AC Coupling/DC Blocking

Introduction

Often in high speed differential pairs, you see ac coupling caps that is in line with the signal wires. This cap is referred as AC coupling or DC blocking capacitor. As the name suggests, the cap functions to pass AC signals (differential signaling) and block DC signal (common mode voltage/DC bias from the driver) from reaching to the receiver.

Design

Capacitance value: the ac capacitor forms a high pass filter which needs to have a cut off frequency to pass up to 3x of PCIE fundamental speed. For gen 3, the recommended value is 220 nF from the specification.

Placement

Capacitor should be placed near the transmitter pin.

Functions

• RC constant can be used to detect presence of a receiver and insertion/removal of the receiver at the end of the lane.

• Allows receiver and driver to be biased at different voltage

• Isolates transmitter and receiver ground if signal travels outside of the transmitter board.