2.2 AC Coupling Capacitors

Most interfaces require AC coupling between the transmitter and receiver. The USB 3.1 specification requires AC coupling capacitors in the range of 75 - 265 nF for Gen 1 and Gen 2 speeds. By default, the EVM is populated with 220 nF AC coupling capacitors on Ports B and C. The 220 nF value was chosen over the more typical 100 nF value to maintain spec compliance when the host or device attached to the EVM also has a capacitor in series. Port A is populated with 0 Ω resistors which can be replaced with capacitors if needed. However, the TMUXHS4212 switch requires a bias voltage so capacitors cannot be placed on both sides of the switch unless an external bias voltage of < 2 V is provided. The TMUXHS4212 Two-Channel Differential 2:1/1:2 USB3.1 Mux/Demux datasheet has additional information about implementing coupling capacitors and bias voltage for various applications.