SMD components. Radioelements produced by printing. Installation of inductors on simple printed circuit boards

“Iron-laser” technology for manufacturing printed circuit boards(ULT) literally in a couple of years has become widespread in amateur radio circles and allows you to obtain printed circuit boards quite High Quality. Hand-drawn printed circuit boards require a lot of time and are not immune to errors.

Special requirements for pattern accuracy are imposed in the manufacture of printed inductors for high-frequency circuits. The edges of the coil conductors should be as smooth as possible, as this affects their quality factor. Manually drawing a multi-turn spiral coil is very problematic, and here the ULT may well have its say.

Rice. 1

Rice. 2

So, everything is in order. Let's launch computer program SPRINT-LAYOUT , for example version 5.0. Set in the program settings:

Grid scale - 1.25 mm;

Line width - 0.8 mm;

Board dimensions - 42.5x42.5 mm;

The outer diameter of the “patch” is 1.5 mm;

The diameter of the hole in the “patch” is 0.5 mm.

Find the center of the board and draw a coil conductor template (Fig. 1)along the coordinate grid using the CONDUCTOR tool, twisting the coil into the right side(for the template you need mirror image, but it can be obtained later, when printing). We install a “patch” at the beginning and end of the coil to connect the coil with the circuit elements.

In the print settings, we set the number of prints on a sheet, the distance between prints and, if it is necessary to “twist” the spool in the other direction, mirror printing of the design. You should print on smooth paper or special film, setting the printer settings to the maximum toner supply when printing.

Next we follow the standard ULT. We prepare foil fiberglass, clean the surface of the foil and degrease it, for example, with acetone. We apply the template with toner to the foil and iron it with a hot iron through a sheet of paper until the toner adheres securely to the foil.

Afterwards, soak the paper under running tap water (cold or room temperature) and carefully remove it in “pellets”, leaving the toner on the foil of the board. We etch the board and then remove the toner from it with a solvent, for example, acetone. A clear conductor of a high quality “printed” inductor remains on the board.

Printed coils with spiral turns using ULT are of slightly worse quality. This is due to the square shape of the image pixels, so the edges of the spiral coil conductor are jagged. True, these irregularities are quite small, and the quality of the reel, in general, is still higher than with manual operation.

Open the SPRINT-LAYOUT version 5.0 program again. In the toolkit, select SPECIAL FORM - a tool for drawing polygons and spirals. Select the SPIRAL tab. Install:

Starting radius (START RADIUS) -2 mm;

Distance between turns (DISTANCE) - 1.5 mm;

Conductor width (TRACK WIDTH) -0.8 mm;

The number of turns (TURNS), for example, is 20.

The size of the board occupied by such a coil is 65x65 mm (Fig. 2).

Printed coils are usually coupled together in bandpass filters (BPFs) using small capacitors. However, their inductive coupling is also possible, the degree of which can be changed by changing the distance between the planes of the coils or eccentrically rotating one relative to the other. Fixed mounting of the coils relative to each other can be achieved

Build using dielectric struts.

The inductance of the coils can be adjusted by shorting the turns, breaking the printed conductor, or partially removing it. This will increase the circuit tuning frequency. A reduction in frequency can be achieved by soldering small-capacity SMD-type capacitors between the turns.

Manufacturing of VHF coils in the form of a meander, straight and curved lines, comb filters, etc. using ULT also adds elegance to the final product and, as a rule, increases their quality factor (due to the “smooth” edges of the printed conductors). However, during production, one should remember the quality of the substrate material (fiberglass), which loses its insulator properties with increasing frequency. In equivalent circuits, the loss resistance in the dielectric should be connected in parallel with the printed coils, and this resistance will be lower, the higher the operating frequency and the worse the quality of the dielectric.

In practice, foil fiberglass can be fully used for the manufacture of printed resonant circuits up to the 2-meter range inclusive (up to approximately 150 MHz). Special high-frequency grades of fiberglass can be used in the range of 70 cm (up to approximately 470...500 MHz). At higher frequencies, foil-coated RF fluoroplastic (Teflon), ceramic or glass should be used.

A printed inductor has an increased quality factor due to a decrease in the interturn capacitance, obtained, on the one hand, due to the small thickness of the foil, and on the other, the “winding” pitch of the coil. A closed frame of grounded foil around the printed coil in its plane serves as a shield from other coils and printed conductors, but has little effect on the parameters of the coil if its periphery is under low RF voltage (connected to a common wire) and its center is under high.

Literature

1. G. Panasenko. Manufacturing of printing reels. - Radio, 1987, No. 5, P. 62.

In our turbulent age of electronics, the main advantages of an electronic product are small size, reliability, ease of installation and dismantling (disassembling equipment), low energy consumption and convenient usability ( from English- Ease of use). All these advantages are by no means possible without technology. surface mount– SMT technology ( S urface M ount T echnology), and of course, without SMD components.

What are SMD components

SMD components are used in absolutely all modern electronics. SMD ( S urface M mounted D evice), which translated from English means “surface-mounted device.” In our case, the surface is a printed circuit board, without through holes for radio elements:

In this case, SMD components are not inserted into the holes of the boards. They are soldered onto contact tracks, which are located directly on the surface of the printed circuit board. The photo below shows tin-colored contact pads on a mobile phone board that previously had SMD components.

Pros of SMD components

The biggest advantage of SMD components is their small size. The photo below shows simple resistors and:

Thanks to the small dimensions of SMD components, developers have the opportunity to place large quantity components per unit area than simple output radioelements. Consequently, the installation density increases and, as a result, the dimensions decrease electronic devices. Since the weight of an SMD component is many times lighter than the weight of the same simple output radio element, the weight of the radio equipment will also be many times lighter.

SMD components are much easier to desolder. For this we need a hairdryer. You can read how to desolder and solder SMD components in the article on how to solder SMDs correctly. It's much more difficult to seal them. In factories, special robots place them on a printed circuit board. No one solders them manually in production, except for radio amateurs and radio equipment repairmen.

Multilayer boards

Since equipment with SMD components has a very dense installation, there should be more tracks on the board. Not all tracks fit on one surface, so printed circuit boards are made multilayer. If the equipment is complex and has a lot of SMD components, then the board will have more layers. It's like a multi-layer cake made from short layers. The printed tracks connecting SMD components are located directly inside the board and cannot be seen in any way. An example of multilayer boards is mobile phone boards, computer or laptop boards ( motherboard, video card, RAM etc).

In the photo below, the blue board is the Iphone 3g, the green board is the computer motherboard.

All radio equipment repairers know that if a multilayer board is overheated, it will swell with a bubble. In this case, the interlayer connections break and the board becomes unusable. Therefore, the main trump card when replacing SMD components is the correct temperature.

