Measurement conditions: Input a sinusoidal AC voltage between the positive and negative terminals of the ADC and observe the sampling output results of the ADC. By analyzing the frequency spectrum of the output signal through Fourier transform, to obtain the signal fundamental frequency, harmonic, and noise components. ADC dynamic AC parameter are calculated based on these components.

Figure 1-2 Diagram of ADC AC Test

• Total Harmonic Distortion (THD)

  Total harmonic distortion refers to the ratio of the total power of all harmonic components in an AC signal to the power of the fundamental frequency signal. The calculation formula in decibel form is as follows.

  Equation 5. $\mathrm{THD}=20{\log }_{10}\left(\frac{\sum {\mathrm{V}}_{\mathrm{Harmonics}}}{{\mathrm{V}}_{\mathrm{Signal}\left(\mathrm{RMS}\right)}}\right)\left(\mathrm{dB}\right)$

  In the experiment of ADC communication parameter measurement, THD is caused by the nonlinearity of the ADC transmission characteristic curve, which means that the output and input of the ADC do not meet strict linear relationship within the sampling range (caused by the static error DNL/INL/EO/EG of the ADC), resulting in the harmonics in the output signal. Generally, the harmonic order is taken from 2 to 10 during calculation.

• Signal-to-Noise Ratio (SNR)

  The signal-to-noise ratio is the ratio of the effective value of the signal to the effective value of the noise. The noise mentioned here is high-frequency noise such as ADC quantization noise and 1/f noise, excluding the harmonic components of the input signal in THD calculation. The calculation formula is as follows.

  Equation 6. $\mathrm{SNR}=20{\log }_{10}\left(\frac{{\mathrm{V}}_{\mathrm{Signal}\left(\mathrm{RMS}\right)}}{{\mathrm{V}}_{\mathrm{Noise}\left(\mathrm{RMS}\right)}}\right)\left(\mathrm{dB}\right)$
• Signal-to-Noise and Distortion (SINAD)

  There is always distortion of the signal caused by internal errors of the ADC, SINAD is used to measure the ratio of the signal to the sum of harmonics and noise, indicating the overall proportion of the effective signal in the ADC output.

  Equation 7. $\mathrm{SINAD}=20{\log }_{10}\left(\frac{{\mathrm{V}}_{\mathrm{Signal}\left(\mathrm{RMS}\right)}}{\sum \left({\mathrm{V}}_{\mathrm{Harmonics}}\right)+{\mathrm{V}}_{\mathrm{Noise}\left(\mathrm{RMS}\right)})}\right)\left(\mathrm{dB}\right)$
• Effective Number of Bits (ENOB)

  ENOB is the effective number of bits of ADC, which represents the actual resolution of ADC and its ability to distinguish small amplitude signals. It can be directly calculated by SINAD.

  The above formula provides the calculation method of ENOB when the input signal is assigned the ADC maximum range. When the input signal is less than the maximum range, the calculation of ENOB is adjusted to:

  Equation 8. $\mathrm{ENOB}=\frac{{\mathrm{SINAD}}_{\mathrm{MEASURED}}-1.76\mathrm{dB}+20{\log }_{10}\left(\frac{\mathrm{Fullscale} \mathrm{Amplitude}}{(\mathrm{Input} \mathrm{Amplitude}}\right)}{6.02}$