• Between every PCIe transmitter (Tx) and receiver (Rx) lane, there’s a series capacitor.
• This capacitor blocks DC voltage but allows high-frequency AC signals (the actual PCIe data) to pass.
  

👉 In other words:

• DC (static offset) = blocked.
• AC (data transitions) = transmitted.

Why PCIe Uses AC Coupling

1. Common Mode Voltage Independence

   • The transmitter may drive its differential pair around one reference level, and the receiver may use a slightly different one.
   • With AC coupling, each side can use its own internal reference voltage, since the capacitor blocks any mismatch in DC levels.

2. Noise & Grounding Benefits

   • If devices are on different boards, slots, or power domains, their ground/reference voltages may differ.
   • AC coupling prevents large DC currents that would otherwise flow due to these mismatches.

3. Signal Integrity

   • High-speed PCIe signaling is all about fast transitions (edges).
   • Since information is carried in the changes, not the static voltage level, blocking DC does not harm data transmission.

The Physical PCIe Link

• Differential Pairs:
  Each lane = 2 wires (positive + negative). Data is encoded as the difference between them → improves noise immunity.

• AC Coupling Capacitors:
  Placed close to the transmitter. Typical values are 75–200 nF.

• Receiver Termination:
  The Rx has resistive termination to bias the signal into the correct range after the capacitor.