Hi,
Thank you for contacting EngineerZone regarding your question about AD9432.
The series R (50ohm) and the C (22pF) form a low pass filter that filters out the higher order harmonics going into the ADC. This is a generic low pass Anti-Aliasing Filter (AAF) design. In this particular case, the cut-off frequency is around 140-150MHz. But if you know your input frequency, you can design a filter with a roll-off that fits your application.
The AD8138 amplifier has a potential to "ring" when connected to capacitive loads like an ADC (AD9432 = 3kohm||4pF). Since this is a high impedance capacitive load the series R's might be necessary. You can however choose to not use the series R's but you could expect some ringing related performance in pulse situations.
If you are trying to use the AD9432 for a new design, I suggest you use the AD9628. This is a much lower power higher performance 12-bit part.
Thanks
Umesh