where should I place the AC coupling capacitor?

[daraemon_liu]

In the differential output circuit ,we usually use the AC couple.My question is in the PCB layout ,where should I place the capacitor? Should I place it near the output or near the input ?Dose these two pattern have some differences?

 

[ckshivaram]

Its different for transmission lines, digital circuits, and high speed circuits, and analog circuits. Usually they are placed near the output of the stage.
Mounting the AC-coupling capacitors right next to each other exacerbates the parasitic body capacitance effect (an analog designer may recognize that as a type of "Miller effect"). With the capacitors staggered, parasitic capacitance of each mounting pad and capacitor body to the reference plane still exists, but at least you eliminate the capacitance directly from one to the other.

AC coupling circuit

 

[biff44]

Well, I am simply ASTOUNDED that a company that I thought was reputable, namely Hewlett Packard, would file a patent for an AC coupling circuit like that in 2004! That is a trivial circuit that pretty much any engineer hooking up a differential circuit would have developed "by inspection" without a second thought. The fact that two HP engineers thought it was patentable, and the fact that HP corporate funded the patent application, is nothing short of pathetic and sad. HP has stooped to a new low!

RE: patent 7719380

 

[permute]

indeed.

Xilinx's GTX appnote suggest placing the caps at the source, or destination, but no in the middle of the transmission line. Likewise, they suggest placing the caps next to each other, citing EMI reasons. There could also be some SI reasons if the caps are very far apart (P/N mismatch). This is for high speed differential pairs.