AC coupling cap in pcb

[samym]

hi,

please tell me why we are using ac coupling caps in pcb design?

in my board have USB and device(Bga) in this i am using ac coupling for Transmitter signals alone not for Receiver signals please tell me whats reason behind?
also why we need to place those AC coupling caps close to device?

 

[marce]

Sound more like series terminations, that are always placed as close to the transmitting device as possible. By close I mean next to the pins, close.