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

Compared to low-frequency circuits, complicated circuit matching is required, and the structure of high-frequency circuits is relatively simple. A simple structure often means that more problems need to be considered. Take the most common AC coupling capacitors, either adding two direct connections between the chips or two between the chip and the connector. Seemingly simple, but everything is different because of the high speed. The high speed makes this capacitor not "ideal". This capacitor is not designed well and may cause the entire project to fail. Therefore, for high-speed circuits, this AC coupling capacitor will not be optimized and will be "fatal."

I will use the previous project experience to analyze some of the problems I encountered with the use of this capacitor.

Begin to understand the role of AC coupling capacitors first. In general, we use AC coupling capacitors to provide DC bias, which is to filter out the DC component of the signal and make the signal symmetrical about the 0 axis. Since this is the role, can this capacitor be placed anywhere on the channel? This is the first question that I encountered when using this capacitor when I was first working on a high-frequency circuit—where the AC coupling capacitor should be placed.

Here we take a typical project to analyze the path.

In low-speed circuit design, this capacitor can be equivalent to an ideal capacitor. In high-frequency circuits, due to the presence of parasitic inductances and the impedance discontinuities caused by the plates, in fact, this capacitor cannot be considered as an ideal capacitor. The signal frequency here is 2.5G, the channel length is 4000mil, and the position of the AC coupling capacitor is located at the distance of 200mil from the sending end and the receiving end, respectively. Let's take a look at the changes in the simulated eye diagram.