Some boards use both sides of the printed circuit board, and the mounting density, as you understand, doubles. This is another advantage of SMT technology. Oh yes, it’s also worth taking into account the fact that the material required for the production of SMD components is much less, and their cost during mass production of millions of pieces literally costs pennies.

Main types of SMD components

Let's look at the main SMD elements used in our modern devices. Resistors, capacitors, low-value inductors, and other components look like ordinary small rectangles, or rather, parallelepipeds))

On boards without a circuit, it is impossible to know whether it is a resistor, a capacitor, or even a coil. The Chinese mark as they please. On large SMD elements, they still put a code or numbers to determine their identity and value. In the photo below these elements are marked in a red rectangle. Without a diagram, it is impossible to say what type of radio elements they belong to, as well as their rating.

The standard sizes of SMD components may be different. Here is a description of the standard sizes for resistors and capacitors. Here, for example, is a yellow rectangular SMD capacitor. They are also called tantalum or simply tantalum:

And this is what SMDs look like:

There are also these types of SMD transistors:

Which have a high denomination, in SMD version they look like this:

And of course, how can we live without microcircuits in our age of microelectronics! There are many SMD types of chip packages, but I divide them mainly into two groups:

1) Microcircuits in which the pins are parallel to the printed circuit board and are located on both sides or along the perimeter.

2) Microcircuits in which the pins are located under the microcircuit itself. This is a special class of microcircuits called BGA (from English Ball grid array- an array of balls). The terminals of such microcircuits are simple solder balls of the same size.

The photo below shows a BGA chip and its reverse side, consisting of ball pins.

BGA chips are convenient for manufacturers because they greatly save space on the printed circuit board, because there can be thousands of such balls under any BGA chip. This makes life much easier for manufacturers, but does not make life any easier for repairmen.

Summary

What should you use in your designs? If your hands don’t shake and you want to make a small radio bug, then the choice is obvious. But still, in amateur radio designs, dimensions do not play a big role, and soldering massive radio elements is much easier and more convenient. Some radio amateurs use both. Every day more and more new microcircuits and SMD components are being developed. Smaller, thinner, more reliable. The future definitely belongs to microelectronics.

The intent of this article is to discuss common mistakes made by PCB designers, describe the impact of these mistakes on quality performance, and provide recommendations for resolving problems that arise.

GENERAL CONSIDERATIONS

Due to the significant differences between analog and digital circuitry, the analog portion of the circuit must be separated from the rest, and special methods and rules must be followed when wiring it. Effects arising from non-ideal characteristics of printed circuit boards become especially noticeable in high-frequency analog circuits, but errors general view, described in this article, may affect the quality characteristics of devices operating even in the audio frequency range.

Printed circuit board - circuit component

Only in rare cases can an analog circuit PCB be routed so that the influences it introduces do not have any effect on the operation of the circuit. At the same time, any such impact can be minimized so that the characteristics of the analog circuitry of the device are the same as those of the model and prototype.

Layout

Developers of digital circuits can correct small errors on the manufactured board by adding jumpers to it or, conversely, removing unnecessary conductors, making changes to the operation of programmable chips, etc., moving very quickly to the next development. This is not the case for an analog circuit. Some of the common errors discussed in this article cannot be corrected by adding jumpers or removing excess conductors. They can and will render the entire printed circuit board inoperative.

It is very important for a digital circuit designer using such correction methods to read and understand the material presented in this article well in advance of submitting the design to production. A little design attention and discussion of possible options will not only prevent the PCB from becoming scrap, but also reduce the cost of gross errors in a small analog part of the circuit. Finding errors and fixing them can result in hundreds of lost hours. Prototyping can reduce this time to one day or less. Breadboard all your analog circuits.

Sources of noise and interference

Noise and interference are the main elements that limit the quality of circuits. Interference can be either emitted by sources or induced on circuit elements. Analog circuitry is often located on a printed circuit board along with high-speed digital components, including digital signal processors (DSPs).

High frequency logic signals generate significant radio frequency interference (RFI). The number of noise emission sources is enormous: key power supplies digital systems, Cell phones, radio and television, lamp power supplies daylight, personal computers, lightning discharges, etc. Even if an analog circuit operates in the audio frequency range, radio frequency interference can create noticeable noise in the output signal.

CATEGORIES OF PRINTED BOARDS

The choice of PCB design is an important factor in determining the mechanical performance of the overall device. For the manufacture of printed circuit boards, materials of varying quality levels are used. It will be most suitable and convenient for the developer if the PCB manufacturer is located nearby. In this case, it is easy to control the resistivity and dielectric constant - the main parameters of the printed circuit board material. Unfortunately, this is not enough and knowledge of other parameters such as flammability, high-temperature stability and hygroscopicity coefficient is often necessary. These parameters can only be known by the manufacturer of the components used in the production of printed circuit boards.

Layered materials are designated by the indices FR (flame resistant) and G. Material with the index FR-1 has the highest flammability, and FR-5 the least. Materials with indexes G10 and G11 have special characteristics. The materials of printed circuit boards are given in table. 1.

Do not use FR-1 category PCB. There are many examples of FR-1 PCBs that have suffered thermal damage from high-power components. Printed circuit boards in this category are more similar to cardboard.

FR-4 is often used in the manufacture of industrial equipment, while FR-2 is used in the manufacture of household appliances. These two categories are standardized in the industry, and FR-2 and FR-4 PCBs are often suitable for most applications. But sometimes the imperfect characteristics of these categories force the use of other materials. For example, for very high-frequency applications, fluoroplastic and even ceramics are used as printed circuit board materials. However, the more exotic the PCB material, the higher the price may be.

When choosing a PCB material, pay special attention to its hygroscopicity, since this parameter can have a strong negative effect on the desired characteristics of the board - surface resistance, leakage, high-voltage insulating properties (breakdown and sparking) and mechanical strength. Also pay attention to operating temperature. Hot spots can occur in unexpected places, such as near large digital integrated circuits that switch at high frequencies. If such areas are located directly below analog components, increased temperatures may affect the performance of the analog circuit.

Table 1

	Components, comments
	paper, phenolic composition: pressing and stamping at room temperature, high hygroscopicity coefficient
	paper, phenolic composition: applicable for single-sided printed circuit boards of household appliances, low hygroscopicity coefficient
	paper, epoxy composition: designs with good mechanical and electrical characteristics
	fiberglass, epoxy composition: excellent mechanical and electrical properties
	fiberglass, epoxy composition: high strength at elevated temperatures, non-flammable
	fiberglass, epoxy composition: high insulating properties, highest strength of fiberglass, low hygroscopicity coefficient
	fiberglass, epoxy composition: high flexural strength at elevated temperatures, high solvent resistance

Once the PCB material is selected, the thickness of the PCB foil needs to be determined. This parameter is primarily selected based on the maximum value of the flowing current. If possible, try to avoid using very thin foil.

