How does harmonic distortion occur in ADCs? We are currently testing an ADC at my workplace and the Total Harmonic Distortion is an important spec. It got me to thinking as to how it occurs in the first place 

1) Can there be harmonic distortion if the input signal is a pure sine wave? In other words if the input is theatrically a clean sine wave , can the adc artifically introduce harmonics? 

2) ive read quantisation noise can introduce or amplify harmonics of a signal. How does this happen? 

Thanks!
There are many distortion mechanisms in ADC but the most important ones are:


"static" distortion due to INL
Track-and-hold distortion


The first one is caused by the fact that the voltage-in-code-out relationship is not perfectly linear (INL/DNL).

An approximation of harmonic distortion based on DNL is given by (Lee, JSSCC 12/2017):

$$
\mathrm{HD3} = -20\log\left( \frac{4}{3\sqrt{3}} \frac{2^B}{\mathrm{INL}_{\mathrm{max}}} \right)
$$

The second one is caused by track nonlinearity. The sampling switch is for example a weakly nonlinear CMOS switch. Its on-resistance is given by (using the simple square law model):

$$
R_{on} = \frac{1}{\mu C_{ox} \frac{W}{L} (V_{DD}-V_{in}-V_t)}
$$

From which is can be seen that the resistance depends on the input signal (see for example Razavi, Data Conversion System Design, page 16).

This distortion can be approximated as

$$
\mathrm{HD3} = 20\log\left(\frac{1}{4} \left(\frac{A}{V_{GS}-V_t}\right)^2 2\pi f_{in} R_{on} C \right)
$$

For a simple CMOS switch this is by far the dominant distortion source and is practically overcome by using a bootstrap sampling switch. However, also this technique has limitations.