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

1 Introduction

In an ideal amplifier circuit, the operational amplifier or op amp can perfectly output the signal of interest without modification to the phase or waveform shape.

GUID-20230825-SS0I-LJNL-QPXV-FPJXTZJBRPDC-low.svg Figure 1-1 Ideal Buffer Amplifier

A Fast Fourier Transform or FFT converts a signal into its spectral components and provides frequency information about the signal. Figure 1-2 shows an FFT for a pure 1 kHz sine wave with an amplitude of 1 V peak seen on the ideal amplifier output. The FFT shows only the fundamental frequency and no noise. This is the ideal case. The FFT shows no other frequency components in addition to the 1 kHz fundamental frequency indicating that the amplifier did not distort the input signal.

GUID-20230919-SS0I-T8XT-32DJ-9XMG52N0CMLH-low.svg Figure 1-2 Ideal FFT of 1 V Peak Amplitude 1 kHz Sine Wave

Real amplifiers have non-linear characteristics and limitations that distort the input signal adding harmonics and noise that can be seen in an FFT as shown in Figure 1-3. The FFT is a measurement made at the output of the amplifier that is configured in a gain of 1 V/V. The y-axis is in units of volts root mean squared or VRMS. The input signal to the amplifier is a 1 kHz sine wave with a magnitude of 6 VRMS. The signal harmonics are seen in Figure 1-3 and are integer multiples of the 1 kHz fundamental frequency. Signal harmonics are highlighted at 2 kHz,
3 kHz, and 4 kHz, however higher order signal harmonics are also visible up to 10 kHz. The test equipment and circuit power supply are connected to a 120 VAC (volts alternating current) 60 Hz power outlet. The 60 Hz power line cycle and integer multiples of 60 Hz are also present in the FFT. In audio circuits, 60 Hz is often described to sound like a hum when it is coupled into the signal chain.

GUID-20230907-SS0I-TBG8-DDQ2-SN6D9T210KNS-low.svg Figure 1-3 Realistic FFT

Total harmonic distortion plus noise is a measurement that provides a figure of merit for a circuits ability to accurately output a signal seen at it's input. THD + N is an important measurement in audio systems. Audio systems with low THD + N provide a more accurate representation of how the audio was intended to sound. Equation 1 mathematically shows that THD + N is defined as the ratio of the harmonic power measurements summed together to the power of the fundamental frequency.

Equation 1. T H D   +   N   ( % )   =   100 %   ×   ∑ i = 2 ∞   V i 2   +   V n 2 V f 2

Where:

Vi RMS voltage of the ith harmonic of the fundamental (i=2,3,4…)
Vn RMS noise voltage of the circuit
Vf RMS voltage of the fundamental