Proper method to filter inputs of a (true) differential ADC
It depends on desired SNR and level of CM noise how to best reduce CM levels.

Balanced paths &amp; impedance
high CMMR Differential Amp with wide bandwidth
Shielded pairs
Active Guarding on shield using buffered CM signal
Baluns, raise CM series impedance followed by shunt caps

no effect on differential mode impedance



General best differential buffers input signals with common mode caps.

Using unity gain buffers gives the highest BW for signal and  CMRR


It is assumed Transmission Line terminal impedance matching for broadband signals on long paths, is a separate issue depending on situation.