[ADS41B29] just a little confused analog input pin is ac-coupling? or dc-coupling ?

Hi Siwon,

The input of the ADS41B29 is not 'fixed'. You are right, it depends on the driving circuit you use.

The EVM for instance uses a double-transformer input circuit which is by default optimised for high-bandwidth applications.

As explained in the EVM user guide (http://www.ti.com/lit/ug/slwu067c/slwu067c.pdf), you can also use an amplifier (for example THS45xx family) to drive the input. And, by removing the transformers and adjusting the driving circuit you can have a DC-coupled ADC.

Hope it helps,

Quentin