Noise Reduction and Isolation
CONTROLLING NOISE

Controlling noise in measurement systems isvital because it can become a serious problem
even in the best instruments and data acquisition hardware. Most laboratories and industrial
environments contain abundant electrical-noise
sources, including AC power lines, heavymachinery, radio and TV stations, and a varietyof electronic equipment. Radio stations generate
high-frequency noise, while computers andother electronic equipment generate noise in all
frequency ranges. Building a completely noisefree environment just for running tests andmeasurements is seldom a practical solution.
Fortunately, simple devices and techniques suchas using proper grounding methods, shielded
and twisted wires, signal averaging methods,
filters, and differential input voltage amplifiers
can control the noise in most measurements.
Some techniques prevent
Figurenoise
10.01from entering
the system, while others remove extraneous
noise from the signal.

THE GROUNDING CONFLICTA non-technical dictionary defines the term
ground as a place in contact with the earth, acommon return in an electrical circuit, and an
arbitrary point of zero voltage potential.
Grounding, or connecting some part of an electrical circuit to ground ensures safety for personnel
and it usually improves circuit operation.
Unfortunately, a safe environment and a robust
ground system often do not happen simultaneously. It takes planning based on systematicallyunderstanding how electricity behaves in various
types of circuits. For example, high redundancy
is one key feature that makes most of the electrical distribution systems around the world safeand operate properly.

Grounding for Safety

Isolated secondaries of step-down power distribution transformers are generally grounded near
the transformer and within the first switching

Control Panel Grounding
Utility
distribution
transformer

Circuit breakerbox

Controlpanel
Hot phase A
Hot phase B
NeutralGroundEarthgroundEarthgroundEnclosuregroundFig. 10.01. Utility power transformers are typicallygrounded to earth ground near the transformer, andagain at the input to the electrical junction box or
first switching panel. The switching panel, in turn,
may be connected to a rod driven into the earth toensure that it too is at true ground potential.

panel in the wired path to the eventual load. (SeeFigure 10.01.) The ground is a point within thepanel connected to a nearby earth ground rod.
Typically, a large or significant structure (building
frame) or metallic system (plumbing) is also
connected to the same point. This minimizes
voltage differences that may develop between awater pipe and an appliance with a three-wiregrounded cord, for example. An electrical fault
such as a non-grounded conductor contacting a
metal object is designed to open a fuse or trip a
breaker rather than leave an electrically energizedappliance at a higher potential than a nearby
water pipe or a sink faucet. If the ground connection in the panel disconnects for any reason, theredundant ground near the transformer will
provide the path for fault currents to open fuses
or trip breakers. Preventing electrical shocks andelectrical fires is the highest priority for groundcircuits, but the redundancies built into manyelectrical grounding systems occasionally limit
certain kinds of connections for input to dataacquisition systems.

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Interfering Ground loop

Common Grounds
VCC

VS

DigitalcircuitsPower
supplyADC

VSS VDD

Sensor+Analogcircuits

Digitalground+Vout

AnaloggroundRL–+
150 mVVin = 2.65V

Vout = 2.50V

Preferredcommon returnDefault groundEarthground

30 mA–

Enclosureground

+

A

–

ES

150 mV+–

30 mA RL
500 ft.

Fig. 10.03. The 150 mV dropped across RL in the bottom
ground return line arises from the 30 mA of current flowing
in 5.2 W of lead wire resistance. The voltage adds to thesensor’s 2.50V signal to yield 2.65V at the input to thesignal-conditioning amplifier and produces a 6.6% error.

Fig. 10.02. Significant current fed to multiple circuits shouldhave individual return paths to a common ground or negative
terminal. This reduces the risk of voltage drops developing overlong ground runs or lead wires that could become input (error)
Figure 10.03
signals to other circuits.

few milliseconds and is usually not a safety hazard. But
the problem can be much more serious if lightning strikes
a safety ground structure and thousands of amperes flow
through the ground system. Potential differences acrosseven a fraction of an ohm can easily exceed 1,000 VAC
and damage equipment and endanger lives.

