ltilayer 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 the clock line is as short as possible.
(7) I/O drive circuit should leave the edge of the printing plate instantly. The signal goes into the printed circuit board and from the high noise zone should be added to filter, reducing the signal reflection by stringing termination resistor.
(8) MCD useless end should be high, or ground, or defined as the output, and the end of power supply in integrated circuit need to be grounded, do not vacant.
(9) Do not make the input end of idle gate circuit vacant; positive input termination of idle op amp is grounded and negative input termination connects output.
(10) Try to use 45 fold lines instead of 90 fold lines in PCB to reduce the external launch and coupling from high-frequency signal.
(11) Printed circuit board is distinguished by the frequency and current switching characteristics, the noise components and non-noise ones should be far.
(12) single-sided and double-sided board are adopting single-point power supply and single-point grounding; power and ground wires is as thick as possible, using multi-layer board to reduce the power supply and capacitive inductance if you are not sensitive to money.
(13) Clock, bus and chip select should be away from I/O lines and connectors.
(14) Analog voltage input line, notice the voltage side stays away from the digital circuit signal lines, especially to the clock.
(15) To the A/D class devices, digital and analog part would rather unite rather than cross.
(16) The interference of clock line which is perpendicular to the I/O line is smaller than the parallel and the clock component pin is far from I/O cable.
(17) Component and decoupling capacitor pins are as short as possible.
(18) The key line should to be as thick as possible with protection ground on both sides. High-speed line is short and straight.
(19) Noise-sensitive lines do not parallel to high-current, high-speed switching lines.
(20) Do not route below quartz crystal and noise-sensitive devices.
(21) Do not form a current loop around weak signal circuit and low frequen