Suppressing the source of EI in electronic devices

writer: G March 15, 2018

1. Ground connection

Analog and digital circuits have independent power and ground paths, try to widen the power and ground of these two circuits, or use separate power and ground planes to reduce the impedance of the power and ground loops and reduce any Potential interference voltage in power and ground loops.

The analog ground and digital ground of a single-working PCB can be connected at a single point near the system ground. If the power supply voltage is the same, the power of the analog and digital circuits is connected at a single point of the power inlet. If the power supply voltage is inconsistent, the two power supplies are close. Place a -1 to 2μf capacitor to provide a path for signal return currents from both sources.

The ideal ground line is a zero-impedance, zero-potential physical entity that is not only the reference point for the signal, but also does not produce a voltage drop when current flows. In an actual electrical and electronic device, such an ideal ground line does not exist, and a voltage drop is inevitable when the current flows through the ground. According to this, the formation mechanism of the interference in the ground can be summarized as the following two points. First, the impedance of the low impedance and the power supply line are reduced. Secondly, the correct choice of grounding method and the ground loop to block, according to the grounding method to have a suspended ground, single-point grounding, multi-point grounding, hybrid grounding. If the interference from the sensitive line mainly comes from the external space or the system enclosure, the floating ground can be used to solve this problem. However, the suspended ground equipment is prone to static electricity accumulation. When the charge reaches a certain level, electrostatic discharge will occur, so the floating ground should not be used. In general electronic equipment.


2. PCB component layout requirements

The layout of circuit components and signal paths must minimize the coupling of unwanted signals:

(1) The low electronic signal channel cannot be close to the high signal channel and the unfiltered power line, including circuits that can generate transients.

(2) High, medium, and low speed logic circuits use different areas on the PCB.

(3) Arrange the circuit to minimize the signal line length.

(4) Ensure that there are no long parallel signal lines between adjacent boards, adjacent layers on the same board, and adjacent cables on the same level.

(5) Electromagnetic interference (EMI) filters should be placed as close to the EMI source as possible and placed on the same circuit board.

(6) DC/DC converters, switching elements, and rectifiers should be placed as close to the transformer as possible to minimize the length of their wires.

(7) Place the voltage regulator and filter capacitor as close to the rectifier diode as possible.

(8) The printed board is zoned according to the frequency and current switching characteristics, and the noise components are further away from the non-noise components.

(9) Noise-sensitive wiring should not be paralleled with high-current, high-speed switch wires.


3. Multilayer design

In a multilayer board design, the power plane should be close to the ground plane and arranged below the ground plane. In this way, the capacitor of the two metal plates can be used as the smoothing capacitor of the power supply, and the ground plane can also shield the radiation current distributed on the power plane; in order to generate the flux cancellation, the wiring layer should be arranged adjacent to the entire metal plane. In the middle layer of the printed lines to form a planar waveguide, the formation of microstrip lines on the surface, the transmission characteristics of the two are different; clock circuit and high-frequency circuit is the main source of interference and radiation, must be arranged separately, away from sensitive circuits; all A printed circuit board with a certain voltage will radiate electromagnetic energy into the space. To reduce this effect, the physical dimensions of the printed circuit board should be 20H smaller than the physical size of the nearest ground plate, where H is the two printed circuit boards. spacing. According to the typical typical PCB size, 20H is generally about 3mm.

In order to avoid electromagnetic crosstalk caused by the relatively small distance between two printed lines, the width of printed lines not less than 2 times the line spacing should be maintained, ie not less than 2W, and w is the width of the printed lines.


4. Set the decoupling capacitor

Good high frequency decoupling capacitors can remove high frequency components up to 1 GHz. Ceramic chip capacitors or multilayer ceramic capacitors have better high-frequency characteristics. When designing a printed circuit board, a decoupling capacitor must be added between each power supply of the integrated circuit and ground. The decoupling capacitor has two functions: on the one hand, the storage capacitor of the integrated circuit provides and absorbs the charge and discharge energy of the integrated circuit when it opens and closes the door; on the other hand, it bypasses the high-frequency noise of the device.


5. Inhibiting electromagnetic coupling between lines

Reduce the loop area of ??the interference source and sensitive circuitry. The best way is to use twisted pair and shielded cable, twist the signal line with the grounding line (or current-carrying circuit) so that the distance between the signal and the grounding line (or current-carrying circuit) is the nearest; increase. The distance between the lines makes the mutual inductance between the interference source and the induced line as small as possible; if possible, the line of the interference source is wired at right angles (or near right angles) to the sensed line, which can greatly reduce the two Coupling between lines;


6. Other ways to reduce noise and electromagnetic interference

(1) Circle the clock area with the ground wire and keep the clock line as short as possible.

(2) Try to provide some form of damping for relays, etc.

(3) Use the lowest frequency clock that meets the system requirements.

(4) The clock generator is as close as possible to the device using this clock. Quartz crystal oscillator housing should be grounded.

(5) The I/O driver circuit should be as close as possible to the edge of the printed board, allowing it to leave the printed board as soon as possible. The signal entering the printed circuit board must be filtered, the signal from the high noise area must be filtered, and the signal reflection can be reduced by using the series termination resistor.

(6) Don't leave unused input terminals of the gate circuit unconnected. Leave unused input terminals of the op amp's positive input grounded, and connect the negative input terminal to the output terminal.

(7) The printed board should use a 45-degree fold line instead of a 90-degree fold line to reduce the external emission and coupling of high-frequency signals.

(8) Keep the clock, bus, and chip select signals away from the I/O lines and connectors.

(9) Analog voltage input lines and reference voltage terminals should be kept away from digital circuit signal lines, especially clocks.

For A/D devices, the digital part and the analog part should be unified and do not cross.

(10) Do not route under quartz crystals and under noise-sensitive devices.


In conclusion 

In the PCB design, it is necessary to take into account the effects of various interferences. A complete design can effectively simulate electromagnetic interference, shorten the product design cycle, and improve system stability and reliability.


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