SBOA580 November   2023 INA1620 , OPA1602 , OPA1604 , OPA1611 , OPA1612 , OPA1612-Q1 , OPA1622 , OPA1632 , OPA1655 , OPA1656 , OPA209 , OPA210 , OPA211 , OPA211-EP , OPA2209 , OPA2210 , OPA2211-EP , OPA2211-HT , OPA2211A

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Violating Linear Operating Ranges
  6. 3How to Measure THD+N of an Op Amp
    1. 3.1 Non-Inverting Measurement
    2. 3.2 Inverting Measurement
    3. 3.3 Observations
    4. 3.4 Interpreting THD + N Curves
  7. 4Summary
  8. 5References
  9. 6Appendix

3.4 Interpreting THD + N Curves

Figure 3-13 shows the three components that make up the THD + N ratio vs output amplitude curve. There is a noise dominated region, a distortion dominated region and the transition region between the two. Three FFT measurements are shown that correspond to three points along the THD + N vs Output Amplitude curve. When the output amplitude is 100 mVRMS, the FFT shows the fundamental tone at 20 kHz and the harmonics are in the noise floor of the op amp and not visibly apparent. This represents the noise dominated region of the curve. The harmonics increase as the output amplitude increases and become greater than the noise floor of the op amp. The FFT for both 6 VRMS and 10 VRMS output amplitude show these harmonics visually.

GUID-20231011-SS0I-ZWVV-VQLN-GLKHMS50VLVX-low.svg Figure 3-13 Interpreting THD + N Ratio vs Output Amplitude Curves