The importance of coupling capacitance in high speed circuit design is analyzed in detail (I)

Compared with low frequency circuits, complex circuit matching is required, and the structure of high frequency circuit is relatively simple. Simple structure often means that more problems need to be considered. Take the most common AC coupling capacitor, or add two direct connections between chips, or add two between the chip and connector. It seems simple, but everything is different because of high speed. High speed makes the capacitor not ideal, and this capacitor is not designed well, which may lead to failure of the whole project. Therefore, for high speed circuits, this AC coupling capacitor will be fatal if it is not optimized.

The following editors, based on previous project experience, take stock of some problems I have encountered in the use of this capacitor.

The first thing to begin with is to understand the role of AC coupling capacitance. Generally speaking, we use AC coupling capacitor to provide DC bias, that is to filter out the DC component of the signal, so that the signal is about 0 axis symmetry. Since this is the role, can this capacitance be placed anywhere in the channel? This is the first problem I encountered when using the high frequency circuit in the first time, where the AC coupling capacitor should be placed.