By Len Staller
Embedded Systems Design
(02/24/05, 05:24:00 PM EST)
Embedded Systems Design
(02/24/05, 05:24:00 PM EST)
Harmonic distortion
Nonlinearity in the data converter results in harmonic distortion when analyzed in the frequency domain. Such distortion is observed as "spurs" in the FFT at harmonics of the measured signal as illustrated in Figure 10. This distortion is referred to as total harmonic distortion (THD), and its power is calculated in Equation 5.
Figure 10: FFT showing harmonic distortion
THD = 20 log | √V22 + V22 + … + V2n / V1| (Equation 5)
The magnitude of harmonic distortion diminishes at high frequencies to the point that its magnitude is less than the noise floor or is beyond the bandwidth of interest. Data sheets typically specify to what order the harmonic distortion has been calculated. Manufacturers will specify which harmonic is used in calculating THD; for example, up to the fifth harmonic is common (see the example ADC specification in Table 1).
Table 1: Example: Silicon Labs C8051F060 16-bit ADC electrical characteristics
Reference:
- http://www.embended.com/
- http://en.wikipedia.org
