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Jun 29, 2003

Keywords: common-mode signals, network,
transmission lines, shielded cables, unshielded cables,
RS-485, RS485, RS-422, RS422,
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parts

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APPLICATION NOTE 2045

Understanding Common-Mode Signals

Abstract: To
understand how common-mode signals are created and then
suppressed, you should first understand the interaction of
shields and grounds in common cable configurations. The
following discussion defines a common-mode signal, reviews the
common cable configurations, considers shielded vs. unshielded
cables, and describes typical grounding practices. The article
discusses methods whereby common-mode signals are created and
rejected.

The primary focus of this discussion is on RS-485/RS-422
cables and signals, but the discussion also applies to
telephone, audio, video, and computer-network signals.

Attend
this brief webcast by Maxim on
TechOnline

Common-Mode Signals Defined
When referenced to the
local common or ground, a common-mode signal appears on both
lines of a 2-wire cable, in-phase and with equal amplitudes.
Clearly, a common-mode signal cannot be present if one of the
lines is connected to local common. Technically, a common-mode
voltage is one-half the vector sum of the voltages from each
conductor of a balanced
circuit to local ground or common. Such signals can arise from
one or more of the following sources:

Radiated signals coupled equally to both lines,

An offset from signal common created in the driver
circuit, or

A ground differential between the transmitting and
receiving locations.

Further details about
these basic conditions appear in a later section. Before
examining those details, however, it is useful to understand
different cable configurations, signal-grounding conventions,
and shield-grounding practices.

General Data-Transmission System
The principal aim of
any data-transmission system is to send data from one location
to another, whether within a single box or enclosure, between
boxes within an enclosure, between enclosures within a
building or defined area, or between buildings. Figure
1 illustrates an RS-485 signaling situation in which the
buildings are supplied from different power
circuits.

Figure
1. This generalized system transmits data between two widely
separated buildings, and shows the earth currents created
between ground points in a single-phase power-distribution
system. Similar currents are created in 3-phase, Y-connected
systems.

Bonding a power-line subscriber's neutral
line to a ground rod sunk in the earth at the power-entry
point establishes the power-line neutral as a safety ground.
From that point, a bare or green-insulated wire carries the
safety-ground reference to all electrical outlets and
installed equipment throughout the premises. Industrial
chassis frames are bonded to the safety ground at the chassis'
power-input point, where it becomes frame
ground.

Circuit common is often connected to the
chassis at one or more points, but a single ground point per
chassis is best. In some cases, the circuit common might be
isolated from the frame ground. Leakage currents that flow in
the safety-ground wire from machine windings to the case, or
more commonly, that flow between the ground and earth due to
AC primary or secondary neutral currents in the
power-distribution system, can produce a potential difference
between the neutral and frame ground.

Ground
differentials can vary from several volts to several tens of
volts. The greatest differentials are found in single-phase or
3-phase Y-distribution systems, in which the portion of
neutral current flowing in the earth can be 10% to 70% of the
total neutral current flowing in the primary circuit. Voltage
measurements between ground points are typically
0.2VRMS to 5VRMS, and (rarely) as high
as 65VRMS between widely separated grounds.

Cables and Noise
Noise signals can appear in a cable
for several reasons: capacitive coupling of nearby electric
fields (E); inductive coupling of local magnetic fields (M);
electromagnetic coupling of radio signals in space (EM); and
conduction through intentional or sneak circuit paths (C). The
coupled signal appears as an additional signal in series with
the line or lines (Figure 2). Table 2 lists the
type and origin of potential noise sources in cables. Twisted-pair
lines intercept coupled signals equally, so the incident
signals appear only as common-mode
signals. Twisted-pair lines are said to be balanced if the
impedances connected from each line to the local common are
identical.

Figure
2. These transmission-cable configurations show the locations
of possible noise sources.

