Glossary of Terms Commonly Used in Data Acquisition and Control

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A/D Channel List Start
Signal used to start the A/D acquisition of channels
in the channel list. The triggering edge of this signal (falling edge) enables the ADC Conversion Start

A/D Conversion
The process of converting a single analog input to a
digital value.

A/D Conversion Start
Signal used to start the conversion process of an
analog input to a digital value. The source of this signal can be either an internal ADC synchronous clock
or an external asynchronous signal. This signal causes
the stepping in the Channel List.

AC – Alternating Current
Electric current that varies in amplitude and direction. The usual waveform of ac is a sine wave, but can
be triangular, or square.

ADC – Analog-to-Digital Converter
(a.k.a. simply an “A/D”)
An electrical circuit that converts analog voltage
into a binary number. A/D converters perform the
measurments in all data acquisition systems

A false lower frequency image of a high-frequency
component that appears in sampled data acquired at
too low a sampling rate

A brand name for a magnetic nickel alloy containing small amounts of manganese, aluminum, and
silicon. Used in thermocouples and thermocouple
extension wire.

Ampere (A)
SI unit of electric current.
A device for increasing the power of a signal by taking power from a power supply and controlling
the output to match the input signal shape but with a
larger amplitude.

The size or magnitude of a signal.
Analog Input
An analog input is an infnitely variable signal. In
most data acquisition systems this signal is connected
to an input amplifer and then to an A/D converter.

Analog Output
A waveform or control signal generated as a continuous function of the measured parameter.
Analog Trigger
A trigger that occurs at a user-selected point on
an incoming analog signal. Triggering can be set to
occur at a specifc level on either an increasing or a
decreasing signal (positive or negative slope).

Anti-alias flter
A low pass flter that allows the desired low frequency component of the input waveform through,
but stops the higher frequency components that can
lead to aliasing errors (See Alias).

An Input parameter to a program.
An avionics interface protocol which is the standard
communications link in virtually all modern commercial aircraft.

American Standard Code for Information Interchange. Coding for text fles.
A program that translates assembly language instructions into machine language instructions.
Assembly Language
A machine oriented language in which mnemonics
are used to represent each machine language instruction for a particular CPU.

American Society for Testing and Materials.
(1) Hardware – A property of an event that occurs at
an arbitrary time, without synchronization to a reference clock.
(2) Software – A property of a function that begins
an operation and returns prior to the completion or
termination of the operation.


B-Type Thermocouple
Platinum-rhodium thermocouple with a temperature range of 600 to >1700 °C.
The primary-level of a hierarchical computer network connected to lower-level nodes in the hierarchy.
Background Acquisition
Background Data Acquisition refers to data that is
acquired while another program or processing routine is running in the foreground without apparent

Background Noise
Interfering signals that can cause disturbance affecting a signal that may distort the intended signal.
Base Address
A memory address that serves as the starting address for programmable registers. All other addresses
are located by adding to the base address.

Batch process
Any process on which operations are carried out on
a limited number of articles, as opposed to continuous process.

A signal range that includes both positive and negative values (for example, -5 V to +5 V).
One binary digit (either 0 or 1).
Block Mode
A high-speed data transfer in which the address of the data is sent followed by a specifed number of
back-to-back data words.

Burst Mode
A high-speed data transfer in which the address of
the data is sent followed by back-to-back data words
while a physical signal is asserted.

The group of conductors that interconnect individual circuitry in a computer. Typically, a bus is the
expansion vehicle to which I/O or other devices are
connected. Examples of PC buses are the ISA and PCI buses.

Bus Master
A type of a plug-in board or controller with the ability to read and write devices on the computer bus,
without using the host CPU.

Eight related bits of data, an eight-bit binary number. Also used to denote the amount of memory
required to store one byte of data. A byte may represent 256 unique numbers (typically from 0 to 255 in the decimal system)


High-speed processor memory that buffers commonly used instructions or data to increase processing throughput.
The process of determining the relationship between the output of a measurement device, and the
input data, and comparing it against a measurement
standard. In most cases, the term calibration also includes the process of adjusting the output of the

measurement device to comply with the measurement standard.
The amount of a stored (or separated) electrical charge for a given electrical potential, measured in
farads (F).

