A Fundamental Rule of Grounding (EE Tip #124)
If your circuit looks like it should work but doesn't, the problem may be caused by electrical noise. In this case, you should make sure that your circuit is properly grounded, isolated, and shielded. There are generally three types of grounds: earth groundsignal groundand chassis ground. The earth ground denotes a physical connection to the earth, which is conventionally defined to be 0 volts. On electrical appliances, this is the third pin on the power plug.
However, many circuit diagrams will use this symbol to designate 0V reference even if there is no physical connection with the ground. The second type, the signal groundis used as a reference for data transmission lines. The signal ground is generally not connected to the earth to isolate it from electrical noise and interference from electric currents and fields in the earth. Thus, there is often a significant voltage difference between the signal ground and the earth ground.
The third kind is known as the chassis groundand is usually connected to the conducting shell or chassis of that particular piece of equipment.
For example, the negative termial of a car battery is wired to the metallic shell of the car. This provides a reference voltage that may or may not be connected to the earth, although for most high-power appliances, it usually is.
Not connecting the chassis ground to the earth ground can result in a voltage difference between the chassis ground and the earth ground, which leaves the risk of electrical current including static electricity finding a path between the chassis and earth through something This is the reason why fuel trucks must be grounded while dispensing fuel—the voltage difference can result in a spark and explosion.
Because of the resistance of our conductors, it is possible for our ground wire to have difference voltages at different points along the wire when current is flowing through it.
To keep wire resistance and voltage differences at a minimum, there are several things we can do:. Power circuitry often causes electrical noise on the power supply. The power supply may also see voltage spikes from commutator brushes or other circuitry. A changing magnetic field can induce a current that can interfere with your signal. The voltage induced by the magnetic field depends on the rate of change of the magnetic field and the area your circuit loop encloses:.
Shielding, Grounding, Noise Suppression, etc.Learn about the basics of grounds, grounding, and ground symbols. Not all grounds are created equal. In this article, we'll discuss earth ground, common ground, analog ground, and digital ground. In electronics and electrical engineering, it is by convention we define a point in a circuit as a reference point. This reference point is known as ground or GND and carries a voltage of 0V. Voltage measurements are relative measurements.
That is, a voltage measurement must be compared to another point in the circuit. If it is not, the measurement is meaningless. The ground reference point is often, but not always—more on this later—represented by a standard ground symbol. See Figure 1. Typically, this reference point is the base for all other voltage measurements within the circuit.
However, not all voltage measurements are taken from this reference point. For instance, if you were to measure the voltage across the upper resistor in a resistive voltage divider, your reference point would not be ground. See Figure 2. A true earth ground, as defined by the National Electrical Code NECconsists of a conductive pipe, or rod, physically driven into the earth to a minimum depth of 8 feet. The earth provides an electrically neutral body, and due to the earth's virtually infinite state of neutrality, it is immune to electrical wavering.
It should be noted, however, that "earth being immune to electrical wavering" is, in fact, a generalization. In reality, earth ground is quite the complex subject given all variables and materials that make up the earth. And, earth's electrical potential does indeed experience some isolated areas of varying due to events such as lightning hits, as an example. Power poles, those that are strung throughout neighborhoods, are also connected to ground.
Figure 3 shows an earth grounding wire attached to a power pole. This outlet connection to earth ground provides a means for, as an example, test equipment to be connected to earth ground—the ground green wire from a power cord is connected to the equipment's internal frame or chassis. And when connecting various pieces of test equipment to earth ground, they are all connected to a common grounding point, and, therefore, have a common reference. You can verify this point by measuring the resistance between the ground terminals of any two pieces of test equipment.
This common reference is brought out, to the user, as a ground lead terminal. Side note: your desktop computer's chassis is also connected to earth ground. For instance, the earth ground symbol is also used as a common ground symbol or a 0V reference. This is a bit misleading because a 0V reference is not actually connected to earth ground.
Digital circuits generate spikes of current when the digital signals change states. Although there are multiple techniques for proper grounding, when it comes to mixed-signal grounding it's of most importance—regardless of which grounding technique is adopted—to separate the "more-noisy" digital return currents from the "less-noisy" analog return currents. Such ground currents—think of them as changing currents—when applied to ground return paths, create voltage variations recall Ohm's Law called noise.
You may have heard the term "a noisy ground. Grounding has always been a major obstacle for design, system, and test engineers. One possible grounding technique, which can be helpful in some, but not all, situations, uses what is called a "star" ground.
