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| The difference between Murata inductance, Murata magnetic beads and zero-ohm resistance |
| 2019-01-04 |
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Inductors are energy storage components, which are mostly used in power supply filter circuits, LC oscillating circuits, low-frequency filter circuits, etc., and their application frequency range rarely exceeds 50MHz. For an inductor, its inductive reactance is proportional to the frequency. This can be illustrated by the formula: XL = 2πfL, where XL is the inductive reactance (in Ω). For example: an ideal 10mH inductor, the inductive reactance is 628Ω at 10kHz and 6.2MΩ at 100MHz. Therefore, at 100MHz, this inductance can be considered an open circuit. At 100 MHz, if a signal is passed through this inductor, the quality of this signal will drop.
Ferrite bead materials are ferro-magnesium or iron-nickel alloys. These materials have high electrical resistivity and magnetic permeability. At high frequencies and high impedance, the capacitance between the coils inside the inductor is minimized. Magnetic beads are usually only suitable for high-frequency circuits, because at low frequencies, they basically retain the complete characteristics of the inductor (including resistive and reactive components), thus causing some micro-loss on the line. At high frequencies, it basically only has a reactive component (jωL), and the resistance component increases as the frequency increases. Like some RF circuits, PLLs, oscillator circuits, including UHF memory circuits (DDR, SDRAM, RAMBUS, etc.), it is necessary to add beads to the power input section. In fact, the magnetic beads are high frequency attenuators of radio frequency energy. In fact, the magnetic beads can be considered as a resistor in parallel with an inductor. At low frequencies, the resistor is "short-circuited" by the inductor, and the current flows to the inductor; at high frequencies, the high inductance of the inductor forces the current to flow to the resistor. Essentially, a magnetic bead is a "dissipative device" that converts high-frequency energy into heat. Therefore, in terms of performance, it can only be interpreted as a resistor, not an inductor.
The role of the zero-ohm resistor is as follows:
1, there is no function in the circuit, just for the convenience of debugging or compatible design on the PCB.
2, can be used for jumpers, if a certain line is not used, directly subsidize the resistance (does not affect the appearance).
3. When the matching circuit parameters are uncertain, replace it with 0 ohm. When the actual debugging, determine the parameters and replace them with specific numerical components.
4, when you want to measure the current consumption of a certain part of the circuit, you can remove the 0ohm resistor and connect the ammeter, which is convenient for measuring current consumption.
5, in the wiring, if the actual cloth can not be passed, you can also add a 0ohm resistor (feeling should be used in-line, should not be surface-mounted (luther.gliethttp)).
6, acting as an inductor or capacitor under high frequency signals. (Related to external circuit characteristics) Inductance, mainly to solve EMC problems. (such as ground and ground, power and IC Pin).
7, single point grounding (refers to protective grounding, working grounding, DC grounding on the equipment separated from each other, each becomes a separate system).
8, fuse action inductor
1 analog ground and digital ground single point grounding
As long as it is ground, it will eventually be received together and then into the earth. If they are not connected together, they are “floating”, there is a pressure difference, and it is easy to accumulate electric charges and cause static electricity. The ground is referenced to 0 potential, all voltages are derived from reference ground, the ground standards are the same, so the various grounds should be shorted together. It is believed that the earth can absorb all the electric charges and always maintain stability, which is the ultimate test site. Although some boards are not connected to the earth, the power plant is connected to the earth, and the power supply on the board will eventually return to the power plant. If the analog ground and the digital ground are directly connected to each other, it will cause mutual interference. It is not short-circuited and not appropriate. There are four ways to solve this problem as above:
1. Connected with magnetic beads; the equivalent circuit of the magnetic beads is equivalent to the band-stopper, which only significantly suppresses the noise of a certain frequency. When using, it is necessary to estimate the noise frequency in advance so that the appropriate model can be selected. For cases where the frequency is uncertain or unpredictable, the beads do not fit.
2, with a capacitor connection; capacitors through the straight through, easy to cause floating.
3. Connected with an inductor; the inductor is bulky, with many stray parameters and instability.
4. Connected with 0 ohm resistor; 0 ohm resistor is equivalent to a very narrow current path, which can effectively limit the loop current and suppress noise. The resistor has an attenuation in all frequency bands (0 ohm resistor also has impedance), which is stronger than the magnetic beads.
2 for current loop when bridging
When the ground plane is divided, the shortest return path of the signal is broken. At this time, the signal loop has to be detoured to form a large loop area, and the influence of the electric field and the magnetic field becomes stronger, which is easy to interfere/interfere. By connecting a 0 ohm resistor across the partition, a shorter return path can be provided to reduce interference.
3 configuration circuit
Generally, there should be no jumpers or dip switches on the product. Sometimes the user will tamper with the settings, which may cause misunderstanding. In order to reduce the maintenance cost, the 0 ohm resistor is used instead of the jumper to solder on the board. The vacant jumper is equivalent to the antenna at high frequencies, and the effect of the chip resistor is good.
4 other uses:
A. Over-line during wiring B. Debug/test C. Temporarily replace other SMD devices D. As a temperature compensation device, it is more often required for EMC countermeasures. In addition, the 0 ohm resistance is smaller than the parasitic inductance of the via, and the via also affects the ground plane (because the hole is to be dug).
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