What is the purpose of connecting a capacitor between the DC power supply and ground?

We all know that a capacitor is connected between the DC power supply and the ground wire, but what is its function?

1. The capacitor connected in parallel between the DC power supply (Vcc) and ground can be called a filter capacitor. The filter capacitor filters out the noise and AC components of the power supply, smoothes the pulsating DC voltage, and stores electrical energy. The value is generally 100-4700uF. The value is related to the load current and the purity of the power supply. The larger the capacity, the better. Sometimes there will be a smaller capacitor among the large capacitors, called a high-frequency decoupling capacitor. It is also a type of filtering used to filter out High-frequency clutter in the power supply prevents the circuit from self-excitation and stabilizes the working state of the circuit. The value is generally 0.1-10uF. The value is related to the frequency of filtering out clutter.

The function of such a connection is generally called "decoupling", also called "decoupling" and "bypass" capacitors, which are often arranged near power supply, IC and functional module circuits. Non-inductive porcelain and monolithic capacitors are better.

Its function is to provide a path for high-frequency signals, reduce the internal resistance of the power supply, remove the influence of "long lines" of power and ground wires on the copper-clad board, prevent "harmful cross-connections" between various parts of the public power supply circuits, etc. 10nF is commonly used.

2. On the development board, there are usually many 0.1uF non-electrolytic capacitors and 10uF electrolytic capacitors between the DC power supply and ground.

The purpose of these capacitors is to provide a low impedance between the power line and the ground line, so that the power supply is close to an ideal voltage source. You can say it's a filtering effect, but you need to figure out what kind of wave it is filtering. It is not the ripple of the power supply that is filtered, but the ripple caused by changes in the current of a certain chip on the power line so that it does not affect other chips.

Use a 0.1uF non-polar capacitor and a 10uF electrolytic capacitor in parallel because the electrolytic capacitor has a relatively large parasitic inductance and poor ability to eliminate high-frequency ripples. Non-polar capacitors have small parasitic inductance and better ability to filter out high-frequency ripples. . However, if the capacity is selected according to the requirements of low frequency, the non-polar capacitor will be too large and costly, while the electrolytic capacitor will be small and cheaper with the same capacity. Therefore, two capacitors are connected in parallel.

If you design your own circuit, you should use it in this way, and be very particular about the location and wiring of each capacitor.

I can only say two principles:

1). The connection lines between the two ends of each small-capacity non-polar capacitor and the power pin and ground pin of the chip should be as short as possible. The shorter the better.

2). The power supply is usually introduced from other circuit boards, and there are usually only one or two electrolytic capacitors on each circuit board. For an electrolytic capacitor, place it where the power enters the circuit board. At this time, the electrolytic capacitor is of course far away from each chip, but since the electrolytic capacitor mainly functions at a lower frequency, it does not matter if it is a little farther away. If two electrolytic capacitors are used on the circuit board, place the other near the chip that consumes the most power.

These are related to the circuit board component layout and ground wiring arrangement (multilayer boards usually have ground layers)