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Do you know everything about communication, decoupling and capacitance?


Connection refers to process of signal transmission from first stage to second stage, and usually refers to an AC connection unless otherwise noted.

Do you know everything about communication, decoupling and capacitance?

Decoupling refers to taking additional measures to filter power supply to eliminate effect of mutual signal interference between two stages through power supply. The coupling constant refers to time constant corresponding to product of coupling capacitance value and second stage input impedance value.

01 Separation has three purposes

1. Remove high frequency ripple in power supply and turn off high frequency signal of multistage amplifier through power supply crosstalk path.

2. When operating with a large signal, power consumption of circuit increases, causing power fluctuations. The effect of power fluctuations on high voltage input/amp stage during large signals is reduced by decoupling.

3. Form a hanging ground or hanging power supply, and complete coordination of various parts of ground wire or power supply in a complex system. The high-frequency switching noise generated by active device when switching will propagate along power line.

The main function of decoupling capacitor is to provide a local DC power supply to active device in order to reduce propagation of switching noise on board and to direct noise to ground.

02 Interference method

The interference signal generated by interference source creates electromagnetic interference in electronic control system through a certain communication channel. The interference coupling method is nothing more than impact on electronic control system through wires, rooms, public lines, etc.

There are basically following types of analysis:

  • Direct link: is most direct way to intervene, as well as most common way in system. If interference signal enters system directly through wire, it will cause interference in system. For this connection method, filtering and decoupling techniques can be used to effectively suppress incoming EMI signals.

  • Common impedance coupling: is also a common coupling method. Often this happens when currents of two circuits share a common path. There are two types of common impedance connections: common ground and power supply impedance. To prevent such coupling, coupling impedance must be close to zero so that there is no common impedance between interferer and interferer.

  • Capacitive coupling. Also known as electric field coupling or electrostatic coupling, this is a method of coupling due to existence of distributed capacitance.

  • Electromagnetic induction coupling. Also known as magnetic field coupling. It is a method of communication due to induction of electromagnetic fields in inner or outer space.TVE. A common method to prevent this communication is to shield devices or circuits that are susceptible to interference.

  • Radiative coupling. Electromagnetic field radiation can also cause interference coupling, which is a kind of random interference. These interferences are easily transmitted to system via power lines. In addition, when signal transmission lines are long, they can emit and receive interference waves, which is called long line effect.

  • Leakage coupling: The so-called leakage coupling is a resistive coupling. This violation often occurs when insulation is reduced.

  • The decoupling capacitor usually has a relatively large capacitance, which avoids interference with other parts; bypass capacitor has a small capacitance and provides a low impedance noise return path.

    Actually, there is nothing wrong with this statement. However, after checking relevant information, I found that there is no fundamental difference between decoupling and bypass, and two terms are interchangeable. The function of deuce is vulgar: it is used as a power source.

    The so-called noise is actually fluctuations in power supply. Fluctuations in power supply come from two aspects: fluctuations in power supply itself, voltage fluctuations caused by changes in current drawn by load, and difference in corresponding power of power supply system. Both decoupling and shunt capacitors refer to noise caused by load changes.

    There is therefore no need to make a distinction between two. In fact, size and magnitude of capacitance value also have a theoretical justification, when chosen, self-oscillations in decoupling capacitor (shunting) and distributed parameters can occur at will in some cases.

    Therefore, decoupling and bypassing are literally based on actual load and power system conditions. There is no need to make a distinction, and there is no significant difference.

    Capacitors are essential components of board design, and their quality has become a very important aspect for us when evaluating quality of boards.

    1. Capacity Function and Representation

    Consists of two metal rods with an insulating medium between them. The characteristics of capacitors are mainly DC blocking and AC coupling, so they are mainly used for interstage coupling, filtering, decoupling, bypassing and signal tuning.

    The capacitance is represented by letter "C" plus numbers in chain, for example C8, which represents capacitance labeled 8 in chain.

    2. Classification of capacitors

    Capacitors are divided into: gas dielectric capacitors, liquid dielectric capacitors, inorganic solid dielectric capacitors, organic soliddifferent dielectric capacitors and electrolytic capacitors depending on different environments.

    Separated by polarity: polarized and non-polarized capacitors. According to their structure, they can be divided into: fixed capacitors, variable capacitors and trimmer capacitors.

    3. Capacitor capacity

    Capacity indicates size that can store electrical energy.

    The slowing effect of capacitance on AC signals is called capacitive reactance. Capacitance is related to frequency and capacitance of AC signals. Capacitance XC=1/2πf c (f represents frequency of AC signals and C represents capacitance).

    4. Capacitor capacitance unit and withstand voltage

    The basic unit of capacitance is F (farad). Other units of measure include millifarad (mF), microfarad (µF), nanofarad (nF) and picofarad (pF).

    Because capacitance of F unit is too large, we usually see units of µF, nF, and pF.

    Conversion ratio: 1F=1000000uF, 1uF=1000nF=1000000pF.

    Each capacitor has its own withstand voltage value, expressed in V.

    Common non-polarized capacitors have relatively high rated withstand voltages: 63V, 100V, 160V, 250V, 400V, 600V, 1000V, etc.

    The withstand voltage of polarized capacitors is relatively low, and common withstand voltage ratings are: 4V, 6.3V, 10V, 16V, 25V, 35V, 50V, 63V, 80V, 100V, 220V, 400V etc.

    5. Capacity marking method and capacity error

    The marking methods of capacitors are divided into: direct marking method, color marking method and digital marking method.

