Maxim > Design=20 support > App = notes=20 > Amplifier=20 and Comparator Circuits > APP = 2045
Maxim > Design = support >=20 App = notes=20 > Audio=20 Circuits > APP 2045
Maxim > Design = support >=20 App = notes=20 > Communications=20 Circuits > APP 2045

Jun 29, = 2003

Keywords: common-mode signals, network, = transmission lines, shielded cables, unshielded = cables,=20 RS-485, RS485, RS-422, RS422,
Related=20 parts

Download,=20 PDF format (103kB)
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APPLICATION NOTE 2045

Understanding Common-Mode Signals

Abstract: To=20 understand how common-mode signals are created and = then=20 suppressed, you should first understand the = interaction of=20 shields and grounds in common cable configurations. = The=20 following discussion defines a common-mode signal, = reviews the=20 common cable configurations, considers shielded vs. = unshielded=20 cables, and describes typical grounding practices. The = article=20 discusses methods whereby common-mode signals are = created and=20 rejected.

The primary focus of this discussion is on = RS-485/RS-422=20 cables and signals, but the discussion also applies to = telephone, audio, video, and computer-network = signals.=20

Attend=20 this brief webcast by Maxim on=20 = TechOnline

Common-Mode Signals Defined

When referenced to = the=20 local common or ground, a common-mode signal appears = on both=20 lines of a 2-wire cable, in-phase and with equal = amplitudes.=20 Clearly, a common-mode signal cannot be present if one = of the=20 lines is connected to local common. Technically, a = common-mode=20 voltage is one-half the vector sum of the voltages = from each=20 conductor of a balanced=20 circuit to local ground or common. Such signals can = arise from=20 one or more of the following sources:=20

Radiated signals coupled equally to both lines,=20
An offset from signal common created in the = driver=20 circuit, or=20
A ground differential between the transmitting = and=20 receiving locations.

Further = details about=20 these basic conditions appear in a later section. = Before=20 examining those details, however, it is useful to = understand=20 different cable configurations, signal-grounding = conventions,=20 and shield-grounding practices.

General Data-Transmission System

The principal = aim of=20 any data-transmission system is to send data from one = location=20 to another, whether within a single box or enclosure, = between=20 boxes within an enclosure, between enclosures within a = building or defined area, or between buildings. = Figure=20 1 illustrates an RS-485 signaling situation in = which the=20 buildings are supplied from different power=20 circuits.

Figure=20 1. This generalized system transmits data between two = widely=20 separated buildings, and shows the earth currents = created=20 between ground points in a single-phase = power-distribution=20 system. Similar currents are created in 3-phase, = Y-connected=20 systems.

Bonding a power-line subscriber's = neutral=20 line to a ground rod sunk in the earth at the = power-entry=20 point establishes the power-line neutral as a safety = ground.=20 From that point, a bare or green-insulated wire = carries the=20 safety-ground reference to all electrical outlets and=20 installed equipment throughout the premises. = Industrial=20 chassis frames are bonded to the safety ground at the = chassis'=20 power-input point, where it becomes frame=20 ground.

Circuit common is often connected to = the=20 chassis at one or more points, but a single ground = point per=20 chassis is best. In some cases, the circuit common = might be=20 isolated from the frame ground. Leakage currents that = flow in=20 the safety-ground wire from machine windings to the = case, or=20 more commonly, that flow between the ground and earth = due to=20 AC primary or secondary neutral currents in the=20 power-distribution system, can produce a potential = difference=20 between the neutral and frame ground.

Ground=20 differentials can vary from several volts to several = tens of=20 volts. The greatest differentials are found in = single-phase or=20 3-phase Y-distribution systems, in which the portion = of=20 neutral current flowing in the earth can be 10% to 70% = of the=20 total neutral current flowing in the primary circuit. = Voltage=20 measurements between ground points are typically=20 0.2V_RMS to 5V_RMS, and (rarely) = as high=20 as 65V_RMS between widely separated = grounds.