2
Grounding for Robust Instrumentation

Several internal, common busses in a data control instrument are arranged to regulate current flows and terminate
all paths at one common point. This approach ensuresthat the current flowing in any path will not force a
voltage drop in a return path for another circuit andappear as an (erroneous) input signal. (See Figure 10.02.)
Usually, this one common point connects through low
impedances to the safety ground connection on the
instrument’s AC power cord. This connection prevents
the internal system from floating at an AC potential
between earth ground and the input AC supply potential.

Symptoms of Ground Loops

Sometimes, a measurement error is mistakenly attributed to a ground loop problem, especially where aground is not strictly involved. The phenomenon
relates to two types of situations; shared current flow
in a circuit path, which produces unintended voltages, and inadvertent circuits that interfere with theproper operation of intended circuits.

GROUND LOOPSHow Ground Loops are Created

A ground loop problem can be illustrated by thefollowing example. An integrated sensor with internalsignal conditioning contains three wires; a positive
power supply lead, a signal output lead, and a negative
lead that serves as both the power return and signal
common. (See Figure 10.03.) The sensor’s internal
circuitry draws about 30 mA and the output signal
ranges from 0 to 5 VDC.

Measuring instruments that contain an earth ground as
described above usually generate a ground loop. A groundloop can become a serious problem even when the groundvoltage on the measured point equals the ground voltage
entering the instrument through the line cord. A voltagethat develops between the two grounds can be either an
AC or a DC voltage of any value and frequency, and as thevoltage and frequency increase, the effects of the groundloop become more troublesome.

The sensor is stimulated and a digital voltmeter reads thecorrect output of 2.50 VDC on the test bench. But when
the three leads are extended by 500 feet with 20 AWG
wire (10.4 W/1,000 ft. at 20˚C), the common lead wire
carrying the 30 mA of power supply current drops about
150 mV. This lead resistance voltage drop adds to thesensors output voltage and delivers 2.65 VDC to thedigital voltmeter. The error amounts to about 6.6% andDangerous and Destructive Ground Loops

A transient current can generate a substantial voltage on
grounded conductors. During an electrical fault when an
energized conductor contacts a safety ground, for example,
a fraction of the supply voltage can end up on the safety
ground before the fuse or circuit breaker supplying thefault opens and removes the voltage. This happens in a

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Single-ended Measurement: Unprotected Wires

Bypassed Ground Loop
150 mV–+

5.00E-03
4.00E-03

30 mA+

Vout =
2.50V

Vin =
2.50V

+

RL

150 mV–A–

3.00E-03
2.00E-03

ES

Volts

Sensor

30 mA

500 ft.

Fig. 10.04. A separate wire run from the sensor ground(or common terminal) bypasses the power supply groundwire voltage drop. The true output signal of the sensor(Vout) reaches the amplifier input terminals because theinput draws negligible current.

1.00E-03
0.00E+00-1.00E-03-2.00E-03-3.00E-03-4.00E-03-5.00E-03

0

0.05

0.1
0.15
Time [seconds]

0.2

0.25

Fig. 10.05. Radiated noise spikes are easily picked up on nontwisted, non-parallel, unshielded lead wires connected to thesingle-ended amplifier input terminals, even when the inputs
are shorted together.

Figure 10.05

what’s worse, it varies widely with the temperature of thewire. The specific application determines whether theerror can be tolerated or not.

When the current drawn by the sensor circuit is not asteady state value, that is, it consists of an ambient levelcombined with a dynamic component, the error introduced will be time varying. It might be a relatively highfrequency, which acts as noise in the measured output,
or it may be in perfect synch with the physical phenomenon being sensed. It then affects the magnitude of thetime-varying output signal. Both types of errors often
appear in data acquisition systems.

supply. Differential input connections used with theanalog common, which is referenced to the power supply
return terminal, eliminates the effect of the ground loops
inherent in this multiple-sensor arrangement.