NUMBER OF PRINTED BOARD LAYERS

Depending on the overall circuit complexity and quality requirements, the designer must determine the number of layers of the PCB.

Single Layer PCBs

Very simple electronic circuits are made on single-sided boards using cheap foil materials (FR-1 or FR-2) and often have many jumpers, resembling double-sided boards. This method of creating printed circuit boards is recommended only for low-frequency circuits. For reasons that will be described below, single-sided printed circuit boards are highly susceptible to interference. A good single-sided PCB is quite difficult to design for many reasons. Nevertheless good boards This type does occur, but their development requires a lot of thought in advance.

Double Layer PCBs

At the next level are double-sided printed circuit boards, which in most cases use FR-4 as the substrate material, although FR-2 is also sometimes found. The use of FR-4 is more preferable, since in printed circuit boards This material makes holes more best quality. Circuits on double-sided printed circuit boards are much easier to wire because In two layers it is easier to route intersecting routes. However, for analog circuits, crossing traces is not recommended. Where possible, the bottom layer (bottom) should be allocated to the ground polygon, and the remaining signals should be routed to the top layer (top). Using a landfill as an earth bus provides several advantages:

the common wire is the most frequently connected wire in the circuit; therefore, it is reasonable to have “a lot” of common wire to simplify wiring.
the mechanical strength of the board increases.
the resistance of all connections to the common wire decreases, which, in turn, reduces noise and interference.
The distributed capacitance for each circuit circuit is increased, helping to suppress radiated noise.
the polygon, which is a screen, suppresses interference emitted by sources located on the side of the polygon.

Double-sided PCBs, despite all their advantages, are not the best, especially for low-signal or high-speed circuits. In general, the thickness of the printed circuit board, i.e. the distance between the metallization layers is 1.5 mm, which is too much to fully realize some of the advantages of a two-layer printed circuit board given above. The distributed capacity, for example, is too small due to such a large interval.

Multilayer PCBs

For critical circuit design, multilayer printed circuit boards (MPBs) are required. Some reasons for their use are obvious:

The distribution of power buses is just as convenient as for the common wire bus; if polygons on a separate layer are used as power buses, then it is quite simple to supply power to each circuit element using vias;
signal layers are freed from power buses, which facilitates the wiring of signal conductors;
Distributed capacitance appears between the ground and power polygons, which reduces high-frequency noise.

In addition to these reasons for using multilayer printed circuit boards, there are other, less obvious ones:

better suppression of electromagnetic (EMI) and radio frequency (RFI) interference thanks to the reflection effect (image plane effect), known back to the time of Marconi. When a conductor is placed close to a flat conducting surface, most of the high frequency return currents will flow along the plane directly below the conductor. The direction of these currents will be opposite to the direction of the currents in the conductor. Thus, the reflection of the conductor in the plane creates a signal transmission line. Since the currents in the conductor and in the plane are equal in magnitude and opposite in direction, some reduction in radiated interference is created. The reflection effect only works effectively with unbroken solid polygons (these can be both ground polygons and power polygons). Any loss of integrity will result in reduced interference suppression.
reduction in overall cost for small-scale production. Although multilayer PCBs are more expensive to manufacture, their potential radiation is lower than that of single- and double-layer PCBs. Therefore, in some cases, using only multilayer boards will allow you to meet the emission requirements set during design, without additional testing and testing. The use of MPP can reduce the level of radiated interference by 20 dB compared to double-layer boards.

Layer order

Inexperienced designers often have some confusion about the optimal order of PCB layers. Let's take for example a 4-layer chamber containing two signal layers and two polygon layers - a ground layer and a power layer. What is the best layer order? Signal layers between polygons that will serve as screens? Or should we make the polygon layers internal to reduce the interference of signal layers?

When addressing this issue, it is important to remember that often the location of the layers does not matter much, since the components are located on the outer layers anyway, and the buses that supply signals to their pins sometimes pass through all the layers. Therefore, any screen effects are just a compromise. In this case, it is better to take care of creating a large distributed capacity between the power and ground polygons, placing them in the inner layers.

Another advantage of placing the signal layers outside is the availability of signals for testing, as well as the possibility of modifying connections. Anyone who has ever changed the connections of conductors located in the inner layers will appreciate this opportunity.

For PCBs with more than four layers, the general rule is to place high-speed signal conductors between the ground and power polygons, and route low-frequency signal conductors to the outer layers.

GROUNDING

Good grounding is a general requirement for a rich, multi-level system. And it should be planned from the first step of design development.

Basic rule: division of land.

Dividing the earth into analog and digital parts is one of the simplest and most effective methods noise suppression. One or more layers of a multilayer printed circuit board are usually dedicated to a layer of ground polygons. If the developer is not very experienced or inattentive, then the ground of the analog part will be directly connected to these polygons, i.e. analog current return will use the same circuit as digital return current. Auto-distributors work in much the same way and unite all the lands together.

If a previously developed printed circuit board with a single ground polygon combining analog and digital grounds is subject to processing, then it is necessary to first physically separate the grounds on the board (after this operation, the operation of the board becomes almost impossible). After this, all connections are made to the analog ground of the analog circuit components (analog ground is formed) and to the digital ground of the digital circuit components (digital ground is formed). And only after this, digital and analog ground are combined at the source.

Other rules for land formation:

The power and ground buses must be at the same potential alternating current, which implies the use of decoupling capacitors and distributed capacitance.
Avoid overlap of analog and digital polygons. Place the analog power rails and polygons above the analog ground polygon (similar to the digital power rails). If there is an overlap between analog and digital areas at any location, the distributed capacitance between the overlapping areas will create an AC coupling, and noise from the digital components will be carried into the analog circuit. Such overlaps invalidate the isolation of the landfills.
Separation does not mean electrically isolating the analog ground from the digital ground. They must be connected together in some, preferably one, low-impedance node. A correct grounding system has only one ground, which is the ground pin for AC powered systems or the common ground pin for AC powered systems. DC voltage(for example, a battery). All signal and power currents in this circuit must return to this ground at one point, which will serve as the system ground. Such a point may be the terminal of the device body. It is important to understand that when connecting the common terminal of the circuit to several points on the chassis, ground loops can be formed. Creating a single common point of land consolidation is one of the most difficult aspects of system design.
Whenever possible, separate connector pins intended to carry return currents—return currents should only be combined at the system ground point. Aging of connector contacts, as well as frequent disconnection of their mating parts, leads to an increase in contact resistance; therefore, for more reliable operation, it is necessary to use connectors with a certain number of additional pins. Complex digital printed circuit boards have many layers and contain hundreds or thousands of conductors. Adding another conductor rarely creates a problem, but adding additional connector pins does. If this cannot be done, then it is necessary to create two return current conductors for each power path on the board, taking special precautions.
It is important to separate the tires digital signals from places on the printed circuit board where the analog components of the circuit are located. This involves isolation (shielding) by polygons, creating short analog signal paths, and careful placement of passive components with adjacent high-speed digital and mission-critical analog signal buses. Digital signal buses must be routed around areas with analog components and not overlap with buses and areas of analog ground and analog power. If this is not done, then the design will contain a new unintended element - an antenna, the radiation of which will affect high-impedance analog components and conductors.