Cables and Noise

Noise signals can appear in a = cable=20 for several reasons: capacitive coupling of nearby = electric=20 fields (E); inductive coupling of local magnetic = fields (M);=20 electromagnetic coupling of radio signals in space = (EM); and=20 conduction through intentional or sneak circuit paths = (C). The=20 coupled signal appears as an additional signal in = series with=20 the line or lines (Figure 2). Table 2 = lists the=20 type and origin of potential noise sources in cables. = Twisted-pair=20 lines intercept coupled signals equally, so the = incident=20 signals appear only as common-mode=20 signals. Twisted-pair lines are said to be = balanced if the=20 impedances connected from each line to the local = common are=20 identical.

Figure=20 2. These transmission-cable configurations show the = locations=20 of possible noise sources.

Table 1. = Electrical=20 Cable Types and Applications=20

Description	Electrical Return Route	Typical = Applications
Single-Wire Line	Earth or frame	Early telephone and telegraph signaling = circuits,=20 automotive power distribution
Single-Wire Shielded Cable	Shield	Single-wire shielded microphone cable or = coaxial=20 cable for video or radio-frequency (RF) = signals
Unshielded Parallel=20 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=20 cables
Shielded Twisted Pair (STP)	Second wire of the pair	Balanced microphone cable, twin-axial RF = cable, or=20 shielded data-transmission cables
Unshielded Multiple Twisted Pair	Second wire of the pair	26-pair telephone cable and 4-pair=20 EIA/TIA-designated Categories 1 through = 6
Shielded Multiple Twisted Pair	Second wire of the pair	Intercom cable and EIA/TIA CAT=20 5 Class=20 D or CAT = 7

Table 2. Noise=20 Sources for Listed Cable Configurations=20

Description	e_n1	e_n2	e_n3	e_n4	Notes
Single-Wire Line, Earth or Frame Return	Radiated E, EM, or M	Earth currents	=E2=80=93	=E2=80=93	Receiving circuit must be insensitive to the = sum of=20 e_n1 + e_n2 at = load.
Single-Wire Shielded Cable	If radiated or conducted noise enters an = unshielded=20 portion of the main conductor or appears between = source=20 common and a ground point at cable ends	Radiated E, EM, or M along length of = shield	=E2=80=93	Conduction by electrical currents flowing in=20 external ground path if both ends of shield are=20 grounded.	Copper shield ineffective for = inductively-coupled=20 noise. e_n1 is insignificant if: = inductive=20 coupling is absent; shielding is complete from = source to=20 load; and ground points are connected directly = to=20 circuit common at source and load.
Unshielded Parallel Pair	Radiated E, EM, or M	Radiated E, EM, or M	=E2=80=93	=E2=80=93	e_n1 and e_n2 will = partially=20 cancel if the lines are parallel and closely=20 spaced.
Unshielded Twisted Pair (UTP), or Unshielded=20 Multiple Twisted-Pair	Radiated E, EM, or M	Radiated E, EM, or M	=E2=80=93	=E2=80=93	Twisted lines make e_n1 and = e_n2=20 equal in amplitude and phase. Receiving circuit = must=20 reject the V_CM signal.
Shielded Twisted Pair (STP), or Shielded = Multiple=20 Twisted-Pair	Radiated M	Radiated M	Radiated E, EM, or M along length of = shield	Conduction by electrical currents is flowing = in the=20 external ground path if both ends of the shield = are=20 grounded.	Neither e_n3 nor e_n4 = appear in=20 the signal path, but could cause circulating = current if=20 both ends of the shield are grounded. Receiving = circuit=20 must reject V_CM signal if = e_n1 and=20 e_n2 are = present.

Circuit and Shield Grounding

Single line = with earth=20 return: The signal common line connects to earth = ground at=20 the source and load by the earth (frame) return path. = Circuit=20 commons must also be connected to the earth (frame)=20 ground.

