I was doing some high-speed PCB layout for a project and ran across DC Blocking capacitors or AC Coupling capacitors. These 2 names are used interchangeably and, in my opinion, remain the same for high-speed design. These caps play an essential role in maintaining signal integrity while enabling proper interfacing between different circuit sections.
A DC blocking capacitor is simply a capacitor connected in series that passes AC signals while blocking DC components in that line. It is effectively a high-pass filter. It removes the unwanted DC bias in the line. These can happen when there is some sort of encoding of signals in the line and it comes up as a non 50% duty cycle of 1s and 0s. Also when on an actual PCB, there may be 2 different chips communicating via a diff pair and each can be of a different DC operating point. So directly connecting them without a blocking capacitor can cause unwanted current flow or blow up the Tx or Rx drivers.
Since these caps are used in series on differential lines on PCBs with a particular line impedance, it causes a line impedance discontinuity. This can cause reflections for high-speed signals. So ideally we want to minimize the impedance changes. For that, we usually prefer using the smallest-sized resistors like 0201 rather than let’s say 0603 because to add a cap physically, you need to widen the controlled impedance traces to accommodate it. Now another option is to remove the return layers right below the capacitor as a slot. This will increase the impedance at the spot as the return layers are much farther. There are a few papers and a nice Intel app note which sums up the size of the slot. It shows how with a slot you effectively make the capacitor invisible(Check images) in terms of impedance changes. The usual chosen caps are 0.1uF or 0.01uF with low ESR.
In summary, DC-blocking capacitors are essential for interfacing high-speed communication lines like USB 3.0+, PCIe, SATA etc. Do take care on the placement position and PCB considerations when using them in your design.
