Don't be fooled by superficial phenomena and beware of design errors in analog or digital circuits

　　Humans are unusual "animals". Sometimes, in some aspects, half-knowledge, arrogance and blind arrogance are more dangerous than ignorance, such as circuit design, which may cause the circuit to not work properly. When seeing experienced engineers hesitate, some people feel that they might as well work with inexperienced people and don't understand why these experienced engineers are in a dilemma. Here are three examples, in which simple analysis can give designers some inspiration to avoid similar problems in future designs.

　　Sometimes, designers often misunderstand how a device works, which leads to strange assumptions and incorrect use of the device. Unfortunately, today's engineering schools focus almost exclusively on digital technology and almost completely ignore analog design. This leaves digital engineers with no analog design experience to gain analog knowledge through trial and error. Some of the results would make Rube Goldberg proud. (Who is Rube Goldberg? He is a Pulitzer Prize-winning cartoonist. Rube Goldberg became famous in the early 20th century for making simple functions complicated through some absurd inventions)

　　Let's consider some situations that are very scary in the eyes of analog engineers. Common misconceptions of digital design are: not realizing how important clean power and ground are to circuit design, and not considering DC impedance matching when connecting circuits. Designs that ignore the laws of physics will eventually lead to system failure.

　　Don't be fooled by appearances

　　Data sheets often state: "The power supply decoupling capacitors should be placed as close as possible to the power pins of the integrated circuit." As shown in Figure 1, the printed circuit board (PCB) does!

　　Figure 1: A printed circuit board layout (PCB), integrated circuits, and capacitors. (Click to enlarge)

　　This board is used for mixed video signals. There are other devices around the integrated circuit shown in Figure 1. These peripheral devices are very critical. This is a four-layer board. The signal path is on the outer two layers. Analog engineers usually place the power supply and ground on the middle two layers respectively. The devices include high-frequency analog/digital converters (ADCs), digital/analog converters (DACs), and signal processing circuits. The device density is moderate, there is no ball grid array (BGA) package, and no more layers or complex layout are required.

　　When testing this design, we found that the video output was very noisy. Moreover, most of the noise came from one integrated circuit. Figure 1 shows the top layer of the board, with very loud noise being tested on the power pins. When running a thin wire through the ground via of the decoupling capacitor to the other side of the board, one lead (not on the inner ground layer) disappeared into another via, which caused some problems.

　　Humans are unusual "animals". Sometimes, in some aspects, half-knowledge, arrogance and blind arrogance are more dangerous than ignorance, such as circuit design, which may cause the circuit to not work properly. When seeing experienced engineers hesitate, some people feel that they might as well work with inexperienced people and don't understand why these experienced engineers are in a dilemma. Here are three examples, in which simple analysis can give designers some inspiration to avoid similar problems in future designs.

　　Sometimes, designers often misunderstand how a device works, which leads to strange assumptions and incorrect use of the device. Unfortunately, today's engineering schools focus almost exclusively on digital technology and almost completely ignore analog design. This leaves digital engineers with no analog design experience to gain analog knowledge through trial and error. Some of the results would make Rube Goldberg proud. (Who is Rube Goldberg? He is a Pulitzer Prize-winning cartoonist. Rube Goldberg became famous in the early 20th century for making simple functions complicated through some absurd inventions)

　　Let's consider some situations that are very scary in the eyes of analog engineers. Common misconceptions of digital design are: not realizing how important clean power and ground are to circuit design, and not considering DC impedance matching when connecting circuits. Designs that ignore the laws of physics will eventually lead to system failure.

　　Don't be fooled by appearances

　　Data sheets often state: "The power supply decoupling capacitors should be placed as close as possible to the power pins of the integrated circuit." As shown in Figure 1, the printed circuit board (PCB) does!

　　Figure 1: A printed circuit board layout (PCB), integrated circuits, and capacitors. (Click to enlarge)

　　This board is used for mixed video signals. There are other devices around the integrated circuit shown in Figure 1. These peripheral devices are very critical. This is a four-layer board. The signal path is on the outer two layers. Analog engineers usually place the power supply and ground on the middle two layers respectively. The devices include high-frequency analog/digital converters (ADCs), digital/analog converters (DACs), and signal processing circuits. The device density is moderate, there is no ball grid array (BGA) package, and no more layers or complex layout are required.

　　When testing this design, we found that the video output was very noisy. Moreover, most of the noise came from one integrated circuit. Figure 1 shows the top layer of the board, with very loud noise being tested on the power pins. When running a thin wire through the ground via of the decoupling capacitor to the other side of the board, one lead (not on the inner ground layer) disappeared into another via, which caused some problems.

　　Looking at the circuit layout, highlighting the nodes of interest, we can see all the connections, as shown in Figure 2.

　　Figure 2. Circuit layout obtained using PCB design software. (Click to enlarge)

  These routings appear to be done by digital circuit automatic routing tools, and the PCB designer may not have experience in analog circuit design. There are no internal ground layers and power layers (refer to AN4345 for grounding techniques and reasonable layout).

　　To those without design experience, this circuit is completely correct, but because all grounds are mixed together, this connection is fine for DC, but at a certain operating frequency, there are large parasitic components in its equivalent circuit, as shown in Figure 3.

