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)
The RC filter highlighted in Figure 25 is an important piece of the AD7946 front-end design. It acts to both reduce the noise that gets into the AD7946's inputs and to reduce voltage transients that occur when the AD7946's internal capacitive DAC reconnects to its input pins after a conversion is complete. This article explains this in more detail if you're interested.
It sounds like the issue you're having is amplifier stability, possibly due to the RC filter we're discussing now. First off, the capacitor there is necessary, for the reasons above. The values of both the capacitor and the resistor can be changed, but they need to fulfill the requirements outlined in the article I referenced above. Unfortunately we don't have any data to say what the THD of the system will be if you increase the resistor past 250ohms.
I would recommend looking at a different option for the ADC driver (amplifier). Table 8 in the AD7946 data sheet gives recommendations for ADC drivers for a variety of application requirements. These products are selected because they are more suitable for driving the RC load and have adequate bandwidth to settle those ADC input voltage transients.
Also, our newer generation parts can simplify this design process. The ADAQ7980/ADAQ7988 product is a subsystem in package that includes the ADC driver with the RC filter and a 16-bit ADC for 1 MSPS or 500 kSPS throughputs. Also, the AD4000 product family is pin-compatible with the AD7946, but is easier to drive, allowing for a wider selection of ADC amplifiers and RC filter component values. Perhaps these options could also be useful for your application?