About the resistance value and the capacitor Value of AD7946

Hi,

I would like to use AD7946.

But, the output of Voltage follower(AD8629) before the ADC oscillated.

So, I have three questions.

#1  Is a capacitor(2.7nF) necessary? Is it possible to reduce the value or not to use a capacitor?(In Figure.25)

#2  How high can the resistance value(33Ω) be increased?(In Figure.25, Figure.28)

    

#3  When the resistance is set to more 250 Ω, what is the THD value?(In Figure.28)

Best Regards,

Yuya