This philosophy builds on the theory that all voltages in a circuit are referred to a single ground point. The method of using single grounding points or star grounds looks great on paper. In practice, however, it can be very difficult to implement depending on the complexity of one's design.
An alternative approach is to use a grounded bus bar. A three-terminal DC power supply, such as the one in Figure 8, may be a little confusing to beginners.In analog design, the relationship of a signal to ground is of fundamental concern and can create issues in digital designs, too. All three indicate connecting to a point of theoretically zero voltagebut within a different context: chassis ground for a device, signal ground for very low voltage signals within a device, and earth ground for a power system.
Figure 1: There are three different electrical symbols for ground, indicating context within a schematic. Source: Wikipedia. But ground as zero voltage is theoretical; only a conductor with zero impedance will have zero voltage. In reality, a ground plane or rail will usually have varying voltages at negligible levels. This is most likely if the circuit or device happens to operate with high amperage draws, or in cases where the ground plane, conductor, or rail has a high impedance i.
Current flow I through any material with resistance R will have a voltage V other than zero. A chassis ground is a ground-collection point that connects to the metal enclosure of an electrical device. A chassis ground may be used for shielding and grounding to prevent electrical shock. For example, with multilayer printed circuit boards, one or more of the conducting layers may be used as a chassis ground. A chassis ground is typically only made at one point.
This prevents a return current path through an available but undesirable means and prevents current circulating through the chassis.
Ground loops, which cause induced EMF noiseare especially problematic for noise-sensitive applications such as instrumentation and audio. Ground loops often occur when connecting multiple electronic devices together because no two grounds are ever exactly at the same potential, which induces flow.
Although the impedance in a loop ground is only a very small fraction of an ohm, this is enough to cause issues such as noise and interference.
Given a path, electricity will flow. Electricity and magnetism are interrelated, a good thing since motors depend on that relationship to work, but not good when current flow is unwanted. A signal ground is a reference point from which a signal is measured. There may be more than one reference ground in a given circuit. A clean signal ground, or a ground connection without injected noise, is essential to electrical equipment that must accurately detect very small voltage levels or differences, such as those in medical equipment.
When there are multiple paths for electricity to flow to ground, the duplicate ground paths pick up interference currents and transform the currents into voltage fluctuations. The ground reference in the system is then no longer a stable potential and noise becomes part of the signal.
Printed circuit boards PCBs can inherit grounding problems from automatic layout programs. Signal ground, or the 0V signal reference voltage, should be on the PCB and not grounded off the PCB where it can pick up external noise. Signal voltages are much smaller than the voltages entering the system on point-of-entry POE power modules, for instance. Common sense says that signal ground is isolated from the chassis or power ground.
The signal ground may also be split between digital and analog sections of a system. Signals can suffer from ground-injected interference when input signal grounding is external to the PCB where the signal lives. Ground-injected interference is possible to ignore if the signal is much larger than the injected noise, however.
Grounding for signal integrity on PCBs is a detailed subject that cannot be covered in this venue, however. A common context for earth ground is in household electrical systems, where current leaves the main circuit panel through a hot wire and flows to receptacles and lights as electricity is consumed or otherwise diverted through a viable pathand a return path is provided back to the panel through a neutral wire.
Grounding adds a third wire a ground wire to provide a path for current that is unable to complete the circuit. It is especially important to have earth ground if high voltages are involved. If electrical equipment has a failed component that causes the live voltage to come in contact with a conducting chassis, for example, the equipment may continue to operate due to the internal isolation of systems, but the first person that touches the chassis becomes a path to ground and will suffer serious injury or even death.
Shielding, Grounding, Noise Suppression, etc.
Even if a fuse is in the path from the live voltage source, it still takes micro or milliseconds for the fuse to blow and open the circuit, preventing flow. Therefore, earth ground and fault interrupters are most often in evidence wherever high voltages are at play. Consequences vary when dealing with very high voltages versus small signals, ground loops can play out in any situation where grounding has an established path, and books have been written on the subject.
Oxford: Newnes, Why capcitors connected Ground to ground? Just a wild guess but maybe there are two isolated grounds on the PCB. Also, this forum is not necessarily for circuit-technical questions. It's pretty much for CircuitStudio-related questions.
Your question came through on main Forum Stream too so there is a pretty good chance that someone smarter than me will respond. What makes you think that both sides of the capacitor are connected to ground?
Try putting an ohm meter across the caps and see what it says. Most of the time these capacitors are not marked well. If you really needed to know the value I would remove one and test it with an LCR meter. So judging by their location right next to connectors and that there is one each side of the board and they connect to an exposed copper area running around the perimeter of the board, presumably for an electrical connection to a chassis that fastens around the board, I would say they are likely AC coupling between the chassis ground and the main ground of the circuit so they are connected as far as AC signals are concerned but they remain isolated as far as DC voltages are concerned.
