(1) Interference source, refers to the components, equipment or signals that generate interference, described in mathematical language as follows: du / dt, di / dt large place is the source of interference. For example: lightning, relays, thyristors, motors, high-frequency clocks, etc. can become sources of interference.
(2) Propagation path, refers to the interference in the path or medium from the source of interference to sensitive devices. Typical interference propagation paths are conduction through the wire and radiation in space.
(3) Sensitive device, refers to the object susceptible to interference. For example: A/D, D/A converter, microcontroller, digital IC, weak signal amplifier, etc. The basic principles of anti-interference design are: suppress the source of interference, cut off the interference propagation path, and improve the anti-interference performance of sensitive devices.
I. Suppression of interference sources
This is the most important principle to be considered in the design of anti-interference, which often results in twice the result with half the effort. The main way to reduce the du/dt of the source is to connect capacitors in parallel with the source. Reducing the di/dt of the source is achieved by connecting an inductor or resistor in series with the source circuit and adding a current-continuity diode.
Common measures to suppress the interference source are as follows.
(1) Relay coil to add a current-continuing diode to eliminate the counter-electromotive force interference generated when the coil is disconnected. Adding only the current-continuing diode will make the relay's disconnection time lag, and adding a voltage regulator diode will allow the relay to operate more times per unit of time.
(2) In the relay contact terminals connected the spark suppression circuit (generally RC series circuit, the resistance is generally selected a few K to tens of K, capacitance selected 0.01uF) to reduce the impact of electric spark.
(3) To the motor filter circuit, pay attention to the capacitor, inductor leads to be as short as possible.
(4) Each IC on the board should be connected to a 0.01μF ~ 0.1μF high frequency capacitor to reduce the impact of IC on the power supply. Pay attention to the wiring of high-frequency capacitors, the connection should be close to the power supply and as short as possible, otherwise, it is equivalent to increasing the equivalent series resistance of the capacitors, which will affect the filtering effect.
(5) Wiring to avoid 90-degree folding line to reduce high-frequency noise emission.
(6) The SCR ends connected to the RC suppression circuit to reduce the noise generated by the SCR (this noise may be serious and cause the SCR breakdown).
II. According to the propagation path of interference can be divided into two categories of conducted interference and radiation interference.
The so-called conducted interference refers to the interference that propagates through the wire to the sensitive device. High-frequency interference noise and useful signal frequency band is different, you can increase the filter in the wire to cut off the transmission of high-frequency interference noise, and sometimes can add isolation opticalcoupler to solve. Power supply noise is the most harmful, to pay special attention to deal with. The so-called radiation interference refers to the interference spread to sensitive devices through space radiation. The general solution is to increase the distance between the source of interference and sensitive devices, using ground wire to isolate them and add a shield on sensitive devices.
Common measures of cut interference propagation path are as follows.
(1) Fully consider the impact of the power supply on the microcontroller. Power supply is well done, the whole circuit of anti-interference will solve a large part. Many microcontrollers are sensitive to power supply noise, to add a filter circuit or voltage regulator to the microcontroller power supply to reduce the power supply noise interference to the microcontroller. For example, you can use magnetic beads and capacitors to form a π-shaped filter circuit, of course, when the conditions are not high requirements 100Ω resistor can also be used instead of beads.
(2) If the microcontroller's I/O port is used to control motors and other noisy devices, isolation should be added between the I/O port and the noise source (adding π-shaped filter circuit). Control motors and other noisy devices, between the I/O port and the noise source should be isolated (add π-shaped filter circuit).
(3) pay attention to the crystal wiring. Crystal and microcontroller pins as close as possible, using ground to isolate the clock area, the crystal shell grounding and fixed. This measure can solve many difficult problems.
(4) board reasonable partition, such as strong, weak signals, digital, analog signals. Keep the source of interference (such as motors, relays) and sensitive components (such as microcontrollers) away as far as possible.
(5) the digital area and the analog area isolated by ground, digital ground and analog ground to be separated, and finally in a point connected to the power supply ground. A/D, D/A chip wiring is also the principle, the manufacturer to allocate A/D, D/A chip pin arrangement has taken into account this requirement.
(6) the ground of the microcontroller and high-power devices to be grounded separately to reduce mutual interference. High-power devices to be put as far as possible on the edge of the board.
(7) In the microcontroller I/O ports, power lines, circuit board connections and other key places to use anti-interference components such as magnetic beads, magnetic rings, power filters, shields, can significantly improve the circuit's anti-interference performance.
III. Improve the anti-interference performance of sensitive devices
Improve the anti-interference performance of sensitive devices is to consider from the sensitive side of the device to minimize the pickup of interference noise, as well as from the normal state as soon as possible to recover the method.
Common measures to improve the anti-interference performance of sensitive devices are as follows.
(1) When wiring, to minimize the area of the loop ring to reduce inductive noise.
(2) When wiring, the power and ground lines should be as thick as possible. In addition to reducing the voltage drop, it is more important to reduce the coupling noise.
(3) For the microcontroller idle I/O port, do not dangle, connected to ground or power supply. The idle terminals of other ICs are grounded or connected to the power supply without changing the system logic.
(4) Use of power supply monitoring and watchdog circuit for microcontrollers, such as: IMP809, IMP706, IMP813, X25043, X25045, etc., can significantly improve the overall circuit anti-interference performance.
(5) In the speed to meet the requirements of the premise, as far as possible to reduce the crystal frequency of the microcontroller and the use of low-speed digital circuit.
(6) IC devices as far as possible directly soldered on the circuit board, less use of the IC seat.
In order to achieve good anti-interference, so we often see the PCB board with ground division of the wiring. But not all digital circuits and analog circuits mixed must be ground plane split. Because this division is to reduce the interference of noise.
Theory: The general frequency in digital circuits will be higher than in analog circuits, and their own signals will form a return current with the ground plane (because in signal transmission, there are various inductances and distributed capacitance between copper wires and copper wires), if we mix the ground lines together, then this return current will be crosstalk in digital and analog circuits. And by separating them we are allowing them to form a return current only within themselves. They are only connected with a zero-ohm resistor or beads because they are the same physical sense of ground, and now the wiring separates them, and finally they should be connected.