Hey Voltnuts,
To measure the Linearity of ADCs over around 1kHz you usually put in a clean Sine Wave, and look at the created Distortion Spectrum (THD) by the ADC.
With very good Analog Oscillators or very expensive Audio Analyzers this gets you down to a THD of around 0.1ppm at 1kHz to 10kHz.
However for near DC linearity Measurements you would need Sine Wave Generators with sub Hz Frequency, which can practically not be created with high linearity and analog circuitry.
But by looking at intermodulation distortion, you don't need very linear Signal Generators, you can take a off the shelf Digital Generator like the Siglent SDG1000X (-80dB THD!) and estimate the THD at high frequencies of ADCs with around 1ppm uncertainty! With some trickery you can even get the static transfer function by this method.
The only limitations is the noise floor of your generators and the ADC.
For intermodulation Distortion Measurement you need to take 2 Sine generators with different Frequency and measure the sum Frequency with your DUT.
A non-linear ADC will now create intermodulation distortion at n*frequency_1+m+frequency_2 (n,m are natural numbers).
So theoretically you could take 2 Calibrators, or DACs with low 1/f Noise, tie one ground to the Output of the other one, tell them to approximate a sub Hz Sine Wave and evaluate the near-DC linearity your ADC!
I attached a typical IMD spectrum of a simulated ADC with around 1ppm x^2 non-linearity tested by 2 Signal Generators with around 10ppm non-linearity.
Have fun!
EDIT: The advantage of this method is, that you can average over Days and get ppb Resolution and test Million of points of the transfer function of your ADC/Meter, rather than just some points.