Conformite Europeene. A mark designating product’s compliance with all applicable European Union
legal requirements.

Channel List
A variable length list of channels and their associated gains specifying which analog input channels to
convert to digital values. In continuous A/D acquisition mode, the list wraps around to the frst channel
after it reaches the end. The channels need not be in
any particular order.

A 90% nickel alloy with about 10% chromium, used with Alumel in K-type thermocouples.
Common-Mode Rejection Ratio. A measure of an instrument’s ability to reject interference from a common-mode signal, usually expressed in decibels (dB).
Code Generator
A software program, controlled from an intuitive user interface, that creates syntactically correct highlevel source code in languages such as C or Basic.
Cold-Junction Compensation
Compensation for ambient temperature when a
thermocouple is used in a data acquisition system.

COM port
A serial device connectivity port on a PC, often RS-

Common-Mode Range
The input range over which a circuit can handle a
common-mode signal.

Common-Mode Signal
The mathematical average voltage, relative to the
computer’s ground, of the signals from a differential input.

Component Software
An application that contains one or more component objects that can freely interact with other
component software. Examples include OLE enabled
applications such as Microsoft Visual Basic and OLE
Controls for virtual instrumentation in Component

Control Register(s)
Registers containing bits that initiate control signals
for various onboard subsystems.

Conversion Time
The time elapsed, in an analog input or output
system, from the moment a channel is interrogated
(such as with a read instruction) to the moment that
accurate data is available.

A circuit such as the Intel 8254 device that counts
external pulses or clock pulses (timing).

The manner in which a signal is connected from one location to another.
An unwanted signal on one channel induced by signal activity on another nearby channel.
Current Drive Capability
The amount of current a digital or analog output
channel is capable of sourcing or sinking while still operating within voltage range specifcations.

Current Sinking
The ability of a DAQ board to dissipate current for
analog or digital output signals.

Current Sourcing
The ability of a DAQ board to supply current for
analog or digital output signals.

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Digital-to-analog converter. An integrated circuit that converts a digital number into a corresponding
analog voltage or current.

DAC Conversion Start
Signal used to start the conversion process of digital value to an analog output. The source of this signal
can be either an internal DAC synchronous clock or
an external asynchronous signal. This is a common
signal fed to both DACs.

Data Acquisition
(1) The process of automatically collecting and
measuring electrical signals from sensors, transducers, and test probes or fxtures and inputting them
to a computer for processing; (2) Collecting and
measuring the same kinds of electrical signals with
A/D and/or DIO boards plugged into a PC, and possibly generating control signals with D/A and/or DIO
boards in the same PC.

Data Logger
A stand alone data acquisition device that acquires
data and stores it in local memory from which it may
then be downloaded to a computer for subsequent
analysis and display.
Data Recorder
Another name for a data logger, though data
recorders are typically higher performance and offer
higher sample rates than a typical data logger.

An abbreviation for Data acquisition.
Decibel. The unit for expressing a logarithmic measure of the ratio of two signal levels: dB=20log10
V1/V2, for signals in volts.

Differential Input
An analog input consisting of two terminals, both
of which are isolated from computer ground, whose difference is measured.

Digital input/output.
Dynamic Link Library. A software module in Microsoft Windows containing executable code and data
that can be called or used by Windows applications
or other DLLs. Functions and data in a DLL are loaded and linked at run time when they are referenced by a
Windows application or other DLLs.

Direct memory access. A method by which data can
be transferred to/from computer memory from/to
a device or memory on the bus while the processor
does something else. DMA is the fastest method of transferring data to/from computer memory.

Differential Non-linearity. A measure in LSB of the worst-case deviation of code widths from their ideal
value of 1 LSB.

Software that controls a specifc hardware device,
such as DAQ boards.

Digital Signal Processor. A specialized microprocessor for digital signal processing.
Dual-Ported Memory
Memory that can be simultaneously accessed by
more than one controller or processor.