CROSSTALK IN DATA ACQUISITION SYSTEMS

Another type of ground-loop error is crosstalk between
channels. This may be defined as an interaction between
readings on two or more channels, which may be static ordynamic. When multiple channels are used and groundloops exist, the simplified errors described previously
most likely will be compounded by contributions fromother channels. The crosstalk may or may not be obvious.

How to Eliminate Ground Loops

Static Crosstalk

A reliable trouble-shooting method analyzes current flow
and predicts its results. The wires from the intended
point of measurement must carry only current associatedwith the bias requirements of the analog input channel.
(See Figure 10.04.) These currents are typically measured
in microamperes. At lower voltage levels, they can bealtered substantially if forced to share extremely longwires carrying merely mA. A detailed wiring diagram anda circuit schematic can provide insight and understanding to help prevent this type of problem before
hundreds of feet of wire are installed.

Consider a group of static channels with steady voltages,that when measured individually, yield accurate readings.
However, when each channel is connected to an input ofthe data acquisition system and the readings change, thechange indicates that crosstalk is generated by a steadystate ground loop. Likewise, when the reading of achannel changes by connecting another channel, crosstalk
exists and the problem is a ground loop.

Dynamic Crosstalk

Dynamic crosstalk is the name given to the situation
where a known dynamic signal on a particular channel
appears in a physically unrelated channel. The steadystate currents drawn by the transducers discussed in theprevious example are idealized for simplicity. These
currents commonly vary with the measured physicalvariables along with the errors.

Frequently, multiple wires running between two locations
cannot be shared. When a common wire is shared, thecurrent in one channel affects the voltage reading inanother channel. In the previous numerical example, afourth wire connected to the lower end of a differential
measurement channel provides an output voltage thatcan be measured accurately with a high degree of confidence. This approach is most effective when the system
supports three wires, and they share a common power

Sequentially reading signals of widely varying magnitudes produce sequential crosstalk in multiplexed dataacquisition systems. Capacitive or inductive coupling3
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Proper Installation and Use of Shields

Single-ended Measurement:
Shielded Wires

Typically, a shield terminates at one end only, unless it
extends to the shield in another span of the samechannel wiring. The shield can terminate at either thetransducer end or the input channel end, but not both.
When the sensor or transducer is in a shielded metallic
enclosure, which is also connected to earth ground, theshield may be connected at the sensor end and remain
open at the input channel terminals. When the sensor iswell insulated, the shield may float and connect to theanalog common of the data acquisition system input
terminals. Occasionally, multiple-conductor cables
composed of a bundle of wires and an overall shield are
acceptable for a group of high-level, DC or low-frequency
signals, but would not be recommended for the generaldata acquisition case. Compromising a well planned
wiring system with low quality wire, shared conductors
or shields, and parallel, untwisted wires will produce lessthan optimum results.

5.00E-03
4.00E-03

Volts

3.00E-03
2.00E-03

1.00E-03
0.00E+00-1.00E-03-2.00E-03-3.00E-03-4.00E-03-5.00E-03

0

0.05

0.1
0.15
Time [seconds]

0.2

0.25

Fig. 10.06. Shielded wires reduce electrical noise pick up at
the input of single-ended amplifier input terminals, but othertechniques often are more effective.

ISOLATION AND FLOATINGDATA ACQUISITION SYSTEMS

between channels generates crosstalk in systems with06 (7.02)improperly or carelessly dressed wires. Generally,
however, these are not attributed to ground loops andare less common.

Isolation

Isolation is defined as the separation of one signal fromanother to prevent unintentional interaction between
them. All multiplexed data acquisition systems containa certain degree of channel-to-channel isolation; relaybased systems have galvanic isolation while solid-statesystems do not. Galvanic isolation is the absence of any
DC path. Most isolation methods eliminate all DC
paths below 100 MW. Three major benefits of galvanic
isolation are circuit protection, noise reduction, andhigh common-mode voltage rejection, especially thosedeveloped by ground loops.

