AC Coupling/DC Blocking

Created:11/05/2020Last Updated: 7/06/2023


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. 


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. 


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. 



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.