3 What is THD + N

THD+N is similar to THD and in low noise environments can look similar. However, as system noise increases, the THD+N metrics becomes increasing more accurate relative to THD alone. Sources of system-level noise can include high frequency interference, crosstalk across channels or devices, power-line ground loop coupling at the 60Hz fundamental or the harmonics (120Hz, 240Hz…), and inter-modulation distortion. Several of these effects can be controlled with adequate shielding, PCB trace spacing, grounding or filtering techniques, but are difficult to eliminate entirely. In addition, semiconductor devices (including multiplexers) contribute thermal noise, flicker noise, and shot noise which can impact the baseline THD + N noise.

Total Harmonic Distortion plus Noise (THD+N) extends the definition of THD by including noise in the measurement:

Where V₁ is the fundamental, V_i is the ith harmonic and V_N denotes the noies. Each amplitude is express in V_RMS.