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Why are AC coupling capacitors used in multi gigabit data links?
AC coupling capacitors are frequently used in multi-gigabit data links. Many current data standards require AC coupling (for example PCIe Gen 3, 10 Gb Ethernet, and so on). In addition, there exist incompatible common mode voltages between drivers and receivers, for which AC coupling is the simplest means to solve this problem.
What is the coupling frequency of a capacitor?
Only very small value capacitors (less than 10 pF) have resonant frequencies above 1 GHz. On the other hand, to preserve low frequency data content, required coupling capacitance is in the range of 0.1 mF to 4.7 mF, with self-resonances in the 100s of MHz.
What is the idle peak voltage of a USB 3?
Electrical Idle Differential Peak – Peak Output Voltage 0 (min) 10 (max) 0 (min) 10 (max) mV V. TX-IDLE-DIFF-DC. DC Electrical Idle Differential Output Voltage 0 (min) 10 (max) 0 (min) 10 (max) mV Voltage shall be low pass filtered to remove any AC component. This limits the common mode error when resuming U1 to U0.
What is the unit interval of USB 3?
UI Unit Interval 199.94 (min) 200.06 (max) 99.97 (min) 100.03 (max) ps The specified UI is equivalent to a tolerance of ±300 ppm for each device. Period does not account for SSC induced variations. V TX-DIFF-PP
Where to place capacitors for high speed differential interfaces?
In you case, Z can be not much smaller than Z0. For 1 GHz, inductive reactance only may be around 6 Ohm (assuming 1 nH ESL, L*2*pi*f). So, for such high frequencies (1 GHz and above) the cap should be ideally located near TX, not near RX.
Why do we need serial AC coupling capacitors?
To provide a removable interface that may be shorted to ground without damaging the output drivers. When combined with differential signaling and transformer coupling, to connect boxes without requiring any DC connection between the two product chassis.
Where to place coupling capacitor in signal line?
Now where to place. Any AC coupling capacitor that you place in your signal line is going to be a lower impedance point and will therefor cause a negative reflection back to the source.