2 What is THD

To be more specific, THD is a measure of the distortion introduced to an input signal in the form of harmonics at the output due to device non-linearities. For a fixed frequency input, THD is calculated as the ratio of the Root Mean Square (RMS) sum of all harmonic amplitudes to the RMS amplitude of the fundamental input signal, and can be expressed as either a percentage, or as a decibel ratio:

Equation 1.

T H D (%) = \frac{\sqrt[]{\sum_{i = 2}^{\infty} V_{i}^{2}}}{V_{1}} \times 100 % T H D (d B) = 20 \times {l o g}_{10} \frac{\sqrt[]{\sum_{i = 2}^{\infty} V_{i}^{2}}}{V_{1}}

Where V₁ is the fundamental, V_i is the ith harmonic. Each amplitude is express in V_RMS.