The design quality of the printed circuit board not only directly affects the reliability of electronic products, but also relates to the stability of the product, and is even the key to the success or failure of the design. Therefore, in the design and drawing of the printed circuit board, in addition to providing the correct electrical connection for the components in the circuit, the interference resistance of the printed circuit board should be fully considered. Based on the principle of electromagnetic compatibility, anti-interference design should include three aspects: first, to suppress the noise source, the second is to cut off the noise transmission path, the third is to reduce the noise sensitivity of the disturbed equipment. Printed board noise suppression should start from the design stage, throughout the circuit schematic design, printed board drawing, component selection, printed board installation leads and a series of links. Although the focus of each link is different, but echo each other, should be treated seriously. This article focuses on how to effectively suppress noise when designing printed boards.
Reduce radiation noise
Printed circuit board in the work will be radiated outward noise and become a source of noise: the signal lines in the circuit board by the ground loop transmission to the chassis, causing resonance, by the chassis outward radiation of strong noise; board signal through the signal cable outward radiation noise; board itself also directly outward radiation noise. To weaken the noise radiation, the following treatment can be made.
(1) carefully selected devices. Selection needs to pay attention to the aging of components, and the selection of thermal feedback impact of small devices. For high-frequency circuits, the appropriate chip should be selected to reduce circuit radiation. In the selection of logic devices, give full consideration to their noise tolerance indicators: when simply consider the noise tolerance of the circuit, it is best to use HTL, if taking into account power consumption, it is appropriate to use VDD ≥ 15V CMOS.
(2) Use multilayer printed circuit boards. This can be obtained from the structure of the ideal shielding effect: the middle layer for the power line or ground, the power line sealed in the board, both sides do insulation treatment, can make the flow through the upper and lower switch current does not affect each other; printed board inner layer made of a large area of conductive area, between the conductor surface has a large electrostatic capacitance, the formation of a very low impedance power supply lines, can effectively prevent the board radiation and receive noise.
(3) printed circuit board “full ground”. Drawing high-frequency circuit board, in addition to as thick as possible to ground the printed wire, the board should not be occupied by all the area as a grounding line, so that the device is better near the ground. This can effectively reduce parasitic inductance, at the same time, a large area of the ground line can strongly reduce noise radiation.
(4) in the printed circuit board attached to one or two sides of the grounding plate. That is, with a piece of aluminum or iron attached to the back of the printed board (welding surface), or the printed board sandwiched between two aluminum or iron plate. The grounding plate is installed as close as possible to the printed board, and it must be connected to the system signal (SG) on the best grounding point, this structure is essentially a simple and easy to do “multi-layer” printed board. If you want to pursue a better suppression effect, you can put the printed board in a completely shielded metal box, so that it does not generate, do not respond to noise.
Proper layout of printed wires
Wiring is a key stage in the graphic design of the printed circuit board. Many factors considered in the design should be reflected in the wiring. The layout of the copper foil conductors on the printed board and the crosstalk between adjacent conductors and other factors will determine the immunity of the printed board, and reasonable wiring will enable the printed board to obtain the best performance. From the anti-interference consideration, wiring should follow the design and process principles are.
(1) As long as the wiring requirements are met, wiring should give priority to the selection of single-sided boards, followed by double-sided and multilayer boards. Wiring density should be integrated structure and electrical performance requirements and other reasonable selection, and strive to wiring simple and uniform; minimum width and spacing of wires should generally not be less than 0.2mm, the wiring density allows, the appropriate widening of the printed wire and its spacing.
(2) the main signal lines in the circuit should preferably be brought together in the center of the board, and strive to be close to the ground, or to surround it with ground lines, signal lines, signal return lines formed by the minimum loop area; to try to avoid long-distance parallel wiring, circuit wiring between electrical interconnection points to strive for the shortest; signal (especially high-frequency signals) line corners should be designed to 135 ° towards, or into a circle, arc, do not draw 90 ° or Smaller angle shape.