Almost all clock signals are high enough frequency signals that even small capacitances between traces and polygons can create significant couplings. It must be remembered that it is not only the fundamental clock frequency that can cause a problem, but also its higher harmonics.

There is only one case where it is necessary to combine analog and digital signals over an analog ground area. Analog-to-digital and digital-to-analog converters are housed in housings with analog and digital ground pins. Taking into account the previous discussion, it can be assumed that the digital ground pin and analog ground pin should be connected to the digital and analog ground buses, respectively. However, in this case this is not true.

The names of the pins (analog or digital) refer only to the internal structure of the converter, to its internal connections. In the circuit, these pins must be connected to the analog ground bus. The connection can also be made inside an integrated circuit, but achieving low resistance of such a connection is quite difficult due to topological restrictions. Therefore, when using converters, it is assumed that the analog and digital ground pins are connected externally. If this is not done, then the parameters of the microcircuit will be significantly worse than those given in the specification.

It must be taken into account that the digital elements of the converter can degrade the quality characteristics of the circuit by introducing digital noise into the analog ground and analog power circuits. When designing converters, this negative impact is taken into account so that the digital part consumes as little power as possible. At the same time, interference from switching logic elements are decreasing. If the digital pins of the converter are not heavily loaded, then internal switching usually does not cause any special problems. When designing a PCB containing an ADC or DAC, careful consideration must be given to decoupling the converter's digital power supply to analog ground.

FREQUENCY CHARACTERISTICS OF PASSIVE COMPONENTS

For proper operation analog circuits are very important right choice passive components. Start your design by carefully considering the high-frequency characteristics of passive components and preliminary placement and layout of them on the board sketch.

A large number of designers completely ignore the frequency limitations of passive components when used in analog circuitry. These components have limited frequency ranges and operating them outside the specified frequency range can lead to unpredictable results. Some may think that this discussion only concerns high-speed analog circuits. However, this is far from true - high-frequency signals have a strong impact on the passive components of low-frequency circuits through radiation or direct communication through conductors. For example, a simple low-pass filter on an op-amp can easily become a high-pass filter when exposed to high frequency at its input.

Resistors

There are three types of resistors commonly used: 1) wirewound, 2) carbon composite, and 3) film. It doesn't take much imagination to understand how a wirewound resistor can be converted into an inductance, since it is a coil of wire made of high-resistance metal. Most electronic device developers have no idea about the internal structure of film resistors, which are also a coil, albeit made of a metal film. Therefore, film resistors also have an inductance that is less than that of wirewound resistors. Film resistors with a resistance of no more than 2 kOhm can be freely used in high-frequency circuits. The resistor terminals are parallel to each other, so there is a noticeable capacitive coupling between them. For high-value resistors, the terminal-to-terminal capacitance will reduce the total impedance at high frequencies.

Capacitors

The high-frequency characteristics of capacitors can be represented by the equivalent circuit shown in Figure 6.

Capacitors in analog circuits are used as decoupling and filtering components.

A 10 µF electrolytic capacitor has a resistance of 1.6 ohms at 10 kHz and 160 µohms at 100 MHz. Is it so?

When using electrolytic capacitors, care must be taken correct connection. The positive terminal must be connected to a more positive constant potential. An incorrect connection causes DC current to flow through the electrolytic capacitor, which can damage not only the capacitor itself, but also part of the circuit.

In rare cases, the DC potential difference between two points in the circuit may change its sign. This requires the use of non-polar electrolytic capacitors, the internal structure of which is equivalent to two polar capacitors connected in series.

Inductance

Printed circuit board

The printed circuit board itself has the characteristics of the passive components discussed above, although not so obvious.

The pattern of conductors on a printed circuit board can be both a source and a receiver of interference. Good wiring reduces the sensitivity of the analog circuit to radiation sources.

The printed circuit board is susceptible to radiation because the conductors and leads of the components form a kind of antenna. Antenna theory is a rather complex subject to study and is not covered in this article. However, some basics are provided here.

A bit of antenna theory

On DC or low frequencies the active component predominates. As the frequency increases, the reactive component becomes more and more significant. In the range from 1 kHz to 10 kHz, the inductive component begins to take effect and the conductor is no longer a low-impedance connector, but rather acts as an inductor.

Typically, traces on a printed circuit board have values from 6 nH to 12 nH per centimeter of length. For example, a 10 cm conductor has a resistance of 57 mOhm and an inductance of 8 nH per cm. At a frequency of 100 kHz, the reactance becomes 50 mOhm, and at higher frequencies the conductor will be an inductance rather than a resistive one.

The rule for a whip antenna is that it begins to noticeably interact with the field at about 1/20 of the wavelength, and maximum interaction occurs at a rod length of 1/4 of the wavelength. Therefore, the 10 cm conductor from the example in the previous paragraph will start to become a pretty good antenna at frequencies above 150 MHz. It must be remembered that although the generator clock frequency A digital circuit may not operate at frequencies above 150 MHz; its signal always contains higher harmonics. If the printed circuit board contains components with pin pins of considerable length, then such pins can also serve as antennas.

The other main type of antenna is loop antenna. The inductance of a straight conductor increases greatly when it bends and becomes part of an arc. Increasing inductance lowers the frequency at which the antenna begins to interact with the field lines.

The theory of signal reflection and matching is close to the theory of antennas.

When the PCB conductor is rotated through an angle of 90°, signal reflection may occur. This is mainly due to changes in the width of the current path. At the apex of the corner, the trace width increases by 1.414 times, which leads to a mismatch in the characteristics of the transmission line, especially the distributed capacitance and the trace's own inductance. Quite often it is necessary to rotate a trace on a printed circuit board by 90°. Many modern CAD packages allow you to smooth the corners of drawn routes or draw routes in the form of an arc. Figure 9 shows two steps to improve the corner shape. Only the last example maintains a constant path width and minimizes reflections.