SHIELDED WIRING
Benefits

Metallic shields placed around equipment and test leads
effectively prevent noise from either entering or leaving
the system. For example, loose or exposed wires become
antennas for radio frequency signal pickup, and canform loops that radiate noise.
To emphasize the need for controlling noise, Figure
10.05 shows a single-ended voltage measurement on ashorted channel. Approximately 6 feet of wire, nottwisted or shielded, was attached to the data acquisition
system. Figure 10.06 shows the noise in a single-ended,shorted channel using shielded cable with obvious
improvement.

Computer-based data acquisition equipment makes
possible an array of multiple channel measurements
previously beyond the economic reach of many applications. This has been accomplished by user acceptance of
two major compromises, multiplexing and non-isolated
inputs. Multiplexing is successful when the sampling
rate is adequately high and the source impedances are
sufficiently low. Lack of isolation places an entirely
different kind of limitation on the type of input signalsthat can be connected.

The best instrumentation wiring schemes consist of carefully grouped lines, twisted in pairs, occasionally covered
with a second shield, and routed through a dedicatedconduit or raceway. A shielded, twisted pair is quite
commonly used in a channel to connect a signal from asource to an input terminal. Shields minimize capacitive
coupling and twisted wires minimize inductive coupling.

Circuit Protection

Isolation separates the signal source from the measurement circuitry that could be damaged by the signal.
Voltages higher than about 10V can distort data or
damage components used in the system. High-voltage
input signals or signals containing high-voltage spikes
should therefore be isolated. The protection also works inthe opposite direction to safeguard a sensitive signal
conditioner from a device failing elsewhere in the system.

Proximity to other wires, especially power wires carrying
high voltages and high currents can couple noise into lowlevel signal conductors. Capacitive coupling can exist
between any two pieces of metal in close proximity,
including two conductors in totally separate circuits.
Likewise, air-core transformer coupling can crop up between
two closed wiring loops in totally separate circuits.

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terminals with the condition that the two input voltages
be identical. In other words, the two input terminals may
be connected together and the common-mode voltage
applied between the shorted inputs and the common
terminal as shown in Figure 10.07. In a practical test andmeasurement situation, the common-mode voltage may
exceed the instrument amplifier’s input rating, which istypically less than 10V. For safe and accurate measurements, common-mode voltages higher than 10V must be
isolated from the instrumentation amplifier while allowing
the measured signal to pass. Common types of isolation
amplifiers use magnetic, optical, or capacitive means tocouple the signal.

Common-Mode Input Voltage
Rf
100 kW
100 kW+–Ri
100 kW
Ri

VCM
–

A

+

VoCM =ACM VCM

95.3 kW
Rf

Magnetic Isolation

0.1% resistors

Common-mode
adjustment

Special instrument amplifiers use transformers thatmagnetically couple analog-type AC signals from the input
section to the output section while effectively sustaining
high common-mode voltages. Transformer coupling also
lets them provide isolated power to the input stage without
using a separate DC/DC converter. A particular instrument
amplifier contains an input op amp with a CMRR of about
130 dB at a gain of 100, and 2000V peak common mode
voltage isolation. Similar instrumentation amplifiers areavailable for powering isolated bridges, cold junction
compensation, linearization, and other special signalconditioning requirements. (See Figure 10.08.)

Fig. 10.07. Common-mode voltage is measured between thetwo input terminals and the common terminal. Because thetwo inputs must have identical voltages, they may be tied
together and connected to one voltage source.

Computer-based data acquisition equipment is most often
connected to a host computer, which is connected toearth ground. The analog inputs of plug-in cards and mosteconomical external systems are not electrically isolated
from earth ground or each other. Many applications arecompatible with this situation, but some applications facea problem with high common-mode voltage.