Single-wire shielded: Signal = current is=20 always carried on the shield, so connections to the = circuit=20 common must be present at both source and load. = Table 3=20 lists the shield-grounding connections for various=20 conditions.

Table 3. Shield Grounding for=20 Single-Wire Shielded Cables=20

When	Shield Is Grounded	Conditions
Source Floats	At load only	Source is battery powered or is an unpowered=20 transducer, such as a microphone, although the=20 microphone case may be connected to the = shield.
Load Floats	At source only	Load is isolated, such as a battery-powered = device.=20 Such a line may be used to transmit signals to a = remote=20 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=20 noise source in the signal path. Double = grounding is=20 thus used only: within a single chassis or = enclosure; or=20 between several enclosures sharing a common=20 iso-potential frame ground or having no frame = ground at=20 all. Audio and video cables within a home = entertainment=20 system are examples of this=20 use.

Two-wire = parallel:=20 Each conductor carries an equal amount of signal = current, but=20 in opposite directions. Table 4 lists = line-grounding=20 connections for various conditions.

Table 4. = Line=20 Grounding for Two-Wire Parallel Cables=20

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=20 nonelectronic (headphones or loudspeaker). Found = in AC=20 power-distribution systems, where the power = enters a=20 user's premises.
Source and Load Grounded	At both ends	Electronic signaling systems such as RS-232.=20 Note, however, that RS-232 commonly utilizes=20 twisted-pair cabling.
Source and Load Floating	At neither end	Found in transformer-coupled signaling systems = (doorbell or other call systems). These systems = are=20 often immune to any low-level noise signals that = may be=20 = present.

Unshielded twisted=20 pair(s): Any driving or receiving circuit probably = includes a connection to local common or frame ground, = but=20 connecting the transmission line itself to frame = ground is=20 unnecessary and undesirable. A differential-mode or=20 balanced-signal source (such as unshielded RS-422=20 and RS-485 data-transmission circuits) transmits data = signals=20 to a remote location where source and load circuits = are both=20 referenced to local ground or common. = Transformer-coupled=20 applications include 10/100 Base-T Ethernet=20 cables.

Shielded twisted pair(s): = Grounding the=20 shield of any shielded pair shunts to ground any = unwanted=20 signals or noise intercepted by the shield. Typical = shield=20 materials (copper and aluminum) shield the internal = conductors=20 from signals coupled capacitively or = electromagnetically, but=20 not from those coupled inductively.

For any = shielded=20 pair(s) carrying balanced signals, you should connect = the=20 shield to ground at one end, usually the receiving = end. If the=20 transmitting-location ground carries a noise signal = different=20 from that at the receiving location, grounding the = shield at=20 both ends causes current flow along the shield. = Grounding at=20 both ends is acceptable if there is no substantial = potential=20 difference between the two ground locations. This=20 configuration includes shielded RS-422 and RS-485=20 data-transmission circuits. RS-485 Application = Guidelines=20 specify connecting the shield to earth ground, either = directly=20 or through a fusible resistor at one or both ends of = the cable=20 shield.

Signal-Mode Definitions

Electrical signals = carried on=20 cables can be described as normal mode, differential = mode, or=20 common mode.=20

A normal-mode signal is any type (other = than=20 common mode) that appears between a pair of wires, = or on a=20 single wire referenced to (or returned through) the = earth,=20 chassis, or shield. Normal-mode signals are read = between two=20 wires in a balanced or unbalanced transmission path. = (For a=20 balanced 2-wire path, one=20 wire is driven positive while the other is = driven=20 negative by an equal amount, both with respect to a = static=20 or no-signal condition in which both lines assume = the same=20 voltage level relative to circuit common.)=20
A differential-mode signal appears = differentially=20 on a pair of wires in an ungrounded cable = configuration.=20
A common-mode signal appears equally = (with=20 respect to local circuit common) on both lines of a = 2-wire=20 cable not connected to earth, shield, or local = common.=20 Usually, but not always, this is an unwanted signal = that=20 should be rejected by the receiving circuit. = Common-mode=20 voltage (V_CM) is expressed mathematically = as the=20 average of the two signal voltages with respect to = local=20 ground or common:

=20

Figure 3 shows a 3V = differential-mode=20 signal riding on a 2.5V common-mode signal. The DC = offset is=20 typical of differential-mode data transmitters = operating from=20 a single supply. The common-mode voltage can be AC, = DC, or a=20 combination of AC and DC. (Figure 3 represents the = simplest=20 case, a DC common-mode voltage with no AC=20 component.)

Figure=20 3. Typical RS-485 transmitters generate a common-mode = DC=20 offset voltage as shown.

When cables are = long (as=20 RS-485 data cables can be), the originating signal's = common or=20 ground may not have the same electrical potential as = that of=20 the receiving location. The RS-485 specification says = to=20 connect the drive-circuit common to frame ground, = either=20 directly or through a 100=CE=A9 resistor. The result = is illustrated=20 in Figure 4.

Figure=20 4. Three types of common-mode signal (e_GD,=20 e_LC, and E_OS) can be present in = a 2-wire=20 data-transmission system.

Signal common can = assume=20 a common-mode voltage equal to the vector sum of the=20 ground-potential difference, the driver-offset = voltage, and=20 any longitudinally coupled noise voltage generated = along the=20 signal path between transmitter and = receiver:

The Origin of Common-Mode Signals

Three = sources of=20 common-mode voltage are represented in Figure 4 as=20 e_GD, e_LC, and E_OS:=20

E_OS is typically a DC offset = introduced by a=20 differential-mode driver operating from a single = supply, as=20 represented in Figure 3.=20
e_GD is a noise signal representing = the=20 difference in ground potentials at the transmitting = and=20 receiving locations. It is usually an AC signal = containing=20 the fundamental and possibly several harmonics of = the=20 power-line frequency.=20
e_LC is a longitudinally coupled noise = signal=20 occurring equally on both transmission lines due to=20 capacitive, electromagnetic, or inductive coupling = from=20 extraneous sources.

Minimizing Common-Mode Signals

E_OS = can be=20 made quite small, or even zero, by operating a=20 differential-mode driver from balanced supplies. In = contrast,=20 e_GD can be minimized only by maintaining a=20 relatively short distance between the transmitting and = receiving locations. e_LC can be minimized = by using=20 a shielded twisted pair: noise introduced within the = cable=20 arises equally on each of two tightly twisted wires.=20 Otherwise, a normal-mode signal would be present due = to the=20 line's asymmetry with respect to disturbing = fields.

The=20 load must also be symmetrical; the resistive and = capacitive=20 load impedances on both lines of the twisted pair must = be=20 matched. Inductively coupled signals can be prevented = only by=20 using magnetic shielding. (Note that any wire carrying = signal=20 current is a source of magnetic radiation.)

Suppressing Common-Mode Signals

Common-mode = signals=20 (V_CM) must be rejected in the receiving = circuit.=20 That rejection is easily accomplished when the = receiving=20 circuit is passive (headphones or loudspeaker), transformer=20 coupled, isolated and battery operated, or otherwise = not=20 referenced in any way to the transmitting-circuit = common=20 (either capacitively or resistively connected). The=20 configurations noted here are inherently immune to = common-mode=20 signals, but receiving circuits referenced to the=20 transmitting-circuit common must be designed to accept = the=20 full range of V_CM presented to them. All = such=20 designs involve differential receivers with high=20 common-mode-rejection (CMR).=20 If the V_CM is of relatively low amplitude, = a=20 high-CMR receiver=20 alone may be adequate.

How High-CMR Receivers Work

All high-CMR = receivers=20 employ either some form of differential pair, or a = traditional=20 instrumentation amplifier consisting of three = amplifiers, as=20 indicated in Figure 5. Each amplifier accepts a = differential input in the presence of a limited = common-mode=20 voltage, with an acceptable V_CM limited to = somewhat=20 less than the supply voltages. Such circuits can = handle analog=20 and digital signals.

