APPLICATION NOTE
Nonideality of Ground
Ground is supposed to be ideal. It should be a black hole forstray currents where the voltage is always zero. Unfortunately,
those stray currents travel through some non-superconducting
material, so small voltages arise. You may not notice smallchanges in ground potential, you may, instead, notice surplus
noise or instability or other unwanted attributes in your system.
We are going to discuss ground. Every circuit is unique andgrounding paths are different for every device on your board
and in your system. Therefore, we are starting with an intuitive
approach to try and give you a feel for the paths currents
choose to travel and how that affects the ideal assumption ofground being zero volts.
First, remember that circuits are only complete when current
has a complete path to travel (that is why a switch can beplaced anywhere along that path to interrupt flow andfunction). The power supply creates a potential difference, apush, for the current if a path exists for it to travel. Assuming asingle-supply system, it's natural for us to ensure that thepositive supply connection is as short and clean as possible.
We add bypass capacitors-sometimes multiple values andtypes of bypass capacitors-to filter the power supply voltage
where it enters every critical chip. Then, since there is a groundpin on our IC, we dutifully hook it to a grounded wire or groundplane. For many of us, that's the last we think about ground…
unless there's a problem.
One of the common methods for tracking down a problem is toprobe important nodes in the circuit with a digital multimeter.
If that doesn't illuminate the problem, try probing ground in avariety of places. You might be surprised with what you find.
The ground in a system is quite like the ground we find innature. There are holes from burrowing animals and there area variety of materials from sand, clay, soil and types of rock.
The consistency and structure of the materials affects how
things like water, animals and tree roots travel through theground. All three of these travelers chose the path of least
resistance. They naturally avoid, or travel around, obstacles intheir path.
In circuits, there are many factors that can affect flow in theground plane. For example, the purity of the metal composing
the plane and the consistency with which is it applied to theprinted circuit board. There are vias connecting to or throughthe ground plane. There are currents from other ICs making
their way through ground to the power supply.
Ever wonder why we place bypass capacitors as close to the IC
as possible? They provide a high-frequency path to short any
high frequencies that may exist or even be gathered on thepower supply line. (A copper wire is a fine dipole antenna. Thefrequencies it picks up are strongly dependent on the length ofthe trace and/or wire.) Earlier we referenced the practice ofprobing nodes with a DMM. When referring to the placementof bypass capacitors, it would be more revealing to probe withan oscilloscope. Not only will you find frequencies thatcirculate in your system, you will likely find signals from local
radio stations or one of the many wireless transmissions
surrounding our everyday lives. These are the signals that

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couple in through longer traces and wreak havoc on theoperations of our systems. Not only do they exist on power
supply lines, they circulate in the ground, too.
An example of bypass capacitor placement is shown inFigure 1. The SOT-23 package pads in the center will hold an
op amp. There is a bypass capacitor connected on each of thedual supply pins. Notice that the opposite sides of thecapacitors are connected to ground. The ground connection is
not a long wire eventually connecting to the ground plane.
Instead, a large rectangle of top layer metal allows highfrequency currents to easily coast through two nearby vias tothe ground plane.
Once the unwanted signals reach the ground plane, they don't
magically disappear. They continue to travel through theground plane. It is on this journey that they can couple into
other signals in the system and affect more than just the IC
with the long trace. To see how the current travels, refer toFigure 2. You'll notice that the op amp layout in Figure 1 is
duplicated twice on the inputs of the circuit in Figure 2. We
now follow the current through the ground connection wehighlighted as a common connection between the bypass
capacitors in Figure 1. The majority of the current takes adirect path to the ground pins of the input connector. However,notice how the density of the current spreads. It travels in a
wider arc that you might imagine. Now superimpose the pathsof signals through every other ground via back to the connector
grounds. (If we tried to draw them all on another figure, itwould look more like fireworks.) That is how signals from one
IC couple to another and how interference signals can couple
into every part of your system.
Notice that two characteristics will affect the spreading of thecurrent and the shape of the return path. The first is distance.
The drive to make consumer products smaller greatly helps
with this aspect. However, as the printed circuit boards get
smaller, the devices get closer together. Distance is reduced,
but now the multiple ground return paths are closer together.
When the fringes of current overlap, the signals intertwine.
What can we do? Careful layout is a must. Keep high frequencycircuitry as compact as possible. You might even choose toisolate these devices with metal traces or cuts in groundplanes. Figure 3 gives you an example. The return current of
input A no longer overlaps with the return current of input B. In
this case, knowing the nonideal nature of ground allows us toimprove the operation of our system. A full ground “plane” will
not work as well as one with cuts to guide the return current. If
we had assumed ground was ideal, we would have not havebeen able to make this improvement.

Page 1 of 3Nonideality of GroundFIGURE 1. THE LOCATION OF BYPASS CAPACITORS ARE HIGHLIGHTED IN A LAYOUT OF AN OP AMP IN A SOT-23 PACKAGEFIGURE 2. AN EXAMPLE PRINTED CIRCUIT BOARD LAYOUT WITH TWO INPUTS TO TWO SEPARATE OP AMPS FEEDING A MULTIPLEXER. THEGREEN DASHED LINES SHOW AN EXAMPLE PATH THAT SIGNALS TAKE THROUGH THE GROUND PLANE BACK TO THE GROUND PINSOF THE INPUT CONNECTOR. THE LONGER THE GREEN DASHES, THE HIGHER THE CONCENTRATION OF GROUND CURRENTFIGURE 3. CUT-OUTS IN THE GROUND PLANCE ELIMINATE ANY CROSSTALK BETWEEN THE RETURN GROUND PATHS OF INPUT A AND INPUT B

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