　　Figure 3. Schematic diagram of "ground bounce noise" in a circuit. (Click to enlarge)

　　Each path and via in Figure 2 has parasitic resistance and inductance. In Figure 3, these distributed parasitic units are equivalent to low-frequency inductors in series with the ground. In the figure, the inductor can be regarded as a mechanical spiral inductor; for the convenience of explanation, it is assumed that the integrated circuit is an operational amplifier , but it can be any circuit.

　　When the current in other circuits changes, the noise from the digital circuits and other circuits on the left and right sides of the "ground bounce noise" symbol will cause the voltage to fluctuate up and down. This directly interferes with the analog signal at many points:

　　1) Noise is coupled to the op amp input through R1.

　　2) Noise is coupled to the ground terminal of the op amp. Some people may want to use the "power supply rejection ratio" to eliminate noise, but don't forget that the ground is its reference potential, which means that the noise will be directly coupled to the output signal.

　　3) Noise is coupled to the op amp input through R2.

　　4) Noise is coupled to the op amp input through the decoupling capacitor and the R1 resistor.

　　Note: The capacitor is a bidirectional device. The function of the decoupling capacitor is to average the high-frequency signals on both sides of the capacitor. If there is noise on the power bus and the ground is very clean, the low-impedance loop to the power supply formed by the decoupling capacitor can effectively reduce the noise. However, if the ground is high-impedance and there is a lot of noise, the decoupling capacitor will add noise to the power supply.

　　As shown above, because the coupled noise signal has a phase difference, the noise is coupled to various nodes around the op amp, making the output very noisy. As shown in the jitter in the figure, all the noise will be superimposed on the output.

　　The output is also affected by the nonlinear distortion of the op amp, whereby the noise component generates sum and difference harmonic components, filling the entire spectrum with noise.

　　The above briefly explains the importance of good power supply and ground layout, which is especially worth noting for engineers who have no analog design experience.

　　Improper layout of RF circuit

　　As another example, let's look at a problem that occurred during the design of an RF transceiver evaluation board. The designer took the circuit and entered it into a PCB auto-routing tool designed for digital logic. As a result, the board did not work at RF, even though the board met the Rube Goldberg requirements.

　　The key paths of the board are scattered and connected through vias (inductors), and the power supply is not properly decoupled. The antenna on the board is oddly shaped, and it is difficult to design a straight antenna. When the designer was asked about the software used to design this antenna, the response was not antenna design software, but the designer said "that's the place for us to put the antenna."

　　Although this designer is a good microprocessor engineer, he did not know that the antenna size is determined by the signal wavelength, nor did he realize that the ground plane is the other half of the antenna. Only under the guidance of an experienced RF engineer can the design be guaranteed.

　　Resonance Principle

　　Musical instruments and radio frequency devices all use resonance to work. Figure 4 shows a set of organ pipes, each pipe is tuned to a fixed note. When we transmit radio from one place to another, resonance will help us select a signal and reject all other signals.

　　Figure 4: A pipe organ in a church in the United States. (Click to enlarge)

　　The RF antenna size is adjusted so that it resonates at the specified frequency, but there is still a problem. When working in an automotive environment, designers place the antenna in the engine compartment in the hope of achieving long-range communication. This is a car accessory that needs to receive and transmit under the metal cover of the car engine.

　　The designers thought that the car's engine compartment would form a resonant cavity at a specified frequency and amplify the signal. Unfortunately, the resonant cavity requires precise design, and different cars have different engine compartment dimensions, making it difficult to achieve resonance. In addition, the designers did not want to spend a lot of money on components to meet the high temperature operating requirements of the car.

　　Designers don't understand that engine compartment temperatures are very high and still expect consumer products to get away with working at temperatures up to 70°C.

　　Mixed-signal devices, such as the MAX541 16-bit D/A converter, usually have one pin for analog ground and another for digital ground. In the MAX541 data sheet, pages 9 and 10 explain how to connect them together and use a star connection. This terminology for describing ground can be misleading, and instead of using analog and digital, it may be easier to use "clean and noisy."

　　As stated in application note 4345, "Good grounding, digital is also analog", because of the threshold effect, digital circuits can ignore some noise, while analog circuits cannot. In data converters, the connection method of digital and analog grounds needs to be very careful, especially when the system consists of many ADCs and DACs. Experience and skills in connecting a ground plane into a star are necessary.

　　At the same time, in every data converter, the analog and digital planes need to be cross-connected in order to return the main current to the power supply, with almost no current consumed at the crossover point. Professional engineers use resistors, beads, or inductors as crossover points to conduct current as a function of frequency. Experience allows for layout minimization, but the only sure way is to repeatedly optimize the circuit layout based on experience to reduce noise.

　　Unfortunately, all devices can be misused or abused, and experience is the best teacher. Students may have to memorize some knowledge when they are in school. Fortunately, by sharing these experiences, we can save other engineers the pain of finding a breakthrough. The time saved can be used to improve the design instead of scratching our heads and worrying about not seeing results quickly.

　　There is no one device that does everything for every engineer, and no matter how Rube Goldberg experiments, one device cannot integrate all the devices for every application. It is a good thing that people have found joy in many of Rube's cartoons.