Another way that you can help yourself to reduce the possibility of ground bounce in your PCB design is to use tools that have been created for this task. You need a PCB design system with signal integrity tools for analysis and high-speed design features for precisely controlling your component placement and routing widths. With Allegro PCB Designer, you have all of this functionality ready to go to work for you.
Types of ground bounce in pcb
One common technique is to use a ground plane, which is a large piece of copper on a PCB. PCB manufacturers will cover all the areas that don’t have a component or trace on them. with the copper ground plane.
In a two-layer board, the standard PCB ground plane rules state. that the ground plane should placed on the board’s bottom layer. while the components and signal traces are on the top layer.
It is best to avoid creating a ring of conductive material formed by the ground plane. as this makes the ground plane more susceptible to electromagnetic interference (EMI). This conductive ring acts as an inductor, and an external magnetic field. may cause an electric current called a ground loop.
You may end up with a conductive ring if placing the ground plane over the whole bottom layer. and then removing the parts that have electronic components. To avoid this issue, make traces as short as possible, and after mapping them. put your ground plane so that it runs underneath them. You may need to adjust the layout of traces. and components to avoid having to create conductive rings.
The ground plane is also often on both sides of the board. In some cases, the plane on the component side kept at the supply voltage. and the plane on the other side of the board grounded. The ground plane connected to the ground pins of the components. and connectors to keep the ground voltage at the same level through the whole PCB.
On a two-layer PCB, you may also use more than one ground plane. Each plane should connect to the power supply. to keep the planes separated and prevent ground loops from occurring.
Ground Plane Vias
If there ground planes on both sides of the PCB. they will connected through vias at many different places on the board. These vias are holes that go through the board and connect the two sides to each other. They allow you to access the ground plane from anywhere you can fit in a via.
Using vias can help you to avoid ground loops. They connect the components to the ground points. which connect through low impedance to all the circuit’s other ground points. They also help to keep the length of return loops short.
Pieces of copper, such as ground planes. may resonate at one-quarter of the wavelength of the frequency of the current. which is flowing into it. Putting stitching vias around the ground plane at specific intervals. can help to control this. A practical rule of thumb is to place ground vias at one-eighth of a wavelength or less. This works because a stub on a trace only starts to become an issue at one-eighth of a wavelength.
To create vias, you drill small holes through the board. and pass thin copper wires. through them before soldering them on each side to form the necessary connections.
All the connectors in a PCB should connected to the ground. In connectors, all signal conducts must run in parallel. Because of this, you must separate connectors using ground pins.
Each board will likely need more than one connector pin leading to the ground. Having one pin may cause issues with impedance mismatch, which can cause oscillations. If the impedance of two connected conductors does not match. the current flowing between them may bounce back and forth. These oscillations can alter the performance of the system and cause it not to work as intended. The contact resistance of each pin of a connector is low but may rise over time. For this reason, it is ideal to use many ground pins. Approximately 30 to 40 percent of the pins in a PCB connector should ground pins.
Connectors come in various pitches and can have different numbers of rows of pins. The pins of a connector may also be parallel to the PCB surface or at a right angle to it.
PCBs contain one or more integrated circuit chips, which need power to operate. These chips have supply pins to connect them to an external power source. They also have ground pins, which connect them to the ground plane of the PCB. Between the supply and ground pins. there is a decoupling capacitor, which serves to smooth out oscillations. in the voltage supplied to the chip. The opposite end of the decoupling capacitor connects to the ground plane.
One of the main reasons for the use of decoupling capacitors. related to functionality. A decoupling capacitor can act as a charge storage device. When the integrated circuit (IC) requires more current. the decoupling capacitor can provide it through a low inductance path. Because of this, it is best to place decoupling capacitors close to the IC power pins.
Another primary purpose is to reduce the noise put into the power and ground plane pairs and reduce EMI. Two main issues can cause this noise. One is a decoupling capacitor that does not provide adequate current resulting. in the lowering of the voltage at the IC power pin . The other is an intentional current sent between the power. and ground planes using a via with a fast-switching signal.
You should choose the placement and number of decoupling capacitors for a design. based on their two functionalities. Often, distributing the capacitors across the entire board is the best approach. try placing some near the IC ground. and power pins to use. Using the highest value of capacitance is also recommended. and it is best to keep all the capacitors at the same value. You may also want to use a combination of high equal series resistance (ESR) and normal capacitors.
Ground bounce history
As system designers begin to use high performance logic families. to increase system performance. they may run into new problems which before did not raise concern. when lower performance devices utilized. These problems can generally avoided by following a few simple rules. This application note discusses. the subject of ground bounce about high performance CMOS logic families. and offers a set of simple guidelines that will drop system problems due to this phenomenon.
Ground bounce has been a concern to some system designers for many years. Its effects can found in most bipolar and CMOS logic families. But, ground bounce has recently become a major issue. Although new advanced CMOS logic families have edge rates. comparable to advanced bipolar logic devices. CMOS outputs swing almost from rail to rail while bipolar outputs. swing from ground to approximately 3.0V. These edge rates, coupled with the greater voltage swings. found in today’s advanced CMOS logic devices. tend to generate more ground bounce noise than their bipolar counterparts.
In 1982,Fairchild Semiconductor. began to develop FACT (Fairchild Advanced CMOS Technology) logic incorporating. more than three years of experience gained. with FAST (Fairchild Advanced Schottky TTL) logic into the groundwork. As a result. Fairchild was able to understand the important trade-offs. associated with high performance in a logic family. In the bipolar world, these trade-offs were between speed and power. in the CMOS world, the tradeoffs are between speed and ease of use. Utilizing experience gained from FAST products. the FACT family objectives defined to provide the greatest solution. allowing greater system performance while minimizing system design problems. Using FACT devices does need more attention toward circuit design. and board layout than older. slower technologies. The resulting advantages—low power and high performance outweigh these considerations.