Figure=20 5. These differential-amplifier circuits exhibit high=20 common-mode rejection.

Additional isolation = must be=20 employed if V_CM is to exceed the receiver's = common-mode range. Most such circuits employ a=20 transformer-coupled isolated supply, plus any one of = the=20 following:=20

Optically coupled circuits=20
Capacitively coupled differential circuits=20
Inductively coupled circuits=20
Resistively coupled differential circuits=20

All the techniques shown in = Figure 6=20 can couple a signal across the isolation barrier in = the=20 presence of high values of V_CM. Each = depends on the=20 use of an isolated power supply, which is usually = transformer=20 coupled. Isolation voltage limits are determined by = the=20 transformer and by the type of isolation chosen. = Isolation to=20 2500V or more is practical with transformer-, = optical-, and=20 capacitive-coupling techniques; resistive coupling is = usually=20 limited to the range of 50V to 100V.

Figure=20 6. The isolation techniques and typical components = shown=20 achieve high common-mode-signal=20 rejection.

Resistive coupling involves = transferring=20 data across a resistive attenuator, which attenuates = both the=20 data and the common-mode signals. Thus, resistive = isolation is=20 limited by the fraction of V_CM that can be=20 accommodated by the receiving circuit, while reliably=20 detecting a small fraction of the original data=20 signal.

In Figure 6, the various isolation = drivers=20 treat the requirement for isolated power in different = ways.=20 Today's inductively-coupled devices make no provision = for the=20 power supply, and therefore require external isolated=20 supplies. Some capacitor-coupled devices include = transformer=20 drivers, but they require external transformers. = Maxim's MAX3157 = and MAX3250 = drivers=20 contain isolated supplies and require external, = low-profile,=20 ceramic charge-pump capacitors. Members of the MAX1480 = and MAX1480E = families,=20 however, contain fully isolated supplies including=20 transformers.

Thus, you can choose the cable = type and=20 isolation technology intelligently, once the source = and=20 magnitude of an intruding common-mode signal is known. = You=20 need only measure or calculate the magnitude of the = disturbing=20 signals, and then select your components to meet the = overall=20 requirements of the system.

References=20

From a 1995 survey of 48 utilities, by the = Minnesota=20 Public Utilities Commission: http://www.strayvoltage.org/stories/index.php3?Story=3D= 20000213_attorneygeneral.inc=20
Documented information is difficult to discover. = The=20 cited numbers were obtained in private communication = with=20 engineers at BC Hydro Power, British Columbia, = Canada.=20
TIA/EIA TELECOMMUNICATIONS SYSTEMS BULLETIN = TSB89,=20 Application Guidelines for TIA/EIA-485-A, = Telecommunications=20 Industry Association, June 1998=20
TIA/EIA STANDARD TIA/EIA-485-A, Electrical=20 Characteristics of Generators and Receivers for Use = in=20 Balanced Digital Multipoint Systems, = Telecommunications=20 Industry Association, March 3, 1998. =

A similar version of this article = appeared in the=20 April 17, 2003 issue of EDN=20 magazine.

Related parts =

MAX1480E=	=C2=B115kV = ESD-Protected,=20 Isolated RS-485/RS-422 Data = Interfaces	Free=20 samples
MAX3157<= /TD>	High CMRR = RS-485=20 Transceiver with =C2=B150V Isolation	Free=20 samples
MAX3250<= /TD>	=C2=B150V = Isolated, 3.0V to=20 5.5V, 250kbps, 2 Tx/2 Rx, RS-232 = Transceiver	Free=20 samples=20

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Download,=20 PDF format (103kB)
APP 2045: Jun = 29, 2003=20
AN2045, AN 2045, = APP2045,=20 Appnote2045, Appnote 2045=20 =