Dynamic Range
The ratio of the largest signal level a circuit can
handle to the smallest signal level it can handle (usually taken to be the noise level), normally expressed in

United Electronic Industries Data Acquisition and Control Glossary


Electrically erasable programmable read-only memory. ROM that can be erased with an electrical
signal and reprogrammed.

Embedded Controller
A control device built into (embedded in) a host device. Monitoring and control are handled by the same CPU as general system operation.
A device that converts linear or rotary displacement
into digital or pulse signals. The most popular type
of encoder is the optical quadrature encoder, which
uses a rotating disk with alternating opaque areas, a
light source, and a photo detector to indicate a rotary

Erasable programmable read-only memory. ROM that can be erased (usually by ultraviolet light exposure) and reprogrammed.
Signals or interrupts generated by a device to notify
another device of an asynchronous event. The contents of events are device-dependent.

External Trigger
A voltage pulse from an external source that triggers an event such as A/D conversion.


First-In First-Out Memory Buffer. The frst data stored is the frst data sent to the acceptor.
A device that allows certain parts of a signal to pass
through while blocking others. In data acquisition
systems, the most common type of flter used is a low
pass, anti-aliasing flter.

A format for processing or storing numbers as digital integers.
A format for processing or storing numbers in scientifc notation (digits multiplied by a power of 10).
A set of software instructions executed by a single
line of code that may have input and/or output parameters, that returns a value when executed.


The factor by which a signal is amplifed, sometimes
expressed in dB.

Gain Accuracy
A measure of deviation of the gain of an amplifer
from the ideal gain.

General Purpose Interface Bus. Also known as IEEE-
488, the GPIB was originally developed by Hewlett
Packard and called the HPIB.

Global Positioning System. A series of satellites
is used to provide accurate location and velocity
information to both ground based and land based
devices. A powerful and often ingored beneft in data acquisition applications is the ability of a GPS system
to generate extremely accurate timing information.
One microsecond accuracy is available with inexpensive units and much tighter specifcations are available
from some vendors.

In terms of data acquisition systems, a ground is
typically where system current is returned. In single
ended systems the negative terminal of the sensor is
typically connected to ground.

Ground Loop
Many measurements are made between an input signal and ground. However, ground is not an absolute reference point and current flowing in wires
between various ground connections can cause the potential at different “ground” points in the system
to be at different potentials. These differences then
manifest themselves as errors in the measurement.

Graphical User Interface. An intuitive, easy-to-use means of communicating information to and from
a computer program by means of graphical screen
displays. GUIs can resemble the front panels of instruments or other objects associated with a computer


A device driver that is installed as part of the operating system of the computer.
The physical components of a computer system, such as the circuit boards, plug-in boards, chassis,
enclosures, peripherals, cables, and so on.


Input/Output. The transfer of data to/from a computer system involving communications channels,
operator interface devices, and/or data acquisition
and control interfaces.

ICP® (piezoelectric vibration sensor)
A simple yet powerful 2-wire interface developed
by PCB Piezo Electronics® to facilitate connection of
Piezo Crystal based vibration sensors to data acquisition systems. Also referred to as IEPE.

Intermodulation Distortion. The ratio, in dB, of the total rms signal level of harmonic sum and difference
distortion products, to the overall rms signal level.
The test signal is two sine waves added together according to the following standards:

Integral Non-linearity. A measure in LSB of the worst-case deviation from the ideal A/D or D/A transfer characteristic of the analog I/O circuitry.
Input Bias Current
The current that flows into the inputs of a circuit.
Input Impedance
The measured resistance and capacitance between
the input terminals of a circuit.

Input Offset Current
The difference in the input bias currents of the two
inputs of an instrumentation amplifer.

Instrumentation Amplifer
A circuit whose output voltage with respect to
ground is proportional to the difference between the
voltages at its two inputs.

Integral Control
A control action that eliminates the steady-state
offset inherent in proportional control.

Integrating ADC
An ADC whose output code represents the average
value of the input voltage over a given time interval.