Optical Isolation

Optical isolation is now the most commonly used method
to couple digital signals. The measured input voltage signal
is converted to a current, which activates a light-emitting
diode within an optical coupler. A light-sensitive transistor
located adjacent to the diode, but on the opposite side of a
voltage barrier, converts the light signal back to a current
that the instrumentation amplifier can handle. The voltage

Rejection of High Common-Mode Voltage

Common-mode input voltage is defined as the voltage
applied between the common terminal and the two input

Transformer Isolation Technique
Input

IN–
IN+
Shunt

VRS= 5 V

IN COM+Viso Out
–Viso Out–+

OutputA

SignalModulator+

Demodulator

HI
VOUT = 5 V
LO

–

Power
Rectifier& filter

Oscillator+15V DCPower returnFig. 10.08. This instrumentation amplifier with transformer coupling is typically used in an industrial process control loop and provides
galvanic isolation in both input and output circuits. It can measure a ±5 VDC signal riding on a common-mode voltage as high as 2000V.

Figure 10.08

5

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Optical Isolation Technique

Modulator/Demodulator Coupling
Input

Analog section
A/D

AC
line

Isolated
power

A

Opto

A

T/Cs

+VC
Vin

Opto's Control
signals

-VC
Com 1
Gnd 1
+VCC1

Digital section

0.1 µF

Output10 Ω

Ps Gnd-VCC1Fig. 10.09. Optical devices are the most widely used component for coupling signals across high-potential barriers and for
low-level signals that are prone to the electrical noise typicallycoupled through different ground potentials.

Sense

C1
Duty cycle
modulator

Duty cycle
demodulator

Sync

Vout
Com 2
-VCC2
Sync
+VCC2

C2
T1
Oscillator
driverRectifiers
Filters

EnableGnd 2Isolation

Fig. 10.10. Low-cost isolation amplifiers couple DC signals
by modulating the signal, coupling its ac equivalent througha substrate capacitor connecting the stages, and then demodulating the signal to restore it to its original DC value.

barrier typically provides as much as several thousand volts(7.08) of isolation between input and output.

Optical devices also are commonly used to isolate theoutput of an ADC, which is usually a serial string ofdata pulses passing througha single
coupler.
Figure 10.10(5.22opticalnew book)(See Figure 10.09.) The serial string is often converted
from several parallel signals (ranging from 8 to 24
output ports, for example) to minimize the number of
optical devices required in a system. Parallel to serial
conversion circuits are less expensive than the 8 to 24
optical devices (one for each bit of output from theparallel ADC). In these instances, the power supply for
the ADC and associated input circuitry are also isolated,usually with a transformer.

Programmable Amplifier
Barrier
Opto couplers
Configuration
µP
Hi
Lo

Capacitive Isolation

Isolation
amplifier
Programmableattenuator
amplifierBypassLPF

MUX

To A/D

DC/DC
A capacitor is a passive device that couples AC voltage
Converterfrom one stage to another while blocking the DC
component. By this definition, it’s a simple but
Typical of each channel
inexpensive isolator. The measured signal to be isolated
Fig. 10.11 All three techniques of isolation are used in a
is modulated and coupled through the capacitor to themulti-channel, programmable amplifier. Optical devices coureceiving side. On the receiving side, the AC signal isple digital signals, capacitors couple analog signals, and thedemodulated to restore the original signal. This technique
DC/DC converter supplies power to the isolated side.
is often applied to low-cost isolation amplifiers where
10.11layer(7.10)
the coupling capacitor is composed of aFigurecommonbetween two isolated IC substrate sections. Signal
capacitively coupled device isolates the analog signal
isolation using these specialized ICs is rated as high aswhile two optical couplers transmit the digital control
1500V. The main benefits of this approach are simplicity,
signals to the floating circuitry.
low cost, and bandwidths as high as 50 kHz. Figure 10.10
illustrates a DC/DC converter often used as the modulator/ Fundamental Application Mistakes
demodulator in an isolation amplifier.
Dangers in measurement can easily surface when a non-isolated analog input to a data acquisition system is erroneously connected to a device operating at high commonmode voltage with respect to earth ground. The mistake,which often precedes this problem, is determining thevoltage range of the desired signal with a handheld digital

