Question E8A11:
Which of the following is a measure of the quality of an analog-to-digital converter?  

Choices:  
A: Total harmonic distortion  
B: Peak envelope power  
C: Reciprocal mixing  
D: Power factor    

Correct Answer is  A: Total harmonic distortion  

To measure the quality of an ADC, we often look at how linear and clean its conversion is. The correct choice is Total Harmonic Distortion (THD).

Why A (Total harmonic distortion) is correct:
- THD quantifies distortion created by the ADC’s nonlinearity when converting a sine wave.
- Defined as the ratio of the RMS sum of harmonic components to the RMS of the fundamental:
  $$\text{THD} = \frac{\sqrt{V_2^2 + V_3^2 + V_4^2 + \cdots}}{V_1}$$
  - $V_1$ is the RMS amplitude of the fundamental.
  - $V_2, V_3, \dots$ are the RMS amplitudes of the harmonics.
- Often expressed in dB:
  $$\text{THD}_{\text{dB}} = 20 \log_{10}(\text{THD})$$
- Lower THD (e.g., −60 dB or better) indicates a higher-quality, more linear ADC.

Why the other choices are incorrect:
- B: Peak envelope power (PEP) — Measures maximum RF output power of a transmitter, not ADC quality.
- C: Reciprocal mixing — A receiver metric related to LO phase noise causing noise-floor rise; not an ADC measure.
- D: Power factor — Ratio of real to apparent power in AC circuits, unrelated to ADC performance.

Useful related metrics and rules of thumb:
- THD+N (total harmonic distortion plus noise) is also common for ADCs; lower is better.
- Ideal SNR for an $N$-bit ADC (full-scale sine):
  $$\text{SNR}_{\text{ideal}} \approx 6.02N + 1.76\ \text{dB}$$
- Effective Number of Bits (ENOB) from measured SNR:
  $$\text{ENOB} \approx \frac{\text{SNR}_{\text{meas}} - 1.76}{6.02}$$
- Practical rule: For high-fidelity conversion, aim for THD < −60 dB (≈0.1%) or better.