Tip for experienced PCB designers: leave the smoothing process for the last stage of work before creating teardrop-shaped pins and filling polygons. Otherwise, the CAD package will take longer to smooth due to more complex calculations.

Capacitive coupling occurs between PCB conductors on different layers when they intersect. Sometimes this can create a problem. Conductors placed one above the other on adjacent layers create a long film capacitor.

For example, a printed circuit board may have the following parameters:
- 4 layers; the signal and ground polygon layers are adjacent,
- interlayer spacing - 0.2 mm,
- conductor width - 0.75 mm,
- conductor length - 7.5 mm.

The typical ER dielectric constant for FR-4 is 4.5.

The capacitance value between these two buses is 1.1 pF. Even such a seemingly small capacity is unacceptable for some applications.

The amplitude of the output signal doubles at frequencies close to the upper limit of the op-amp's frequency range. This, in turn, can lead to oscillation, especially at antenna operating frequencies (above 180 MHz).

This effect gives rise to numerous problems, for which there are, however, many ways to solve them. The most obvious of them is reducing the length of the conductors. Another way is to reduce their width. There is no reason to use a conductor of this width to connect the signal to the inverting input, because Very little current flows through this conductor. Reducing the length of the trace to 2.5 mm and the width to 0.2 mm will lead to a decrease in capacitance to 0.1 pF, and such capacitance will no longer lead to such a significant increase in the frequency response. Another solution is to remove part of the polygon under the inverting input and the conductor that goes to it.

The width of the PCB conductors cannot be reduced indefinitely. The limit width is defined as technological process, and the thickness of the foil. If two conductors pass close to each other, a capacitive and inductive coupling is formed between them.

Signal conductors should not be routed parallel to each other, except in the case of differential or microstrip lines. The gap between conductors should be at least three times the width of the conductors.

Capacitance between traces in analog circuits can create problems with large resistor values (several megohms). The relatively large capacitive coupling between the inverting and non-inverting inputs of an op-amp can easily cause the circuit to oscillate.

For example, with d=0.4 mm and h=1.5 mm (fairly common values), the inductance of the hole is 1.1 nH.

Remember that if there are large resistances in the circuit, then special attention should be paid to cleaning the board. During the final operations of manufacturing a printed circuit board, any remaining flux and contaminants must be removed. IN Lately When installing printed circuit boards, water-soluble fluxes are often used. Being less harmful, they are easily removed with water. But at the same time, washing the board with insufficiently clean water can lead to additional contamination that worsens the dielectric characteristics. Therefore, it is very important to clean the high-impedance circuit board with fresh distilled water.

SIGNAL ISOLATION

As already noted, interference can penetrate into the analog part of the circuit through the power supply circuits. To reduce such interference, decoupling (blocking) capacitors are used to reduce the local impedance of the power buses.

If you need to lay out a printed circuit board that has both analog and digital parts, then you need to have at least a small understanding of electrical characteristics logical elements.

A typical output stage of a logic element contains two transistors connected in series with each other, as well as between the power and ground circuits.

These transistors ideally operate strictly in antiphase, i.e. when one of them is open, then at the same moment in time the second is closed, generating either a logical one or a logical zero signal at the output. In the steady state logic state, the power consumption of the logic element is small.

The situation changes dramatically when the output stage switches from one logic state to another. In this case, for a short period of time, both transistors can be open simultaneously, and the supply current of the output stage increases greatly, since the resistance of the current path from the power bus to the ground bus through two series-connected transistors decreases. The power consumption increases abruptly and then also decreases, which leads to a local change in the supply voltage and the occurrence of a sharp, short-term change in current. These changes in current result in the emission of radio frequency energy. Even on a relatively simple printed circuit board there may be tens or hundreds of considered output stages of logic elements, so the total effect of their simultaneous operation can be very large.

It is impossible to accurately predict the frequency range in which these current surges will occur, since the frequency of their occurrence depends on many factors, including the propagation delay of switching transistors of the logic element. The delay, in turn, also depends on many random reasons that arise during the production process. Switching noise has a broadband distribution of harmonic components over the entire range. There are several methods for suppressing digital noise, the application of which depends on the spectral distribution of the noise.

Table 2 shows the maximum operating frequencies for common capacitor types.

table 2

From the table it is obvious that tantalum electrolytic capacitors are used for frequencies below 1 MHz; at higher frequencies, ceramic capacitors should be used. It must be remembered that capacitors have their own resonance and their incorrect choice may not only not help, but also aggravate the problem. Figure 15 shows typical self-resonances of two common capacitors - 10 μF tantalum electrolytic and 0.01 μF ceramic.

Actual specifications may vary between different manufacturers and even from batch to batch within the same manufacturer. It is important to understand that for efficient work capacitor, the frequencies it suppresses must be in a lower range than the frequency of its own resonance. Otherwise, the nature of the reactance will be inductive, and the capacitor will no longer work effectively.

Do not be mistaken that one 0.1 µF capacitor will suppress all frequencies. Small capacitors (10 nF or less) can operate more efficiently at higher frequencies.

IC power decoupling

Decoupling the power supply of integrated circuits to suppress high-frequency noise consists of using one or more capacitors connected between the power and ground pins. It is important that the conductors connecting the leads to the capacitors are short. If this is not the case, then the self-inductance of the conductors will play a significant role and negate the benefits of using decoupling capacitors.

A decoupling capacitor must be connected to each chip package, no matter how many operational amplifiers located inside the case - 1, 2 or 4. If the op-amp is powered bipolar power supply, then it goes without saying that decoupling capacitors must be located at each power pin. The capacitance value must be carefully selected depending on the type of noise and interference present in the circuit.

In particularly difficult cases, it may be necessary to add an inductance connected in series with the power output. The inductance should be located before, not after, the capacitors.

Another, cheaper way is to replace the inductance with a resistor with low resistance (10...100 Ohms). In this case, together with the decoupling capacitor, the resistor forms a low-pass filter. This method reduces the power supply range of the op-amp, which also becomes more dependent on power consumption.

Typically, to suppress low-frequency noise in power circuits, it is sufficient to use one or more aluminum or tantalum electrolytic capacitors at the power input connector. An additional ceramic capacitor will suppress high-frequency interference from other boards.

ISOLATION OF INPUT AND OUTPUT SIGNALS

Many noise problems result from directly connecting input and output pins. As a result of the high-frequency limitations of passive components, the response of a circuit when exposed to high-frequency noise can be quite unpredictable.

