cuit board has a 520nH distributed inductance. A double-stranded 24-pin integrated circuit socket leads in distributed inductance of 4 ~ 18nH.These small distribution parameters are negligible for microcontrollers in these lower frequency systems, but special attention must be paid to high speed systems.
Component layout should be in rationalization partition
The anti-electromagnetic interference should be fully considered in components placement on the PCB board, one of the principles is making the lead between the components as short as possible. In layout, the analog signal part, high-speed digital circuit part and the noise source part (such as the relay, high current switch, etc.) are reasonably separated, so that the signal coupling is the smallest.
Handle the ground wire
The power and ground wire are the most important parts for printed circuit boards. To overcome the electromagnetic interference, the main method is to ground. For double-sided board, the ground layout is special. By single-point grounding, power and ground are connected to the board from both ends of the power supply, and each of them has a junction.
The printed circuit board should have many returned ground wires, which will be gathered the junction back to the power. This is the so-called single point of grounding. The separation of analog ground, digital ground and high-power device, refers to the wiring separating and finally coming together to the ground junction. When connecting to a signal other than a printed circuit board, a shielded cable is usually used. For high frequency and digital signals, both ends of the shielded cable are grounded. The shielded cable used in low-frequency analog signal should be grounded on one end. The Circuits that are very sensitive to noise and interference or circuits or the ones with severe high frequency noise should be shielded with metal covers.
Use a good decoupling capacitor
A good high-frequency decoupling capacitor can remove high-frequency components up to 1GHZ. Ceramic chip capacitors or multilayer ceramic capacitors have high frequency characteristics. When designing printed circuit board, a decoupling capacitor should be added between each integrated circuit power supply and ground. Decoupling capacitors have two effects: On the one hand, it provides and absorbs the charge and discharge energy of integrated circuit; on the other hand reducing high-frequency noise. The typical decoupling capacitor with 0.1uf capacitance in the digital circuit has a 5nH distributed inductor, whose parallel resonant frequency is about 7MHz, this means that the noise below 10MHz has a better decoupling effect and is almost ineffective to the noise above40MHz. 1uf, 10uf capacitor, whose parallel resonance frequency is above 20MHz, effect of removing the high frequency noise is better.
It is often advantageous to place the power supply into the printed circuit board with a 1uf or 10uf high-frequency capacitor, even if the battery-powered system also needs this capacitor.
About 10 integrated circuits should be added a storage capacitor, whose capacitor size is 10uf. You’d better quit electrolytic capacitors, as electrolytic capacitors are rolled up by two layers of thin film. This structure at high frequencies performs as an inductor, and it is best to use the gallium capacitor or polycarbonate capacitor.
Decoupling capacitor value selection is not strict, according to C = 1 / f calculation; namely, 10MHz (0.1uf), for the system composed of micro-controller, you can select between 0.1 ~ 0.01uf.
Reduce the noise and electromagnetic interference
(1) Prefer low-speed chip to high-speed; high-speed chip is used in key places.
(2) By a string of resistance, reducing and controlling the rate of change when circuits go up and down.
(3) Provide relay with some form of damping.
(4) Use the lowest frequency clock that meets the system requirements.
(5) The clock generator is as close as possible to the device using the clock. The shell of Quartz crystal oscillator should be grounded.
(6) Circle up clock with the ground wire and t