Figure 10.11 illustrates a typical multiple-channel,
programmable isolation amplifier in a data acquisition
system that uses all three types of isolation: transformers,
optical devices, and capacitors. A transformer-based
DC/DC converter supplies power to the isolated side. A

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fault resulting from this arrangement effectively shorts
out the supply line and can be a hazard to operatorsand destroy the equipment.

Isolation in Motor Drives
Isolation transformer

AC voltage
supply lines

Power switching circuit using
SCR’s or MOSFET transistors
Internal drive control common

An isolation transformer lets a user safely connect a dataacquisition system input channel, grounded through thehost computer, to a low voltage signal in an ac-powered
device. The preferred isolation transformer for such use
has a grounded electrostatic shield between the primaryand secondary windings to minimize capacitive coupling
and potentials with respect to earth ground. Thisapproach works only when the AC neutral in the electrical system is grounded to earth. Isolation transformers
cannot interrupt the path for the safety ground carriedby the ground prong of a standard 3-wire cord.

Motor
drive

Speed adjustment potentiometer
HostcomputerDataacquisition
systemUnisolated
analogoptionComputer Digital
Analog Analog CH-HI
common common common common COMz
Tied
Low impedanceEarthgroundCurrent sense
amplifierMotor

A laptop computer running on an internal battery andnot connected to other peripherals that are tied to earth
ground (such as printers) can be a floating host safely
connected to a data acquisition system. But a betteroverall approach is to isolate the input signal source.

Shunt

Fig. 10.12. Common-mode voltage is defined as the voltage
between a signal and the localized common, which is usually
earth ground. Common-mode voltages can be quite high, depending on the signal source. For example, a current sensing shunt in
a DC motor drive might be expected to be at line-voltage potential, although the actual signal level is less than 50 mV acrossthe shunt terminals. Measurement techniques for small signals
at high common mode potentials call for isolation.

An isolation transformer usually doesn’t supply all theisolation intended for a data acquisition system, because
the digital common in most computers provides a low
impedance path to earth ground as part of its ESD (electrostatic discharge) protection scheme. But some dataacquisition devices that communicate with the hostcomputer through serial data links such as RS232 andRS485 use communication isolators specifically designed
for that protocol. By comparison, most Ethernet interfaces
are transformer isolated and have ESD protection networks
on both sides of the isolation barrier referenced to chassis,
plus earth ground through the host computer.

or analog multimeter, and neglecting the relationship of
the signal with respect to earth ground. If connected withthe dotted lines, as shown in Figure 10.12, the motor drive
can be damaged as soon as power is applied. This is because
the control circuitry in many AC and DC motor drives isnecessarily referenced to high voltage with respect to earth
ground. When motor current is sensed with a shunt or lowvalue resistor, the control common bus is generally at highcommon-mode potential with respect to earth ground.

Analog Isolators

The ideal solution for measuring high common-mode
signals is the analog isolator. The isolator safely measures
low-level analog input signals containing as much as
1500V common mode, through magnetic, optical, or
capacitive devices. The amplifiers provide both channelto-system isolation and channel-to-channel isolation.
By contrast, most solid-state multiplexers have nochannel-to-channel isolation beyond the standard ±10V
signal range.

Some drives use non-contacting current sensors, transformers, and optical couplers to achieve galvanic isolation of the control circuitry. However, unless the drive is
specifically known to have an isolated analog interface,assume it does not.

