In many high-speed serial signals, AC coupling capacitors are used. When designing high-speed serial circuits, any small difference will cause signal integrity problems. Why do you add an AC coupling capacitor to the serial link? What? This capacitance not only causes the signal edges to become slow, but also may cause impedance discontinuities.
When we connect two systems with different supply voltages or when we are hot-swapping, we may need to use AC coupling capacitors. First of all we have to understand is that here we use AC capacitors in order to make two levels of Better communication, since it is strung between two systems, the signal of communication must be the signal of communication. AC coupling capacitors also provide DC bias and overcurrent protection.
What is the benefit of adding an AC capacitor in the middle?
The main thing is to improve the noise margin. Sometimes we may choose small capacitors when selecting capacitors. We feel that this may filter DC components cleaner, but this can cause signal distortion and cause baseline drift. If you choose a larger capacitor, the time required to stabilize the voltage on the capacitor side needs a long time, and the original small capacitor requires so much. How do we choose this capacitor? The first thing to look at is the frequency and temperature characteristics of the capacitor and choose the low ESR/ESL capacitor. The choice of capacitance value is selected by calculation as follows:
Here, F is the cutoff frequency (the frequency at which the defined voltage is 0.707 times the input voltage), R is the impedance of the transmission line, and C is the AC coupling capacitance.
Of course, when selecting a capacitor, the minimum frequency of the pass band is smaller than the minimum frequency of the transmitted signal. Assuming that the minimum frequency of the signal is Fmin, the frequency value is F:
F=Fmin/20
When the value is F, 99.88% of the signals will pass.
As mentioned earlier, although the AC coupling capacitor has its advantages, it will also cause the edge to become sluggish. When an AC coupling capacitor is placed, it will cause a change in impedance, and there will be an impedance discontinuity point. At the same time, pattern-dependent jitter will also be introduced. When a continuous “1” or “0” occurs in the signal transmitted by the circuit, the DC level voltage drop shown in the figure below will occur, which will affect the eye height.
How can we reduce this DC voltage drop?
This is related to the RC time constant. The larger the RC value (the longer the charging time is, the less the capacitor charging per unit time, and the smaller the partial voltage). The more DC components that can pass through, the smaller the DC voltage drop. Because the equivalent resistance in the link is relatively fixed, only the coupling capacitance can be adjusted. The larger the capacitance shown in the figure below, the lower the pressure drop.
The curve shows that the value of the purple capacitor is the largest, the value of the red capacitor is the second, and the value of the pink capacitor is the smallest.
Then let's increase the capacitance infinitely!
The answer is: No, no! Because the actual installed capacitor is not an ideal capacitor, there is a series resonant frequency in addition to ESR, ESL, and installed inductors. The capacitance is capacitive before the series resonant frequency and is then inductive. As shown below:
The larger the capacitance, the smaller the resonant frequency, and the capacitance will be inductive at lower frequencies. This will increase the attenuation of the high-frequency component of the signal. It will also cause the eye height to decrease, the rising edge to slow down, and jitter to increase.
Therefore, when selecting an AC coupling capacitor, the above two considerations must be considered. Generally, the industry recommends 0.01uF~0.2uF, and the most common is a 0.1uF capacitor. The choice of capacitor package is not recommended for packages larger than 0603, preferably 0402, or smaller.
