Table of contents
1. Capacitance Calculation Method
2.1 View the capacitor device specification
2.2 Definition of crystal oscillator load capacitance
Taking the 51 single-chip microcomputer as an example, how big is the bypass capacitor for the 12M crystal oscillator?
1. Capacitance Calculation Method
Refer to the manufacturer's data for parameter conversion|Calculation formula of load capacitance of equivalent series crystal oscillator|Matching capacitance|Technical support|YXC Yangxing official website
Calculation formula: load capacitance of the crystal oscillator=[(Cd*Cg)/(Cd+Cg)]+Cic+△C; where Cd and Cg are the capacitances connected to the two pins of the crystal oscillator and to the ground respectively; Cic (integrated circuit internal capacitance); △C (capacitance on PCB) is generally 3 to 5pf.
1. Matching capacitance: Load capacitance refers to the capacitance required for the crystal oscillator to oscillate normally. Generally, an external capacitor is used to make the equivalent capacitance at both ends of the crystal oscillator equal to or close to the load capacitance. In the case of high requirements, the ground capacitance of the ic input terminal should also be considered. Generally, the capacitance connected to both ends of the crystal oscillator is twice the required load capacitance. In this way, the parallel connection is close to the load capacitance.
2. Load capacitance refers to the total external effective capacitance across the two ends of the crystal in the circuit. It is a test condition as well as a usage condition. In application, the precise frequency can be obtained by adjusting near the given load capacitance value. The size of this capacitor mainly affects the load resonance frequency and the equivalent load resonance resistance.
2. Capacitance calculation
2.1 View capacitor specifications
This calculation is 11.0592M crystal oscillator, check the specification as follows:
2.2 Crystal Oscillator Load Capacitance Calculation
The load capacitance of the crystal component refers to the total external effective capacitance across the two ends of the crystal in the circuit, which is the capacitance required for the crystal oscillator to oscillate normally. The size of this capacitor mainly affects the load resonance frequency and the equivalent load resonance resistance . This is the parameter of the crystal oscillator itself, not the external resonant capacitor. Generally, it will be written in the data sheet, and it is "CL" in the above picture.
In general, increasing the load capacitance will decrease the oscillation frequency, while decreasing the load capacitance will increase the oscillation frequency.
In the formula, Cd and Cg are the capacitances connected to the two pins of the crystal oscillator and to the ground respectively. In general applications, Cd=Cg ; in order to maintain the load balance of the crystal, in practical applications, it is generally required that CG=CD
CS (integrated circuit internal capacitance); and shunt capacitance C0 (shunt capacitance) in the above figure
In the formula, CG=CD, the maximum CS of this device is 7pf, and CL1=20pF, then the above formula can get the following results:
CG = CD = （CL-CS）X 2
CG = CD =（20-7）X 2 >= 26pf
According to the components of CG, you can get:
CG=Ci+CPCB+CL1=22.4pf
Therefore, CG = CD takes at least 26pf.
Ci—— need to add the parasitic capacitance Ci from the pin core of the external crystal oscillator main chip to GND, that is, the C1 parameter in the above figure , and the value is 3.7pf
CPCB— The capacitance on the PCB is generally 3 to 5pf.
Usually choose 30pf.
Other crystal oscillator calculations, such as 32.768K crystal oscillators, are used for clocks, and the calculated bypass capacitor is 15pf.