Bypass capacitor and decoupling capacitor function and difference

Today, when I watched the CAN bus data, I suddenly saw the can schematic. TJA1050 CAN transceiver power supply pin external power supply time, a capacitor to the ground, suddenly remembered yesterday, my colleague said to me that the power supply must first connect the capacitor and then the chip power supply. The pin was not known at the time, but today I met again to start my "thinking"...

I asked a series of questions:

What is the use of this capacitor?

Why use a 0.1uf capacitor? Is there a requirement for this value? Check Baidu and find that he is called "bypass capacitor". If it is placed in another position, it is called "decoupling capacitor". It is amazing!

Below we are "bypass capacitor" and "decoupling capacitor": (a bit of copying Baidu's rhythm)

1. Definition and difference

Bypass capacitor: the high frequency component of the input signal is used as a filtering object;

Decoupling capacitor: Also known as decoupling capacitor, it is the filtering object of the output signal .

Both the decoupling capacitor and the bypass capacitor act as anti-interference. The position of the capacitor is different, and the calling is different.

The high-frequency bypass capacitor is generally small, according to the resonant frequency is generally 0.1u, 0.01u, etc., and the decoupling capacitor is generally larger, is 10u or more

Second, the role

Decoupling capacitor:

Decoupling capacitors have two main functions:

(1) removing high frequency signal interference;

(2) energy storage; (in fact, the capacitor near the chip also has the function of energy storage, which is the second)

When the high-frequency device is working, its current is discontinuous and the frequency is high, and the device VCC has a certain distance from the total power supply. Even if the distance is not long, the impedance is Z=i*wL+ at a high frequency. R, the inductance of the line will also be very large, which will cause the device to be supplied in time when current is needed. Decoupling capacitors can make up for this deficiency. This is one of the reasons why many boards place small capacitors on the VCC pin of the high-frequency device (a decoupling capacitor is usually connected in parallel with the vcc pin so that the AC component is grounded from this capacitor.)

Additional:

The so-called combination: the high-frequency switching noise generated by the active components of the device that transmits signals between the front and rear stages without affecting each other's static operating points will propagate along the power line. The main function of the decoupling capacitor is to provide a local DC power supply to the active device to reduce the propagation of switching noise on the board and direct the noise to ground. From the circuit, there is always a source of drive and a driven load. If the load capacitance is relatively large, the drive circuit must charge and discharge the capacitor to complete the signal transition. When the rising edge is steep, the current is relatively large, so the driven current will absorb a large supply current, due to the circuit. inductance, resistance (in particular pins on the chip inductance will produce rebound), with respect to such current situation, it is actually normal a noise will affect the normal operation of the preceding stage. This is the coupling. The decoupling capacitor acts as a battery to meet the changes in the drive circuit current and avoid mutual coupling interference.

3. Why is the capacitor of 0.1uf size used? Is there any requirement for this value?

The high frequency switching noise generated by the active device during switching will propagate along the power line. The main function of the decoupling capacitor is to provide a local DC power supply to the active device to reduce the propagation of switching noise on the board and direct the noise to ground.

The decoupling capacitor has two functions between the integrated circuit power supply and ground: on the one hand, the storage capacitor of the integrated circuit, and on the other hand, bypassing the high frequency noise of the device. A typical decoupling capacitor value in a digital circuit is 0.1 μF. The typical value of the distributed inductance of this capacitor is 5μH. The 0.1μF decoupling capacitor has a distributed inductance of 5nH, and its parallel resonant frequency is about 7MHz. The calculation method is

In other words, it has a good decoupling effect for noise below 10MHz, and has little effect on noise above 40MHz. 1μF, 10μF capacitor, parallel resonance frequency above 2MHz, the effect of removing high frequency noise is better. For every 10 or so ICs, add a charge and discharge capacitor, or a storage capacitor, about 10μF. It is best not to use electrolytic capacitors, which are rolled up by two layers of film. This rolled structure behaves as an inductor at high frequencies. Use tantalum or polycarbonate capacitors. The selection of decoupling capacitors is not strict. The capacitance value can be calculated as C=1/F, that is, 0.1μF for 10MHz and 0.01μF for 100MHz.