Wednesday, December 29, 2010

【 Weak current College 】 digital circuit how anti-jamming 】

In the design of electronic systems, in order to make detours and save time, you should take full account of and meet the requirements of the immunity, to avoid design is finished and then go to a antijamming remedies. Form the basic elements of disturbance are three:
(1) the source of interference, means produce interference components, equipment, or a signal, using a mathematical language description: du/dt and di/dt Grand place is the source of interference. For example: Thunder and lightning, relays, SCR, motor, high-frequency clock, etc can become the source of interference.
(2) propagation path means interference from sources of interference to access sensitive devices or media. Typical interference propagation path through the wires of conduction and radiation in space.
(3) sensitive devices that can easily be interference. Example: A/D and D/A Converters Microcontroller and digital ICS, weak signal amplifiers, etc. Anti-jamming design basic principle is: Suppression of interference sources, to cut off the noise propagation path, improve anti-interference performance sensitive devices. (Similar to infectious disease prevention)
1 sources of interference
Suppression of interference is possible to reduce the interference source du/dt and di/dt. This is a antijamming design of the highest priority and the most important principle, often has a multiplier effect. Reduce interference sources du/dt, mainly through the interference source to achieve parallel capacitance at both ends. Reduce the interference of the di/dt is interference source loop inductance or resistance and increase continued flow diode.
Suppression of interference sources of common measures are as follows:
(1) relay coil increase continued flow diode, eliminating broken coil of back-EMF. Only with continued flow diode will enable relay disconnect time lag, increase the Zener diode rear relay in unit time action more often.
(2) the ends of the relay contacts and add spark suppression circuit (typically RC series circuit, general elections a few K resistor to dozens of K, capacitors selected 0.01uF), decrease the EDM.
(3) to the motor and filter circuit, capacitors, inductors leads to short.
(4) the Board each IC to and received a 0.01 μ F ~ than 0 μ F high-frequency capacitors to reduce the effect of the power supply IC. Note the high-frequency capacitance of wiring, wiring should be close to the power supply side and short thick as possible, otherwise, equal to increase the capacitance of the equivalent series resistance, affect the filter effect.
(5) wiring avoid 90 ° bend, reduce the high frequency noise emission.
(6) thyristor suppression received on both sides and RC circuits, reduce the noise generated by Silicon controlled rectifier (this noise serious may be SCR breakdown). By interference propagation path can be divided into conducted interference and radiated interference.
The so-called conducted interference is defined through the wire to sensitive devices. High-frequency interference noise and useful signal frequency bands, can lead to increase filter method for cutting high-frequency interference noise transmission, sometimes can also be combined with optical coupling isolation. Power noise hazards maximum, paying special attention to the process. The so-called radiated interference is propagated through space radiation sensitive devices. The general solution is to increase the interference sources and sensitive devices distance ground their isolation and in sensitive devices on the short hood. Cut the noise propagation path of common measures are as follows:
(1) take full account of the impact of on-chip power supply. The power to do well, the entire circuit interference most of the settlement. Many of the microcontroller is sensitive to the power supply noise, to give single-chip power plus filter circuit or regulators to reduce power noise interference on the microcontroller. For example, you can use beads and capacitance constituted π-shaped filter circuit, course conditions requirements is also available in 100 Ω resistor substitute beads.
(2) if the microcontroller I/O port used to control the motor noise, and the i/o port should be added between the noise source isolation (increase the π-filter). Control motor, noise, noise in the I/O port and should be added between the source isolation (increase the π-filter).
(3) Note Crystal wiring. Crystal oscillator and a microcomputer PIN as close as possible to that ground the clock district isolation, Crystal shell grounding and fixed. This measure can solve many problems.
(4) reasonable partition, such as circuit boards, weak signal, digital and analog signals. As far as possible sources of interference (e.g. motors, relays) and sensitive components (such as SCM) away.
(5) with a ground to digital and analog DC isolation, digital and analog to detach, point final in received power. A/D and D/A chip wiring also as a principle, manufacturers assign A/D and D/A chip pin arrangement is already considering this request.
(6) MCU and high-power devices ground to separate ground to reduce interference. High-power devices in the circuit board edge as possible.
(7) on-chip I/O port, power cables, printed circuit board connectors, and other key parts of the anti-jamming components such as beads, steepness, power filter, shielding enclosures, can significantly improve the performance of the circuit. 3 improve anti-interference performance sensitive devices
Improve anti-interference performance sensitive devices is measured from the sensitive devices here consider minimizing interference noise pickup, and never a normal status as soon as possible.
Improve sensitive devices anti-interference performance of common measures are as follows:
(1) wiring to minimize the loop area, central to reduce induced noise.
(2) wiring, power and ground to rough. In addition to reduce pressure drop, and more importantly reduce coupling noise.
(3) in the case of single-chip I/O port are idle, not vacant, to ground or power supply. Other IC free-without altering the system logic ground or power supply.
(4) on-chip power monitoring and watchdog circuit, such as: IMP809, IMP706, IMP813, X25043, X25045, can greatly improve the performance of the entire circuit.
(5) in speed to meet the requirements, minimize the use of Crystal andLow-speed digital circuits.
(6) IC devices try to direct welding at the Board, the use of IC socket.

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