Q2. Why was the C0G capacitor used instead of X8R for the analog input of the ADC?
Answer:
Figure 1. X8R and C0G characteristic curve
In summary, the capacitor used is a C0G type because of its high linearity, as other types can cause significant distortion at the input of the ADC.
References:
How to Leverage Input High-Z Technology to Reduce Solution Power and Size | Analog Devices