    For relatively large volume capacitors, direct marking method is often used. If it's 0.005, that means 0.005uF = 5nF. If it's 5n it means 5nF.

    Number notation usually uses three digits to represent capacity, first two digits represent significant digits, and third digit represents a power of 10. For example: 102 means 10x10x10 PF=1000PF, 203 means 20x10x10x10 PF.

    The color scale method is to use different colors to represent different numbers along direction of capacitor's lead. The first and second rings indicate capacitance, and third color indicates number of zeros after effective number (in pF). .

    Value represented by color: black = 0, brown = 1, red = 2, orange = 3, yellow = 4, green = 5, blue = 6, purple = 7, gray = 8, white = 9. .

    Capacity error is represented by symbols F, G, J, K, L, and M, and tolerances are ±1%, ±2%, ±5%, ±10%, ±15%, ±20%, respectively.

    6, Distinguishing and measuring positive and negative capacitance

    The black block with a mark on capacitor isnegative pole. In place of capacitor on PCB, there are two semi-circles, and contact corresponding to colored semi-circle is negative pole. Also, use length of pin to distinguish between positive and negative poles: long leg is positive and short leg is negative.

    If we don't know positive and negative poles of capacitor, we can use a multimeter to measure it. The medium between two poles of a capacitor is not an absolute insulator, and its resistance is not infinite, but has a finite value, usually above 1000 MΩ.

    The resistance between two poles of a capacitor is called insulation resistance or leakage resistance. Only when positive pole of electrolytic capacitor is connected to positive of power supply (black test pen when electricity is blocked), and negative terminal is connected to negative of power supply (red test pen when electricity is blocked), leakage current of electrolytic capacitor is small (leakage resistance is large). On contrary, leakage current of electrolytic capacitor increases (the leakage resistance decreases).

    So we first assume that certain pole is "+" pole, and multimeter uses R*100 or R*1K gear, then connect supposed "+" pole to black probe of multimeter, and connect other electrode to red probe multimeter. Connected, write down scale where arrow stops (the resistance of arrow is greater on left side), for a digital multimeter, reading can be read directly.

    Then discharge capacitor (touch two wires), then swap two test leads and repeat measurement. In two dimensions, black probe is connected to positive electrode of electrolytic capacitor when last position of clock hand is to left (or resistance value is high).

    7. Some experience with capacitors and four misunderstandings

    [Some experience]

    If it is not possible to determine polarity of line in circuit, it is recommended to use a non-polar electrolytic capacitor. The ripple current through electrolytic capacitor must not exceed allowable range.

    If specified value is exceeded, a capacitor with high ripple current resistance should be selected. The operating voltage of a capacitor cannot exceed its rated voltage.

    When welding a capacitor, electric soldering iron must be kept at a certain distance from plastic case of capacitor so that plastic case does not break due to overheating. In this case, welding time should not exceed 10 seconds, and welding temperature should not exceed 260 degrees Celsius.

    [Four misunderstandings]

    (1) The bigger capacity, better

    Many people tend to use large capacitors when replacing condensertori. We know that although larger capacitance, stronger current compensation capability provided for IC.

    Not to mention increase in volume caused by increase in capacity, which increases cost and affects airflow and heat dissipation. The bottom line is that there is a parasitic inductance on capacitor, and capacitor discharge circuit will resonate at a certain frequency.

    At resonance point, impedance of capacitor is small. Therefore, impedance of discharge circuit is smallest, and energy replenishment effect is best. But when frequency exceeds resonance point, impedance of discharge circuit begins to increase, and current capacity of capacitor decreases.

    The larger capacitance of capacitor, lower resonant frequency and smaller frequency range in which capacitor can effectively compensate for current. From point of view of ensuring ability of a capacitor to provide high-frequency current, opinion that larger capacitor, better, is incorrect. In general circuitry, there is a reference value.

    (2) For capacitors of same capacity, more small capacitors connected in parallel, better

    Voltage resistance, temperature resistance, capacitance, ESR (equivalent resistance), etc. are some important parameters of capacitors. The lower ESR, better.

    ESR depends on capacitance, frequency, voltage, temperature, etc. of capacitor. When voltage is fixed, larger capacitance, lower ESR. The use of several small capacitors in parallel in board design is limited by PCB space, so some people think that more small resistors connected in parallel, lower ESR and better effect.

    Theoretically, this is true, but given impedance of soldering capacitor leads, effect of connecting several small capacitors in parallel may be negligible.

    (3) The lower ESR, better effect

    Combined with our improved power supply circuit above, for input capacitor, input capacitor needs to be larger. Compared to bandwidth requirements, ESR requirements can be reduced accordingly.

    Because input capacitor is mainly designed to withstand voltage, followed by absorption of switching pulse of MOSFET. For output capacitor, withstand voltage and capacitance requirements can be reduced accordingly.

    The ESR requirements are slightly higher because there is enough current to be provided. But it should be noted here that ESR is not as low as it can be, and low ESR capacitors will cause switching circuit to oscillate. The complication of vibration elimination scheme will also lead to an increase in cost.

    In board design, a reference value is usually used here, which is used as a component selection parameter, in order to avoid increasing cost, causingbathroom vibration elimination circuit.

    (4) Good capacity means high quality

    The capacity-only theory was once very popular, and some manufacturers and media have also intentionally made it an important selling point. In board design, level of circuitry is key.

    Just as some manufacturers may use a two-phase power supply to produce more stable products than some manufacturers use a four-phase power supply, blindly using expensive capacitors may not necessarily produce good products.

    When measuring a product, it must be viewed from all sides and from all angles, and role of capacitors must not be intentionally or unintentionally exaggerated.