(3) adjacent wiring surface wires to take the form of mutual vertical, oblique or bending alignment to reduce parasitic coupling; high-frequency signal wires should not be parallel to each other to avoid signal feedback or crosstalk, can be set up between two parallel lines to increase a ground line.
(4) properly lay out the external signal lines, as short as possible to shorten the input lead, improve the input impedance. Analog signal input lines are best shielded, when the board at the same time, analog, digital signals, it is appropriate to isolate the two ground lines to avoid mutual interference.
(5) Properly handle the redundant inputs of logic devices. With / with the non-gate excess input connected to “1” (avoid hanging), or / or non-gate excess input connected to Vss, counters, registers and D flip-flops and other idle reset / reset terminal through the appropriate resistor connected to Vcc, flip-flop excess input must be grounded.
(6) Select standard component package. If you need to create a component package, the pad hole spacing should be consistent with the device pin spacing to reduce lead impedance and parasitic inductance. Layout of wires should be minimized when metallization over holes to improve the reliability of the entire printed circuit board.
Layout of power and ground lines
Suppression of power line and ground impedance
caused by the oscillation
Design assembly density of the board should pay attention to reduce the power line and ground impedance, common impedance, crosstalk and reflection caused by waveform distortion and oscillation phenomenon need to take the necessary measures. When the board has more integrated circuit devices work at the same time, the board power supply voltage and ground potential fluctuations easily, resulting in signal oscillations, causing circuit misoperation. Especially when the inrush current flow through the printed wire, there will be a transient voltage drop, the formation of power supply spike noise, which is dominated by the interference caused by the wire inductance. In the actual design, the inductance should be avoided as far as possible on the circuit: in each integrated circuit between the power and ground line access bypass capacitors, respectively, to shorten the flow path of the switching current; the power and ground lines are designed as shown in Figure 1 (b) lattice shape, rather than the comb shape shown in Figure 1 (a), this is because the lattice shape can significantly shorten the line loop, reduce the line impedance and reduce interference.
When the printed circuit board is equipped with multiple integrated circuits, and some components consume more power, the ground appears larger potential difference, the formation of public resistance to interference, it is appropriate to design the ground as shown in Figure 1 (d) closed loop, this loop without potential difference, than the way shown in Figure 1 (c) has a higher noise tolerance; should try to shorten the lead, the GND of each integrated circuit to the shortest distance connected to the board entrance ground, the Reduce the spike pulse generated by the printed wire; let the ground and power lines to the same direction as the data transmission to improve the board’s noise tolerance.
The use of a large number of high-speed logic circuit often use multi-layer printed circuit board to reduce the ground potential difference, reduce power line impedance and crosstalk between signal lines. When there is no multilayer board and have to use double-sided, must try to widen the ground line, usually the ground line should be thickened to pass three times the actual flow of electricity through the wire is appropriate; or the use of small bus bar way, the public power lines and ground lines as far as possible, respectively, in the edge of the printed board on both sides. When the printed board plug has more than one plug contact piece, should have more than a few lead plugs for ground use, as shown in Figure 1 (b), and according to the total load current size, access 1 ~ 10mF tantalum capacitor at the plug to the power bus decoupling, and in parallel with the decoupling capacitor next to a 0.01 ~ 0.1mF high-frequency ceramic capacitor.
Correct use of anti-disturbance devices
To carry out the EMC design of the printed board, according to the different characteristics of the noise, the correct choice of anti-disturbance devices: absorb surge voltage with diodes and varistors, isolate power supply noise with isolation transformers, filter out certain frequency bands of interference signals with line filters, resistors, capacitors, inductors and other components of the combination of interference voltage or current bypass, absorption, isolation, filtering, decoupling and other processing. If the anti-interference devices are not used properly, then not only can not effectively reduce interference, and even become a new source of interference.