[FAQ] DS320PR810: How should PCIe signals be AC coupled?

PCIe is fundamentally an AC-coupled signal type. This means that a capacitor is placed in series with a PCIe signal line to attenuate the low frequency content of the signal while allowing the higher frequency content to pass through. In general, the main purpose of AC coupling on high speed signals is to remove the DC component of the transmitted signal so that the receiver can re-bias it to its preferred level, but there are other functions and benefits as well. For PCIe Gen 4 and Gen 5, the typical and recommended AC coupling capacitor size is 0.22 μF (i.e. 220 nF).

Although redrivers are simple ICs that can operate "transparently" in the PCIe link between the root complex and endpoint, because redrivers are still active devices with amplifiers, they do split the direct electrical connections between the root complex and endpoint into indirect electric connections that pass through the redriver. So we should pay attention to the treatment of AC coupling in redriver projects. The input signal into the redriver RX is one PCIe line, and the output signal out of the redriver RX is another PCIe line. 

Each PCIe line should be AC-coupled exactly one time, and it's important to double check this during the schematic and layout design phases of a project using PCIe redrivers:

• If AC capacitors are forgotten and a data line is accidentally not AC-coupled at all, there could be major signal quality issues
  • Mismatches can happen when the receiver tries to work on a signal that already has a nonzero DC component, particularly if the internal voltages of the transmitter and receiver are not the same
  • We have seen that the signal quality issues can prevent any PCIe data from being successfully understood at all, resulting in no link up.
    • This is an especially difficult problem to deal with when discovered during board testing because it is hard or impossible to add capacitors onto data lines that have no pads for them. So a new board revision would be required before anything else about the signal paths (such as generic SI problems) can be tested and corrected.
• It is better to avoid accidentally AC-coupling a data line two times
  • If two identical AC coupling capacitors are in series, their combined capacitance is half of the individual value.
  • The combined value might be outside of the allowable capacitance range in the PCIe specification. Even if it is not, it would lead to some changes in the frequency content of the signal.
  • Although double coupling is not as likely to cause significant problems as no coupling, there is still no benefit, and when working with signal conditioning devices it is preferrable to keep the electrical conditions as consistent and simple as possible.

No TI PCIe redrivers have integrated AC coupling capacitors on the receivers. Therefore, signals that are entering the redriver RX pins must have AC coupling applied already at some point on the signal path. The majority of TI PCIe redrivers do not have integrated AC coupling capacitors on the transmitters and therefore external capacitors should be added; the exceptions are the DS160PR1601 and the DS320PR1601 16-lane redrivers, which have integrated TX AC-coupling capacitors. When using these products there is no need to apply external AC capacitors on the TX side.

It's important to know the typical convention for PCIe CEM connectors in relation to AC coupling:

• Signals should be AC-coupled before they enter a PCIe CEM connector
• Following from the above point, signals that exit a PCIe CEM connector can be assumed to be already AC-coupled, and should not be AC-coupled again

This has been universal in our experience, but there's no guarantee that it would always be followed by everyone. Alternative connector types such as MCIO cables may not follow this convention at all. So, it's important to check the specifications and design of all partner devices/boards that will be used in your design.

AC coupling should always be checked for every project. Different designs can have different combinations of board-to-board, board-to-connector, and board-to-cable connections that follow different conventions or no conventions. Some devices (endpoints, CPUs, redrivers) may have integrated AC coupling capacitors, others may not and require external capacitors instead. Thankfully, the majority of customer projects we have seen have correct AC coupling, so this is usually a simple task for the project designers to check. This FAQ is motivated by several customer cases in the past year where AC coupling capacitors were forgotten and boards had to be redesigned to fix them, adding costs and delays to that could have been easily prevented.