If the ground traces are too thin, maybe the designer wanted to reduce the AC impedance without creating a DC ground loop. If there is no value, it usually means the designer wasn't sure what value would work best. The designer may have wanted the option to replace the cap with a short or simply leave it unpopulated to see what worked better. I could be wrong, but I doubt they are. Are you using a multimeter in continuity mode to 'ring out' the connection?
If so, check to see what resistance and below will ring. I've used meters that ring anything below ohms, which usually rings on power to ground connections.
Chassis capacitors to power and ground
It's just to get rid of any static charges, when the board is inserted into the chassis. This is because the edge is going to be the first bit that touches the chassis, and the capacitors have physically been placed with one end close to the chassis, and the other end connected to the PCB ground. As rachaelp says, its for the AC signal, that brief spike and then in theory the board ground and the chassis ground should be at a closer potential. In the duplicate thread which makes it awkward to provide a good response, because it is only by chance that I saw itit showed the portion of the schematic, with the ground connection on each side.
Show 7 replies.Quantum mechanics notwithstanding, our world is analog. And so despite our fascination with everything digital, we need interfaces to provide bridges for our analog reality to cross over to the digital paradigm and then back again.
One may ask: Is there a common denominator that binds these two worlds together regardless of their many conceptual differences? In my mind, it is grounding. The fundamental rule for grounding is depicted in Figure 1. Yet there are still systems that are sufficiently insensitive to ground potential differences.
They use the chassis for the signal and power returns. At one time, this was the way cars had been wired. The diodes and the capacitor between the planes limit potential differences due to ground bounce, etc.
Broken lines inside boxes 1 and 3 indicate ground referenced, non-symmetrical inputs and outputs. Figure 1a shows circuits sharing a common ground run. Notice that the output or the highest current drawing stage 1 must be the closest to the common point to minimize the voltage developed by that stage current over the grounding conductor.
Also notice that the input signal and its return must be tied to the input block 3. Internal signal returns grounds are shown by broken lines. Returning inputs or outputs anywhere else would superimpose the noise from stages 1, 2, and 3 on the input signal. Figure 1b shows the approach often used in RF equipment. There is no sharing of grounds; they are all individually tied to a single point. The grounds come together at the point G, where the chassis is also connected.
Where there are a few inches of wire tying the individual grounds together, it is a good idea to insert fast signal diodes and a capacitor as shown between the separate ground runs.
Any potential difference developed between the separate grounds due to finite impedance of wiring, as shown in Figure 1, will be attenuated and clamped by the three components.
The following are typically used: pF, 1 nF, 10 nF, 0. In safety-critical systems such as aircraft comprising two or more subsystems enclosed in metal cabinets, such as shown in Figure 2, only currents from lightning or other interference suppressed by the EMC blocks is allowed to be returned to the chassis.Forums New posts Search forums. Articles Top Articles Search resources. Members Current visitors. Log in Register. Search titles only.
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Ground rules: earth, chassis, and signal ground
Chassis capacitors to power and ground. Thread starter Rusttree Start date Jun 1, Rusttree Member. I'm looking at a circuit that has a capacitor between the metal chassis and power, as well as a capacitor between the metal chassis and ground. See the attached image. I'm having trouble figuring out the theory for why that's a beneficial circuit. I would have just put a large resistor between the chassis and ground and been done with it.
What exactly is happening with these two capacitors? It's so the chassis can exhibit RF shielding without being connected to the line. Shielding requires surprisingly high currents, but can still work with a DC offset. The caps provide low-impedance AC paths to ground, yet isolate the DC potentials. Ah, that makes sense. You don't want the AC noise to get into your ground or power plane, so you wouldn't ground the chassis directly.Remember Me?
Forum Analog Design Power Electronics Why connect power supply ground with chassis ground through a capacitor? Why connect power supply ground with chassis ground through a capacitor? I'm wondering about the capacitor in the schematic marked with red.
Why is there a need for it? It connects power supply ground dangerous side to the chassis ground safe side. And why is there sometimes also a resistor connected across it several mega ohms? I suppose the resistor discharges the capacitor after power off. Re: Why connect power supply ground with chassis ground through a capacitor? I think you should search for X and Y caps. It is called Y cap. Similar Threads Why -ve terminal of supply need to connect to ground???? Connect PCB Ground to chassis ground 3.
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