In a situation where the frequency range of the induced noise is significantly different from the frequency range of the circuit, the solution is simple and obvious - placing a passive RC filter to suppress high-frequency interference. However, when using a passive filter, you need to be careful: its characteristics (due to the non-ideal frequency characteristics of passive components) lose their properties at frequencies 100...1000 times higher than the cutoff frequency (f3db). When using series-connected filters tuned to different frequency ranges, the higher frequency filter should be closest to the source of interference. Ferrite ring inductors can also be used to suppress noise; they retain the inductive nature of the resistance up to a certain frequency, and above their resistance becomes active.

The interference on an analog circuit can be so large that it is impossible to get rid of (or, according to at least, reduce) from them is possible only through the use of screens. To operate effectively, they must be carefully designed so that the frequencies that cause the most problems cannot enter the circuit. This means that the screen should not have holes or cutouts larger than 1/20 of the wavelength of the radiation being screened. It is a good idea to allocate sufficient space for the proposed shield from the very beginning of the PCB design. When using a shield, you can optionally use ferrite rings (or beads) for all connections to the circuit.

OPERATIONAL AMPLIFIER CASES

One, two or four operational amplifiers are usually placed in one package.

A single op amp often also has additional inputs, for example to adjust the offset voltage. Dual and quad op amps have only inverting and non-inverting inputs and output. Therefore, if it is necessary to have additional adjustments, it is necessary to use single operational amplifiers. When using additional outputs, you must remember that by their structure they are auxiliary inputs, so they must be controlled carefully and in accordance with the manufacturer's recommendations.

In a single op amp, the output is located on the opposite side of the inputs. This can make it difficult to operate the amplifier at high frequencies due to the long conductors feedback. One way to overcome this is to place the amplifier and feedback components on different sides of the PCB. This, however, results in at least two additional holes and cuts in the ground polygon. Sometimes it is worth using a dual op amp to solve this problem, even if the second amplifier is not used (and its pins must be connected properly).

Dual op amps are especially common in stereo amplifiers, and quad op amps are used in multistage filter circuits. However, there is a rather significant disadvantage to this. Even though modern technology provides decent isolation between amplifier signals on the same silicon chip, there is still some crosstalk between them. If it is necessary to have a very small amount of such interference, then it is necessary to use single operational amplifiers. Crosstalk does not only occur when using dual or quad amplifiers. Their source can be the very close proximity of passive components of different channels.

Dual and quad op-amps, in addition to the above, allow for more dense installation. The individual amplifiers appear to be mirror-image relative to each other.
It is necessary to pay attention to the fact that the conductors of the half-supply voltage driver are located directly under the integrated circuit housing, which makes it possible to reduce their length. This example illustrates not what should be, but what should be done. The average level voltage, for example, could be the same for all four amplifiers. Passive components can be sized accordingly. For example, frame size 0402 planar components match the pin spacing of a standard SO package. This allows conductor lengths to be kept very short for high frequency applications.

When placing op amps in DIP packages and passive components with lead wires, vias must be provided on the printed circuit board to mount them. Such components are currently used when there are no special requirements for the dimensions of the printed circuit board; They are usually cheaper, but the cost of the printed circuit board increases during the manufacturing process due to drilling additional holes for component leads.

In addition, when using external components, the dimensions of the board and the length of the conductors increase, which does not allow the circuit to operate at high frequencies. Vias have their own inductance, which also limits the dynamic characteristics of the circuit. Therefore, overhead components are not recommended for implementing high-frequency circuits or for analog circuits located close to high-speed logic circuits.

Some designers, trying to reduce the length of the conductors, place resistors vertically. At first glance it may seem that this shortens the length of the route. However, this increases the path of current through the resistor, and the resistor itself represents a loop (turn of inductance). The emitting and receiving ability increases many times over.

Surface mounting does not require a hole for each component lead. However, problems arise when testing the circuit, and it is necessary to use vias as test points, especially when using small components.

UNUSED OP-AMP SECTIONS

When using dual and quad op-amps in a circuit, some sections may remain unused and must be connected correctly in this case. Incorrect connections can lead to increased power consumption, more heat, and more noise from the op amps used in the same package. The pins of unused op-amps can be connected like this: the output of the amplifier is connected to the inverting input.

CONCLUSION

Remember the following basic points and keep them in mind at all times when designing and wiring analog circuits.

think of the PCB as a component electrical diagram;
have an awareness and understanding of sources of noise and interference;
model and layout circuits.

Printed circuit board:

use printed circuit boards only from high-quality material (for example, FR-4);
circuits made on multilayer printed circuit boards are 20 dB less susceptible to external interference than circuits made on double-layer boards;
use separated, non-overlapping polygons for different lands and feeds;
Place the ground and power polygons on the inner layers of the PCB.

Components:

Be aware of the frequency limitations introduced by passive components and board traces;
try to avoid vertical placement of passive components in high-speed circuits;
For high-frequency circuits, use components designed for surface mounting;
conductors should be shorter, the better;
if a larger conductor length is required, then reduce its width;
Unused pins of active components must be connected correctly.

Wiring:

place the analog circuit near the power connector;
never route conductors transmitting logic signals through the analog area of the board, and vice versa;
make the conductors suitable for the inverting input of the op-amp short;
make sure that the conductors of the inverting and non-inverting inputs of the op-amp are not located parallel to each other over a long distance;
try to avoid using extra vias, because... their own inductance may cause additional problems;
do not route the conductors at right angles and smooth the tops of the corners if possible.

Interchange:

use the correct types of capacitors to suppress noise in power supply circuits;
to suppress low-frequency interference and noise, use tantalum capacitors at the power input connector;
To suppress high-frequency interference and noise, use ceramic capacitors at the power input connector;
use ceramic capacitors at each power pin of the microcircuit; if necessary, use multiple capacitors for different frequency ranges;
if excitation occurs in the circuit, then it is necessary to use capacitors with a lower capacitance value, and not a larger one;
in difficult cases, use series-connected resistors of low resistance or inductance in power circuits;
Analog power decoupling capacitors should only be connected to the analog ground, not the digital ground.