Isolation Transformers

Not all AC line-powered devices contain the internal
isolation transformers that step the voltage down tolower operating levels needed for electronic circuits andsimultaneously protect users from external ground
faults. The power supply common bus in transformerless devices often connects to one side of the AC line
cord. If the system is not protected from reversing theline cord connector in the socket, the device common
line (and hence the enclosure) can be raised to a voltage
level which is higher than the common terminal ofother devices in the vicinity or other instruments
connected to the data acquisition system. The ground

The most common analog isolators are plug-in modules.
These 3-port devices require a DC power supply andprovide operating voltages for signal conditioning andmodulation circuitry on the input side. They also provide
voltage for demodulation and signal reconstruction
circuits on the output side. Most devices also provide
inherent low-pass filtering and scaling to 0 to 5V output
levels. The wide array of available options in thesemodules can simplify many complex measurement
requirements and still provide data to the overall dataacquisition system of choice, as all data acquisition
manufacturers have products that accept these modules.

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Volts

Noise Reduction: Signal Averaging

Single-Pole RC Filter

5.00E-03
4.00E-03
3.00E-03
2.00E-03
1.00E-03
0.00E+00

R-1.00E-03-2.00E-03-3.00E-03-4.00E-03-5.00E-03

Vin

C

F-3dB =

0

0.05

0.1
0.15
Time [seconds]

Raw Signal

0.2

Vout

1
2πRC

Fig. 10.14. Passive single pole filters are moderately effectivein passing a band of frequencies that contain the measured
signal, while attenuating other frequencies containing noise.

0.25

Averaged 16 values

Fig. 10.13. Signal-averaging using software techniques virtually reduce the effective electrical noise pick up before the dataacquisition system processes the measured signal.

NOISE REDUCTION

Signal Averaging
Figure 10.14 (7.04) Some noise reduction techniques prevent the noise fromentering the system initially, and others remove extraneous noise from the signal. Another technique averages
several signal samples through software. Depending on
the nature of the noise and the specific averaging
method, noise can be reduced by the square root of thenumber of averaged samples (RMS). But this may require
abundant samples to obtain an acceptable measurement.
Figure 10.13 shows the voltage across the shorted
channel when only 16 samples of data are averaged.

The isolation modules are relatively expensive and are
not likely to be used in low-cost data acquisition systems.
Low-cost systems typically don’t contain analog isolators, but many applications require isolators in at least a
few channels. System-wise, the best place to address ahigh common-mode signal channel is at the source forsafety and signal integrity.

Wireless Techniques

Although averaging is an effective technique, it has
several drawbacks. The noise present in a measurement
sequence decreases as the square root of the number ofmeasurements. Therefore, in the above example, reducing
the RMS noise to a single count by averaging alone
would require 3,500 samples. As such, averaging suits
only low-speed applications, and it eliminates only
random noise. It does not necessarily eliminate manyother types of annoying system noise, such as periodic
noise from switching power supplies.

Not all data acquisition systems can connect to sensors
on the test specimen with wires. They require a form ofradio communication called telemetry. Radio transmitters and sensors are located on the device under test
and receivers are located at the data acquisition system.
For example, the rotating member of a large motor or
generator can be monitored remotely and safely. The
system can monitor temperature, vibration, deflection,and speed in rpm without the type of slip rings used in
the past.

Analog Filtering

A relatively new protocol called Bluetooth is increasingly
being used for remote measurement and control. It is a
short-range wireless system that lets devices recognize,
connect, and transfer data among them. The devices areequipped with special Bluetooth chips and transmit overa short range, typically 10 m. They can transfer data at a
rate of 720 KB/sec over a frequency band of 2.40 to
2.48 GHz. Another system is the 802.11 Ethernet-based
wireless system. It often provides physical and electricalisolation on the factory floor and for high voltage utilitylines and demolition test sites. It operates in the same
frequency range as Bluetooth and can handle higher datatransfer rates of 1 to 11 MB/sec.

