AC Coupling/DC Blocking

Introduction

In high-speed differential pairs, AC coupling capacitors are commonly used to pass AC signals while blocking DC signals. These capacitors, also known as AC coupling or DC blocking capacitors, prevent common mode voltage or DC bias from reaching the receiver, ensuring proper signal transmission. 

Design

The capacitance value of the AC coupling capacitor is crucial for achieving the desired filtering characteristics. It forms a high-pass filter, and its cutoff frequency should be set to allow the passage of signals up to three times the fundamental speed of the specific standard, such as PCIe Gen 3. The recommended capacitance value specified for Gen 3 is typically 220 nF. 

Placement

To maintain impedance continuity and preserve signal integrity, the AC coupling capacitor should be placed close to the transmitter pin. Since the capacitor behaves as a series element in the transmission line, placing it near the transmitter helps ensure consistent impedance control throughout the signal path. 

Functions

• Presence detection: The RC time constant formed by the AC coupling capacitor and the receiver impedance can be used to detect the presence of a receiver at the end of the signal lane. Changes in this RC constant can indicate the insertion or removal of the receiver device.

• Bias voltage separation: AC coupling capacitors enable the receiver and driver to be biased at different voltages. This separation prevents interference between the transmitter and receiver grounds, particularly when the signal travels outside of the transmitter board or between separate devices.
  
  

Summary

By utilizing AC coupling capacitors in the appropriate manner, designers can enhance signal integrity, maintain impedance control, detect receiver presence, and isolate voltage biases in high-speed differential pairs.