Multilayer PCBs

For critical circuit design, multilayer printed circuit boards (MPBs) are required. Some reasons for their use are obvious:

The distribution of power buses is just as convenient as for the common wire bus; if polygons on a separate layer are used as power buses, then it is quite simple to supply power to each circuit element using vias
signal layers are freed from power buses, which facilitates signal wiring
distributed capacitance appears between the ground and power polygons, which reduces high-frequency noise

In addition to these reasons for using multilayer printed circuit boards, there are other, less obvious ones:

better electromagnetic suppression ( EMI) and radio frequency ( RFI) interference due to the reflection effect ( image plane effect), known back in the time of Marconi. When a conductor is placed close to a flat conducting surface, most of the high frequency return currents will flow along the plane directly below the conductor. The direction of these currents will be opposite to the direction of the currents in the conductor. Thus, the reflection of the conductor in the plane creates a signal transmission line. Since the currents in the conductor and in the plane are equal in magnitude and opposite in direction, some reduction in radiated interference is created. The reflection effect only works effectively with unbroken solid polygons (these can be both ground polygons and power polygons). Any loss of integrity will result in reduced interference suppression.
reduction in overall cost for small-scale production. Although multilayer PCBs are more expensive to manufacture, their potential radiation is lower than that of single- and double-layer PCBs. Therefore, in some cases, using only multilayer boards will allow you to meet the emission requirements set during design, without additional testing and testing. The use of MPP can reduce the level of radiated interference by 20 dB compared to double-layer boards.

Layer order

For PCBs with more than four layers, the general rule is to place high-speed signal conductors between the ground and power polygons, and route low-frequency signal conductors to the outer layers.

Grounding

Good grounding is a general requirement for a rich, multi-level system. And it should be planned from the first step of design development.

Basic rule: division of land.

Dividing the ground into analog and digital parts is one of the simplest and most effective methods of noise reduction. One or more layers of a multilayer printed circuit board are usually dedicated to a layer of ground polygons. If the developer is not very experienced or inattentive, then the ground of the analog part will be directly connected to these polygons, i.e. analog current return will use the same circuit as digital return current. Auto-distributors work in much the same way and unite all the lands together.

Other rules for land formation:

Figure 4 shows possible variant placement of all components on the board, including the power supply. This uses three separate and isolated ground/power planes: one for the source, one for the digital circuit, and one for the analog circuit. The ground and power circuits of the analog and digital parts are combined only in the power supply. High-frequency noise is filtered out in the power circuits by chokes. In this example, the high frequency signals of the analog and digital parts are separated from each other. This design has a very high probability of a favorable outcome, since it ensures good placement of components and adherence to the rules of circuit separation.

It must be taken into account that the digital elements of the converter can degrade the quality characteristics of the circuit by introducing digital noise into the analog ground and analog power circuits. When designing converters, this negative impact is taken into account so that the digital part consumes as little power as possible. At the same time, interference from switching logic elements is reduced. If the digital pins of the converter are not heavily loaded, then internal switching usually does not cause any special problems. When designing a PCB containing an ADC or DAC, careful consideration must be given to decoupling the converter's digital power supply to analog ground.

Frequency characteristics of passive components

Correct selection of passive components is essential for proper operation of analog circuits. Start your design by carefully considering the high-frequency characteristics of passive components and preliminary placement and layout of them on the board sketch.

Resistors

The high-frequency characteristics of resistors can be represented by the equivalent circuit shown in Figure 5.

Capacitors

The high-frequency characteristics of capacitors can be represented by the equivalent circuit shown in Figure 6.

Capacitors in analog circuits are used as decoupling and filtering components. For an ideal capacitor, the reactance is determined by the following formula:

Therefore, a 10 µF electrolytic capacitor will have a resistance of 1.6 ohms at 10 kHz and 160 µohms at 100 MHz. Is it so?

When using electrolytic capacitors, care must be taken to ensure correct connection. The positive terminal must be connected to a more positive constant potential. An incorrect connection causes DC current to flow through the electrolytic capacitor, which can damage not only the capacitor itself, but also part of the circuit.

Inductance

The high-frequency characteristics of inductances can be represented by the equivalent circuit shown in Figure 7.

Inductance reactance is described by the following formula:

Therefore, a 10 mH inductance will have a reactance of 628 ohms at 10 kHz, and a reactance of 6.28 megohms at 100 MHz. Right?

The printed circuit board itself has the characteristics of the passive components discussed above, although not so obvious.

The pattern of conductors on a printed circuit board can be both a source and a receiver of interference. Good wiring reduces the sensitivity of the analog circuit to radiation sources.

A bit of antenna theory

At direct current or low frequencies, the active component predominates. As the frequency increases, the reactive component becomes more and more significant. In the range from 1 kHz to 10 kHz, the inductive component begins to take effect and the conductor is no longer a low-impedance connector, but rather acts as an inductor.

The formula for calculating the inductance of a PCB conductor is as follows:

The rule for a whip antenna is that it begins to noticeably interact with the field at about 1/20 of the wavelength, and maximum interaction occurs at a rod length of 1/4 of the wavelength. Therefore, the 10 cm conductor from the example in the previous paragraph will start to become a pretty good antenna at frequencies above 150 MHz. It must be remembered that despite the fact that the clock generator of a digital circuit may not operate at frequencies above 150 MHz, higher harmonics are always present in its signal. If the printed circuit board contains components with pin pins of considerable length, then such pins can also serve as antennas.

Experienced PCB designers with a reasonable understanding of loop antenna theory know not to design loops for critical signals. Some designers, however, do not think about this, and the return and signal current conductors in their circuits are loops. The creation of loop antennas is easy to demonstrate with an example (Fig. 8). In addition, the creation of a slot antenna is shown here.

Let's consider three cases:

Option A is an example of bad design. It does not use an analog ground polygon at all. The loop circuit is formed by the ground and signal conductors. When a current passes, an electric field and a magnetic field perpendicular to it arise. These fields form the basis of the loop antenna. The loop antenna rule states that for best efficiency, the length of each conductor should be equal to half the wavelength of the received radiation. However, we should not forget that even at 1/20 of the wavelength, the loop antenna is still quite effective.

Option B is better than Option A, but there is a gap in the polygon, probably to create a specific place for routing signal conductors. The signal and return current paths form a slot antenna. Other loops form in the cutouts around the chips.

Option B is an example of a better design. The signal and return current paths coincide, negating the effectiveness of the loop antenna. Note that this design also has cutouts around the chips, but they are separated from the return current path.

The theory of signal reflection and matching is close to the theory of antennas.

For example, a printed circuit board may have the following parameters:

4 layers; signal and ground polygon layers are adjacent
interlayer spacing - 0.2 mm
conductor width - 0.75 mm
conductor length - 7.5 mm

The typical ER dielectric constant for FR-4 is 4.5.

It can be seen that the amplitude of the output signal doubles at frequencies close to the upper limit of the frequency range of the op-amp. This, in turn, can lead to oscillation, especially at antenna operating frequencies (above 180 MHz).