A filter is an analog circuit element that selectively attenuates a particular band of frequencies in an incoming
signal. Filter circuits can be passive or active. Dependingon whether the filter is low or high-pass, it determines
the frequencies that are attenuated above or below thecutoff frequency. For example, as a signal frequency
increases beyond the cutoff point of a single-pole, lowpass filter, its attenuation increases slowly. Multiple-polefilter attenuation also increases slowly. Multiple-polefilters provide greater attenuation beyond the cutoff
frequency, but they may introduce phase shifts that
could affect some applications. The frequency where thesignal is 3 dB down is given by the equation shown in
Figure 10.14.

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Comparing Single-Pole andThree-Pole Filters

Differential-Input Amplifiers
5.00E-03
4.00E-030-10

3.00E-03
2.00E-03-20

Amplitude in dBV

Volts

-30
-40
-50
-60
-70
-80
-90
-110
1
10
Frequency in kHz
1-pole

0

0.05

0.1
0.15
Time [seconds]

0.2

0.25

Fig. 10.17. The biggest improvement in reducing unwanted
noise signals from the measured variable comes from differential-input amplifiers. It works so well because most of the noiseon the high side duplicates the noise on the low side of the
input and the algebraic sum of the two equal parts.

-100
-120
0.1

1.00E-03

0.00E+00-1.00E-03-2.00E-03-3.00E-03-4.00E-03-5.00E-03

100

3-pole

The three-pole filter has a much greater attenuation for
Fig. 10.15. The three-pole filter attenuates thefrequencies
Figure10.17 (7.07)
frequencies exceeding the cutoff. The improvement inafter the cutoff point more effectively than does the singlesignal quality provided by low-pass filtering is demonpole filter.

05)

strated in Figure 10.16 in which a signal containing wideband noise passes through a three-pole filter with a 1 kHz
cutoff frequency. The deviation from the average signal isplotted in volts. The maximum deviation is 6 counts, andthe RMS noise is 2.1 counts.

Volts

Noise Reduction: Low-Pass Filter
5.00E-03
4.00E-03
3.00E-03
2.00E-03
1.00E-03
0.00E+00-1.00E-03-2.00E-03-3.00E-03-4.00E-03-5.00E-03

0

0.05

0.1
0.15
Time [seconds]

Raw data

0.2

The three-pole filter shown in the example has an active
input with changeable configurations. The active, threepole filter can be a Butterworth, Bessel, or Chebyshev
with corner frequencies up to 50 Hz. Filter properties
depend on the values of the resistors and capacitors,
which the user can change. Filters also use switchedcapacitors. This type requires a clock signal to set thecutoff frequency. The primary advantage of this filter isthe ease of programming the cutoff frequency.

0.25

Filtered output

Differential Voltage Measurement

Fig. 10.16. An active, three-pole low-pass filter, configured asa Butterworth, Bessel, or Chebyshev filter, reduces the noise
signal more effectively without significant attenuation thandoes a passive circuit.

Differential input amplifiers are most often used in dataacquisition systems because they provide a high gain for
the algebraic difference between their two input signals or
voltages, but a low gain for the voltages common to both
inputs. Making differential voltage measurements is
another means of reducing noise in analog input signals.
This technique is effective because often, most noise on
the high-side input lead closely approximates the noise on
the low lead. This is called common-mode noise. Measuring
the voltage difference between the two leads eliminates
this common-mode noise.

Passive vs. Active Filters
0.16 (7.06)

A passive filter is a circuit or device consisting entirely of
non-amplifying components, typically inductors andcapacitors, which pass one frequency band while rejecting
others. An active filter, on the other hand, is a circuit ordevice composed of amplifying components such as
operational amplifiers, and suitable tuning elements, typically resistors and capacitors, which pass one frequency
band while rejecting others. Figure 10.15 compares theamplitude of a single-pole, low-pass filter with a threepole filter. Both types are set for a 1 kHz cutoff frequency.

The improvement gained with differential voltage
measurements is illustrated in Figure 10.17. It shows thesame signal as Figure 10.05, but using a differential input
rather than a single-ended input.

9
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