APPLICATION NOTE
AN1681
Rev 0.00
Oct 25, 2011

Grounding Techniques
Ground is taken for granted. We stand on it, we dig into it, wemake mud pies out of it. The ground isn't supposed to move.
We don't have to think about it; it just is. When it comes togrounding a circuit, we assume that our connections are as
solid as the turf below our scuffed shoes. Many times, this is areasonable assumption-but not always. How do we know when
there is a problem with a circuit's ground? What practices will
ensure we construct a good ground?

TABLE 1. COMPARISON OF VARIOUS TYPES OF ELECTRIAL GROUNDSGROUND
CONNECTED TOTERMINOLOGY DEFINITION EARTH GROUND
Floating Ground

Local
reference
potential

NO

Medical systems toprotect patient

Virtual Ground

Node heldat or neargroundpotential by
feedbackNO

Negative input
terminal onInverting
Op Amp

AC Ground

Node held
at aconstant DC
potentialNO

VSUPPLY, Bias
voltage, Bandgap
voltage

Reference/Earth Ground

Systemground

YES

DIRECT CONNECTION
to earth groundNo longer to be taken for granted, we define ground in ideal
and real situations. Ground configurations and printed circuitboard (PCB) examples will be presented.

Ground Terminology
Many nodes are called “ground”. There are floating grounds,
virtual grounds, AC grounds and earth grounds. For clarity, let'slook at the difference.
• Floating grounds are reference points within an isolatedsystem. They are a reference point and only equal to 0 voltsthrough luck.
• Virtual grounds exist in a negative feedback circuit at theinverting terminal of an op amp. When the noninverting
input is held at zero volts, the feedback (in a stable circuit)will cause the inverting terminal to match. The value is only
held by feedback and is not a stable return for other circuitcurrents.
• AC grounds are nodes with low impedance DC values. That
DC voltage is stable with small circuit perturbations. Since
the node has a DC value, it is not useful as a proper ground.
However, since it is stable, it is useful as a reference point.
AC (or small-signal) analysis considers these points to beunchanging, thus like a ground.
• Earth ground is exactly what the words suggest. Every househas a copper pole sunk into the ground to deplete surplus
currents. Without the presence of a buried battery, the top
layer of turf is fairly homogeneous. A house down the street
from yours might have a difference in ground voltage of ahundred microvolts. This is the type of ground we are goingto discuss.

EXAMPLE

What is Ground?
I asked a handful of fellow engineers to define “ground”. A
chorus of answers responded “Zero volts” or “Zero reference”.
A more extensive definition can be found on Answers.com1], “a large conducting body (as the earth) used as a common
return for an electric current and as an arbitrary zero ofpotential.” This definition is particularly helpful because it
refers to the role of ground in terms of voltage AND current.
To explore this dual role, let's look at an extremely simple
circuit, Figure 1a. This circuit could represent a plethora ofdifferent systems-a flashlight, a coffee maker, or even an iPod.
Figure 1b represents those circuits just as well. Of course, we
don't typically think of ungrounding a circuit to turn it off, wethink of disconnecting the power. The truth is, they are equallyeffective. The load and the switch are in series, so the order isirrelevant. An open switch stops the flow of current; zerocurrent through the load means zero power in the load. (Inpractice, the circuit in Figure 1a is more common thanFigure 1b for a practical reason-it is more likely that thesystem could accidentally become connected to ground (aloose wire to the chassis, for instance) causing unwanted
power in the load than to accidentally become connected tothe power supply.) The principle, however, is useful.
Those lines representing the connection between the source
and load could be of a wide variety of lengths. They could be a
few inches long, as in a household appliance, or they could bemiles long, as in the power grid. In small systems, theassumption of a lossless wire is reasonable. As system sizes
increase, the connections (wires, board traces, and interfaces)
can no longer be considered lossless. An extreme example isthe lines from the power company. Between the repeaters, thepower is transmitted with high voltage (hundreds of kilovolts)
and minimal current to negate losses caused by seriesresistance.

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V+Circuit

V+(a)Circuit(b)

FIGURE 1. SIMPLE CIRCUIT. SWITCH CAN STOP THE FLOW OF CURRENT TO THE CIRCUIT OR AWAYFLOATING GROUNDCIRCU ITCIR CUIT

MULTIPOINT GROUNDCIRC UITCIRCU ITSINGLE POINT GROUND/STAR GROUNDCIRCUITCIRCU IT

FIGURE 2. GROUNDING TOPOLOGIES: FLOATING GROUND, MUTLIPOINTGROUND, SINGLE-POINT GROUNDFIGURE 3. TEST CIRCUIT USED IN LAYOUTS FOR FIGURES 4-6

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PCB Examples
A dual op amp is chosen as the layout example. For clarity, asingle layer of copper is used for all routing (this is not the best
choice for speed, isolation, coupling, etc.). Figure 3 shows thecomplete schematic used for each of the PCB examples.
Capacitors C1 and C2 are included to bypass the supply for thedual op amp package. Resistors R1, R4, R5, and R8 arematching resistors for terminating cables, assuming we areoperating in a 50Ω environment. Resistors R2, R3, R6, and R7
set the amplifiers in a non-inverting configuration with a gainof 2. And lastly, because a single layer is used and a bit ofwire-jumping is necessary, a few zero ohm resistors (R9-R12)
have been included as well.

Figure 4 shows the first attempt to layout our circuit. The groundconnection travels right down the center, providing a spine to ourbilaterally symmetric creation. Sadly, though, both the positive
and negative supplies travel in a large loop around both sides ofthe device. It's great that the layout is symmetric; it's not so greatthat the power lines loop around the board. These long, thin
traces will act as antennas, picking up as much of the local RF
noise as they can. In addition, the traces may have measurable
IR drop in them-another downside.

FIGURE 4. WIRED GROUND LAYOUT

FIGURE 5. OUTER GROUND PLANE

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FIGURE 6. SPLIT/STAR GROUND PLANE

The next attempt at layout is shown in Figure 5. In a single-layer
board, this is a close approximation to having a “ground plane”.
This time the ground loops around the outside of the board. Aconnection to ground is made every time it is needed. This could
be considered a multi-point ground. Instead of a thin trace,
though, the ground connection is wide (considerably lowerimpedance). However, any noise that couples into either power
supply will still be radiated from the area of the loop created by
the power trace and the ground connection.

References[1] Answers.com. October 20, 2011.
<http://www.answers.com/topic/ground>.

Figure 6 shows our best layout. Ground once again forms a planearound the device, but there are 5 slices keeping it from being
continuous. Notice that each of the ground planes connects tothe others, but only at one place. This is an example of asingle-point or star ground. Currents cannot travel haphazardly
throughout the ground plane; they are steered through specificpaths we create and control. This type of set-up will minimize anyantenna characteristics, both emitting and receiving.
These examples offer a bit of insight into the types of grounding
schemes and how they appear on a printed circuit board. The
best advice is to “follow the currents”:• The longer a current has to travel, the more trouble it cancause. It is the best policy to keep wide, low-impedance groundconnections as close to the devices as possible.
• Multiple currents can interact. If multiple loops overlap, breakup the ground connection to minimize coupling.
• Don't forget the return currents! Every current leaving a node
must return via some path. If the forward and return paths arevery close (enclosed in a small area) the external fields cancel.
If not, you have an antenna.

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