OPA1622 Datasheet, PDF (21/36 Pages) Texas Instruments – OPA1622 SoundPlus™ High-Fidelity, Bipolar-Input, Audio Operational Amplifier

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OPA1622
SBOS727A – NOVEMBER 2015 – REVISED NOVEMBER 2015
Application Information (continued)
8.1.2 Total Harmonic Distortion Measurements
The distortion produced by OPA1622 is below the measurement limit of many commercially-available distortion
analyzers. However, a special test circuit, as shown in Figure 51, can be used to extend the measurement
capabilities.
R1
R2
Gain_(Signal)
1 ^R2
R₁
Gain_(Distortion)
1 ^R2
R₁ || R₃
R3
Generator
Output
Device
± ^{Under Test}
+
Analyzer
Input
Audio
Analyzer
Load
Figure 51. Distortion Test Circuit
Consider op amp distortion an internal error source that is referred to the input. Figure 51 shows a circuit that
causes the op amp distortion to be 101 times (approximately 40 dB) greater than that normally produced by the
op amp. 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 a factor of 101, thus extending the resolution by 101. Note that the input signal and load
applied to the op amp are the same as with conventional feedback without R₃. Keep the value of R₃ small to
minimize its effect on the distortion measurements.
Verify the validity of this technique by duplicating measurements at high gain or high frequency where the
distortion is within the measurement capability of the test equipment.
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