Detailed analysis of the importance of coupling capacitors in high-speed circuit design (II)

Obviously, the integrity of the signal when the AC coupling capacitor is close to the receiving end is better than that at the transmitting end. My understanding is this: non-ideal capacitor impedance is not continuous, the signal reflected by the channel after the reflected energy is less than the direct reflection of energy, so the vast majority of serial links require this AC coupling capacitor on the receiving end. However, there are some exceptions. I previously encountered this problem when I was doing board-to-board connection. I checked the PCIE specification and found that if two boards are usually placed on the transmitting end, another function of the AC coupling capacitor is also used - overvoltage protection. For example SATA, so it is usually required to place near the connector.

Solved the problem of placement, another problem that plagued everyone is the choice of the value of the value. In this case, the equivalent resistance R of our entire serial link is fixed, then the selection of the AC coupling capacitor C will be related to the time constant (RC). The larger the RC, the greater the DC component, the higher the DC voltage. The lower the drop. In this case, AC coupling capacitance can increase infinitely? Obviously it is not enough.

In the same position, it can be seen that the eye height becomes lower after increasing the coupling capacitance. The reason is that "high speed" makes the capacitance unsatisfactory. The induced inductance will generate series resonance. The larger the capacitance, the lower the resonance frequency. The AC coupling capacitance is inductive at low frequencies, so the attenuation of high frequency components increases, the eye height decreases, the rising edge slows, and the corresponding JITTER will also Increase. It is usually recommended that the AC coupling capacitance is between 0.01uf and 0.2uf, and 0.1uf is more common in the project. The 0402 package is recommended.