Table 1. Electrical
Cable Types and Applications

Description

Electrical Return Route

Typical Applications

Single-Wire Line

Earth or frame

Early telephone and telegraph signaling circuits,
automotive power distribution

Single-Wire Shielded Cable

Shield

Single-wire shielded microphone cable or coaxial
cable for video or radio-frequency (RF) signals

Unshielded Parallel
Pair

Second wire of the pair

Signaling or AC power distribution

Unshielded Twisted Pair (UTP)

Second wire of the pair

Single-line telephone, signaling, or data
cables

Shielded Twisted Pair (STP)

Second wire of the pair

Balanced microphone cable, twin-axial RF cable, or
shielded data-transmission cables

Unshielded Multiple Twisted Pair

Second wire of the pair

26-pair telephone cable and 4-pair
EIA/TIA-designated Categories 1 through 6

Shielded Multiple Twisted Pair

Second wire of the pair

Intercom cable and EIA/TIA CAT
5 Class
D or CAT 7

Table 2. Noise
Sources for Listed Cable Configurations

Description

en1

en2

en3

en4

Notes

Single-Wire Line, Earth or Frame Return

Radiated E, EM, or M

Earth currents

–

–

Receiving circuit must be insensitive to the sum of
en1 + en2 at load.

Single-Wire Shielded Cable

If radiated or conducted noise enters an unshielded
portion of the main conductor or appears between source
common and a ground point at cable ends

Radiated E, EM, or M along length of shield

–

Conduction by electrical currents flowing in
external ground path if both ends of shield are
grounded.

Copper shield ineffective for inductively-coupled
noise. en1 is insignificant if: inductive
coupling is absent; shielding is complete from source to
load; and ground points are connected directly to
circuit common at source and load.

Unshielded Parallel Pair

Radiated E, EM, or M

Radiated E, EM, or M

–

–

en1 and en2 will partially
cancel if the lines are parallel and closely
spaced.

Unshielded Twisted Pair (UTP), or Unshielded
Multiple Twisted-Pair

Radiated E, EM, or M

Radiated E, EM, or M

–

–

Twisted lines make en1 and en2
equal in amplitude and phase. Receiving circuit must
reject the VCM signal.

Shielded Twisted Pair (STP), or Shielded Multiple
Twisted-Pair

Radiated M

Radiated M

Radiated E, EM, or M along length of shield

Conduction by electrical currents is flowing in the
external ground path if both ends of the shield are
grounded.

Neither en3 nor en4 appear in
the signal path, but could cause circulating current if
both ends of the shield are grounded. Receiving circuit
must reject VCM signal if en1 and
en2 are present.

Circuit and Shield Grounding
Single line with earth
return: The signal common line connects to earth ground at
the source and load by the earth (frame) return path. Circuit
commons must also be connected to the earth (frame)
ground.

Single-wire shielded: Signal current is
always carried on the shield, so connections to the circuit
common must be present at both source and load. Table 3
lists the shield-grounding connections for various
conditions.

Table 3. Shield Grounding for
Single-Wire Shielded Cables

When

Shield Is Grounded

Conditions

Source Floats

At load only

Source is battery powered or is an unpowered
transducer, such as a microphone, although the
microphone case may be connected to the shield.

Load Floats

At source only

Load is isolated, such as a battery-powered device.
Such a line may be used to transmit signals to a remote
ungrounded load as, for example, an antenna with
isolated ground plane.

Source and Load Grounded

At both ends

Source and load grounds are at equal voltages,
otherwise circulating currents in the shield create a
noise source in the signal path. Double grounding is
thus used only: within a single chassis or enclosure; or
between several enclosures sharing a common
iso-potential frame ground or having no frame ground at
all. Audio and video cables within a home entertainment
system are examples of this
use.

Two-wire parallel:
Each conductor carries an equal amount of signal current, but
in opposite directions. Table 4 lists line-grounding
connections for various conditions.

Table 4. Line
Grounding for Two-Wire Parallel Cables

When

Shield Is Grounded

Conditions

Source Floats

At load only

Source is battery-powered.

Load Floats

At source only

Source is electronic and the load is passive or
nonelectronic (headphones or loudspeaker). Found in AC
power-distribution systems, where the power enters a
user's premises.

Source and Load Grounded

At both ends

Electronic signaling systems such as RS-232.
Note, however, that RS-232 commonly utilizes
twisted-pair cabling.

Source and Load Floating

At neither end

Found in transformer-coupled signaling systems
(doorbell or other call systems). These systems are
often immune to any low-level noise signals that may be
present.

