = +1, VO = 3 VRMS, BW = 80kHz)
throughout the audio frequency range, 20 Hz to 20
kHz, with a 2-kΩ load (see Figure 7 for characteristic
performance).
The distortion produced by the OPA166x series op
amps is below the measurement limit of many
commercially available distortion analyzers. However,
a special test circuit (such as Figure 47 shows) can
be used to extend the measurement capabilities.
CAPACITIVE LOADS
The dynamic characteristics of the OPA1662 and
OPA1664 have been optimized for commonly
encountered gains, loads, and operating conditions.
The combination of low closed-loop gain and high
capacitive loads decreases the phase margin of the
amplifier and can lead to gain peaking or oscillations.
As a result, heavier capacitive loads must be isolated
from the output. The simplest way to achieve this
isolation is to add a small resistor (RS equal to 50 Ω,
for example) in series with the output.
Op amp distortion can be considered an internal error
source that can be referred to the input. Figure 47
shows a circuit that causes the op amp distortion to
be gained up (refer to the table in Figure 47 for the
distortion gain factor for various signal gains). The
addition of R3 to the otherwise standard noninverting
amplifier configuration alters the feedback factor or
noise gain of the circuit. The closed-loop gain is
unchanged, but the feedback available for error
correction is reduced by the distortion gain factor,
thus extending the resolution by the same amount.
Note that the input signal and load applied to the op
amp are the same as with conventional feedback
without R3. The value of R3 should be kept small to
minimize its effect on the distortion measurements.
This small series resistor also prevents excess power
dissipation if the output of the device becomes
shorted. Figure 25 illustrates a graph of Small-Signal
Overshoot vs Capacitive Load for several values of
RS. Also, refer to Applications Bulletin AB-028
(literature number SBOA015, available for download
from the TI web site) for details of analysis
techniques and application circuits.
R1
R2
SIGNAL DISTORTION
R1
R2
R3
GAIN
+1
GAIN
101
¥
1 kW
10 W
R3
OPA166x
VO = 3 VRMS
-1
101
4.99 kW 4.99 kW 49.9 W
549 W 4.99 kW 49.9 W
R2
R1
Signal Gain = 1+
+10
110
R2
Distortion Gain = 1+
R1 II R3
Generator
Output
Analyzer
Input
Audio Precision
System Two(1)
Load
with PC Controller
(1) For measurement bandwidth, see Figure 7 through Figure 12.
Figure 47. Distortion Test Circuit
16
Copyright © 2011, Texas Instruments Incorporated
Product Folder Link(s): OPA1662 OPA1664
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