The width of the PCB conductors cannot be reduced indefinitely. The maximum width is determined by both the technological process and the thickness of the foil. If two conductors pass close to each other, then a capacitive and inductive coupling is formed between them (Fig. 12).

For example, with d=0.4 mm and h=1.5 mm (fairly common values), the inductance of the hole is 1.1 nH.

Remember that if there are large resistances in the circuit, then special attention should be paid to cleaning the board. During the final operations of manufacturing a printed circuit board, any remaining flux and contaminants must be removed. Recently, when installing printed circuit boards, water-soluble fluxes are often used. Being less harmful, they are easily removed with water. But at the same time, washing the board with insufficiently clean water can lead to additional contamination that worsens the dielectric characteristics. Therefore, it is very important to clean the high-impedance circuit board with fresh distilled water.

Signal isolation

If you need to lay out a printed circuit board that has both analog and digital parts, then you need to have at least a small understanding of the electrical characteristics of the logic elements.

A typical output stage of a logic element contains two transistors connected in series with each other, as well as between the power and ground circuits (Fig. 14).

Table 2 shows the maximum operating frequencies for common capacitor types.

table 2

Actual specifications may vary between different manufacturers and even from batch to batch within the same manufacturer. It is important to understand that for a capacitor to operate effectively, the frequencies it suppresses must be in a lower range than its own resonance frequency. Otherwise, the nature of the reactance will be inductive, and the capacitor will no longer work effectively.

Do not be mistaken that one 0.1 µF capacitor will suppress all frequencies. Small capacitors (10 nF or less) can operate more efficiently at higher frequencies.

IC power decoupling

A decoupling capacitor must be connected to each chip package, regardless of whether there are 1, 2, or 4 op-amps inside the package. If the op amp is dual-supplied, then it goes without saying that decoupling capacitors should be located at each power pin. The capacitance value must be carefully selected depending on the type of noise and interference present in the circuit.

In particularly difficult cases, it may be necessary to add an inductance connected in series with the power output. The inductance should be located before, not after, the capacitors.

Isolation of input and output signals

In a situation where the frequency range of the induced noise is significantly different from the frequency range of the circuit, the solution is simple and obvious - placing a passive RC filter to suppress high-frequency interference. However, when using a passive filter, you must be careful: its characteristics (due to the non-ideal frequency characteristics of passive components) lose their properties at frequencies 100...1000 times higher than the cutoff frequency (f 3db). When using series-connected filters tuned to different frequency ranges, the higher frequency filter should be closest to the source of interference. Ferrite ring inductors can also be used to suppress noise; they retain the inductive nature of the resistance up to a certain frequency, and above their resistance becomes active.

The interference on an analog circuit can be so large that it is only possible to get rid of it (or at least reduce it) by using screens. To operate effectively, they must be carefully designed so that the frequencies that cause the most problems cannot enter the circuit. This means that the screen should not have holes or cutouts larger than 1/20 of the wavelength of the radiation being screened. It is a good idea to allocate sufficient space for the proposed shield from the very beginning of the PCB design. When using a shield, you can optionally use ferrite rings (or beads) for all connections to the circuit.

Op amp housings

One, two, or four operational amplifiers are usually placed in one package (Fig. 16).

In a single op amp, the output is located on the opposite side of the inputs. This can make it difficult to operate the amplifier at high frequencies due to the long feedback lines. One way to overcome this is to place the amplifier and feedback components on different sides of the PCB. This, however, results in at least two additional holes and cuts in the ground polygon. Sometimes it is worth using a dual op amp to solve this problem, even if the second amplifier is not used (and its pins must be connected properly). Figure 17 illustrates the reduction in the length of the feedback circuit conductors for an inverting connection.

Dual and quad op-amps, in addition to the above, allow for more dense installation. The individual amplifiers appear to be mirror-image relative to each other (Fig. 18).

Figures 17 and 18 do not show all connections required for normal operation, such as the mid-level driver when unipolar power supply. Figure 19 shows a diagram of such a shaper when using a quad amplifier.

The diagram shows all the necessary connections to implement three independent inverting stages. It is necessary to pay attention to the fact that the conductors of the half-supply voltage driver are located directly under the integrated circuit housing, which makes it possible to reduce their length. This example illustrates not what should be, but what should be done. The average level voltage, for example, could be the same for all four amplifiers. Passive components can be sized accordingly. For example, frame size 0402 planar components match the pin spacing of a standard SO package. This allows conductor lengths to be kept very short for high frequency applications.

Unused sections

When using dual and quad op-amps in a circuit, some sections may remain unused and must be connected correctly in this case. Incorrect connections can lead to increased power consumption, more heat, and more noise from the op amps used in the same package. The pins of unused operational amplifiers can be connected as shown in Fig. 20a. Connecting pins with additional components (Fig. 20b) will make it easy to use this op-amp during setup.

Conclusion

Remember the following basic points and keep them in mind at all times when designing and wiring analog circuits.

Are common:

think of a PCB as an electrical circuit component
have an awareness and understanding of sources of noise and interference
model and layout circuits

Printed circuit board:

use PCBs only made of quality material (for example, FR-4)
circuits made on multi-layer printed circuit boards are 20 dB less susceptible to external interference than circuits made on two-layer boards
use separated, non-overlapping polygons for different lands and feeds
Place the ground and power polygons on the inner layers of the PCB.

Components:

Be aware of the frequency limitations introduced by passive components and board traces
try to avoid vertical placement of passive components in high-speed circuits
For high frequency circuits, use surface mount components
conductors should be shorter, the better
if a larger length of conductor is required, then reduce its width
Unused pins of active components must be connected correctly

Wiring:

Place the analog circuit near the power connector
never route wires carrying logic signals through the analog area of the board, and vice versa
make the conductors suitable for the inverting input of the op-amp short
make sure that the conductors of the inverting and non-inverting inputs of the op-amp are not parallel to each other over a long distance
try to avoid using extra vias, because... their own inductance can cause additional problems
do not route conductors at right angles and smooth the corners if possible

Interchange:

use the correct types of capacitors to suppress noise in power circuits
To suppress low-frequency interference and noise, use tantalum capacitors at the power input connector
To suppress high-frequency interference and noise, use ceramic capacitors at the power input connector
use ceramic capacitors at each power pin of the microcircuit; if necessary, use multiple capacitors for different frequency ranges
if excitation occurs in the circuit, then it is necessary to use capacitors with a lower capacitance value, and not a larger one
in difficult cases, use series-connected resistors of low resistance or inductance in power circuits
Analog power decoupling capacitors should only be connected to analog ground, not digital ground

Bruce Carter
Op Amps For Everyone, chapter 17
Circuit Board Layout Techniques
Design Reference, Texas Instruments, 2002