Unshielded twisted
pair(s): Any driving or receiving circuit probably
includes a connection to local common or frame ground, but
connecting the transmission line itself to frame ground is
unnecessary and undesirable. A differential-mode or
balanced-signal source (such as unshielded RS-422
and RS-485 data-transmission circuits) transmits data signals
to a remote location where source and load circuits are both
referenced to local ground or common. Transformer-coupled
applications include 10/100 Base-T Ethernet
cables.

Shielded twisted pair(s): Grounding the
shield of any shielded pair shunts to ground any unwanted
signals or noise intercepted by the shield. Typical shield
materials (copper and aluminum) shield the internal conductors
from signals coupled capacitively or electromagnetically, but
not from those coupled inductively.

For any shielded
pair(s) carrying balanced signals, you should connect the
shield to ground at one end, usually the receiving end. If the
transmitting-location ground carries a noise signal different
from that at the receiving location, grounding the shield at
both ends causes current flow along the shield. Grounding at
both ends is acceptable if there is no substantial potential
difference between the two ground locations. This
configuration includes shielded RS-422 and RS-485
data-transmission circuits. RS-485 Application Guidelines
specify connecting the shield to earth ground, either directly
or through a fusible resistor at one or both ends of the cable
shield.

Signal-Mode Definitions
Electrical signals carried on
cables can be described as normal mode, differential mode, or
common mode.

A normal-mode signal is any type (other than
common mode) that appears between a pair of wires, or on a
single wire referenced to (or returned through) the earth,
chassis, or shield. Normal-mode signals are read between two
wires in a balanced or unbalanced transmission path. (For a
balanced 2-wire path, one
wire is driven positive while the other is driven
negative by an equal amount, both with respect to a static
or no-signal condition in which both lines assume the same
voltage level relative to circuit common.)

A differential-mode signal appears differentially
on a pair of wires in an ungrounded cable configuration.

A common-mode signal appears equally (with
respect to local circuit common) on both lines of a 2-wire
cable not connected to earth, shield, or local common.
Usually, but not always, this is an unwanted signal that
should be rejected by the receiving circuit. Common-mode
voltage (VCM) is expressed mathematically as the
average of the two signal voltages with respect to local
ground or common:

Figure 3 shows a 3V differential-mode
signal riding on a 2.5V common-mode signal. The DC offset is
typical of differential-mode data transmitters operating from
a single supply. The common-mode voltage can be AC, DC, or a
combination of AC and DC. (Figure 3 represents the simplest
case, a DC common-mode voltage with no AC
component.)

Figure
3. Typical RS-485 transmitters generate a common-mode DC
offset voltage as shown.

When cables are long (as
RS-485 data cables can be), the originating signal's common or
ground may not have the same electrical potential as that of
the receiving location. The RS-485 specification says to
connect the drive-circuit common to frame ground, either
directly or through a 100Ω resistor. The result is illustrated
in Figure 4.

Figure
4. Three types of common-mode signal (eGD,
eLC, and EOS) can be present in a 2-wire
data-transmission system.

Signal common can assume
a common-mode voltage equal to the vector sum of the
ground-potential difference, the driver-offset voltage, and
any longitudinally coupled noise voltage generated along the
signal path between transmitter and receiver:

The Origin of Common-Mode Signals
Three sources of
common-mode voltage are represented in Figure 4 as
eGD, eLC, and EOS:

EOS is typically a DC offset introduced by a
differential-mode driver operating from a single supply, as
represented in Figure 3.

eGD is a noise signal representing the
difference in ground potentials at the transmitting and
receiving locations. It is usually an AC signal containing
the fundamental and possibly several harmonics of the
power-line frequency.

eLC is a longitudinally coupled noise signal
occurring equally on both transmission lines due to
capacitive, electromagnetic, or inductive coupling from
extraneous sources.

Minimizing Common-Mode Signals
EOS can be
made quite small, or even zero, by operating a
differential-mode driver from balanced supplies. In contrast,
eGD can be minimized only by maintaining a
relatively short distance between the transmitting and
receiving locations. eLC can be minimized by using
a shielded twisted pair: noise introduced within the cable
arises equally on each of two tightly twisted wires.
Otherwise, a normal-mode signal would be present due to the
line's asymmetry with respect to disturbing fields.

The
load must also be symmetrical; the resistive and capacitive
load impedances on both lines of the twisted pair must be
matched. Inductively coupled signals can be prevented only by
using magnetic shielding. (Note that any wire carrying signal
current is a source of magnetic radiation.)

Suppressing Common-Mode Signals
Common-mode signals
(VCM) must be rejected in the receiving circuit.
That rejection is easily accomplished when the receiving
circuit is passive (headphones or loudspeaker), transformer
coupled, isolated and battery operated, or otherwise not
referenced in any way to the transmitting-circuit common
(either capacitively or resistively connected). The
configurations noted here are inherently immune to common-mode
signals, but receiving circuits referenced to the
transmitting-circuit common must be designed to accept the
full range of VCM presented to them. All such
designs involve differential receivers with high
common-mode-rejection (CMR).
If the VCM is of relatively low amplitude, a
high-CMR receiver
alone may be adequate.

How High-CMR Receivers Work
All high-CMR receivers
employ either some form of differential pair, or a traditional
instrumentation amplifier consisting of three amplifiers, as
indicated in Figure 5. Each amplifier accepts a
differential input in the presence of a limited common-mode
voltage, with an acceptable VCM limited to somewhat
less than the supply voltages. Such circuits can handle analog
and digital signals.

Figure
5. These differential-amplifier circuits exhibit high
common-mode rejection.

Additional isolation must be
employed if VCM is to exceed the receiver's
common-mode range. Most such circuits employ a
transformer-coupled isolated supply, plus any one of the
following:

Optically coupled circuits

Capacitively coupled differential circuits

Inductively coupled circuits

Resistively coupled differential circuits

All the techniques shown in Figure 6
can couple a signal across the isolation barrier in the
presence of high values of VCM. Each depends on the
use of an isolated power supply, which is usually transformer
coupled. Isolation voltage limits are determined by the
transformer and by the type of isolation chosen. Isolation to
2500V or more is practical with transformer-, optical-, and
capacitive-coupling techniques; resistive coupling is usually
limited to the range of 50V to 100V.

Figure
6. The isolation techniques and typical components shown
achieve high common-mode-signal
rejection.

Resistive coupling involves transferring
data across a resistive attenuator, which attenuates both the
data and the common-mode signals. Thus, resistive isolation is
limited by the fraction of VCM that can be
accommodated by the receiving circuit, while reliably
detecting a small fraction of the original data
signal.

In Figure 6, the various isolation drivers
treat the requirement for isolated power in different ways.
Today's inductively-coupled devices make no provision for the
power supply, and therefore require external isolated
supplies. Some capacitor-coupled devices include transformer
drivers, but they require external transformers. Maxim's MAX3157 and MAX3250 drivers
contain isolated supplies and require external, low-profile,
ceramic charge-pump capacitors. Members of the MAX1480 and MAX1480E families,
however, contain fully isolated supplies including
transformers.

Thus, you can choose the cable type and
isolation technology intelligently, once the source and
magnitude of an intruding common-mode signal is known. You
need only measure or calculate the magnitude of the disturbing
signals, and then select your components to meet the overall
requirements of the system.

References

From a 1995 survey of 48 utilities, by the Minnesota
Public Utilities Commission: http://www.strayvoltage.org/stories/index.php3?Story=20000213_attorneygeneral.inc

Documented information is difficult to discover. The
cited numbers were obtained in private communication with
engineers at BC Hydro Power, British Columbia, Canada.

TIA/EIA TELECOMMUNICATIONS SYSTEMS BULLETIN TSB89,
Application Guidelines for TIA/EIA-485-A, Telecommunications
Industry Association, June 1998

TIA/EIA STANDARD TIA/EIA-485-A, Electrical
Characteristics of Generators and Receivers for Use in
Balanced Digital Multipoint Systems, Telecommunications
Industry Association, March 3, 1998.

A similar version of this article appeared in the
April 17, 2003 issue of EDN
magazine.

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APP 2045: Jun 29, 2003

AN2045, AN 2045, APP2045,
Appnote2045, Appnote 2045

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