A software utility that executes source code from
a high-level language such as Basic, C, or Pascal, by
reading one line at a time and executing the specifed

A computer signal indicating that the CPU should suspend its current task to service a designated activity.
Interprocess Communication. Protocol by which processes can pass messages. Messages can be either
blocks of data and information packets, or instructions and requests for process to perform actions. A
process can send messages to itself, other processes

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on the same machine, or processes located anywhere
on the network.

Isolation Voltage
The voltage that an isolated circuit can normally
withstand, usually specifed from input to input and/
or from any input to the amplifer output, or to the
computer bus.




Kilo, the standard metric prefx for 1,000, or 103, used with units of measure such as volts, Hertz, and

Kilo, the prefx for 1,024, or 210, used with byte in
quantifying data or computer memory.

A unit for data transfer that means 1,000 or 103 bytes/s.


A popular data acquisition programming language developed by National Instruments®
The adherence of device response to the equation R = KS, where R =response, S = stimulus, and K = a

Least signifcant bit. In terms of data acquisition and measurement systems, the LSB is the smallest
change in input that can be detected by the system’s
A/D converter.

Designed as the Successor to GPIB, LXI is a standard developed by the LXI Consortium, and is an abbreviation for LAN eXtensions for Instrumentation. More
information can be found at the LXI Standard Website


(1) mega, the standard metric prefx for 1 million or 106, when used with units of measure such as volts
and Hertz;
(2) mega, the prefx for 1,048,576, or 220, when
used with byte to quantify data or computer memory.

A unit for data transfer that means 1 million or 106

Man-Machine Interface, also Human-Machine Interface(HMI): The means by which an operator
interacts with an industrial automation system; often
a GUI.

A defacto standard communications protocol used in a wide variety of Industrial Control and DAQ (data
acquisition) systems industrial control and monitoring equipment. Modbus was developed by Modicon in 1979 as a means to facilitate communication in
their programmable controller products.

A property of an operating system in which several
processes can be run simultaneously.

Multiplexer. A switching device with multiple inputs that sequentially connects each of its inputs to its
output, typically at high speeds, in order to measure
several signals with a single analog input channel.
Most data acquisition systems utilize multplexers to
allow a single A/D converter to measure or monitor
multiple inputs.


An undesirable electrical signal. Noise comes from
external sources such as the AC power line, motors,
generators, transformers, fluorescent lights, soldering irons, CRT displays, computers, electrical storms,
welders, radio transmitters, and internal sources such as semiconductors, resistors, and capacitors.


Object Linking and Embedding. A set of system services, developed by Microsoft, that provides a means
for applications to interact by linking and embedding
document objects. Based on the underlying Component Object Model, OLE is object-enabling system
software. Through OLE Automation, an application
can dynamically identify and use the services of other applications, to build powerful solutions using packaged software. OLE also makes it possible to create
compound documents consisting of multiple sources of information from different applications.

Operating System
Base-level software that controls a computer, runs
programs, interacts with users, and communicates
with installed hardware or peripheral devices.

Optical Isolation
The technique of using an optoelectric transmitter
and receiver to transfer data without electrical continuity to eliminate high-potential differences and

Output Settling Time
The amount of time required for the analog output
voltage to reach its fnal value within specifed limits.

Output Slew Rate
The maximum rate of change of analog output voltage from one level to another.
The amount of computer processing resources, such as time and/or memory, required to accomplish
a task.


See Programmable Automation Controller
A technique used for extending the address range of a device to point into a larger address space
Peripheral Component Interconnect. A high-performance expansion bus architecture originally
developed by Intel to replace ISA and EISA. It offers a
theoretical maximum transfer rate of 132 Mbytes/s.
More information can be found at the PCI-SIG Website

PID Control
A three-term control mechanism combining proportional, integral, and derivative control actions.
Also see proportional control and integral control.

A high-performance processor structure in which the completion of an instruction is broken into its
elements so that several elements can be processed
simultaneously from different instructions.

See Programmable Logic Controller.
Plug and Play ISA
A specifcation prepared by Microsoft, Intel, and
other PC-related companies intended to create PCs
with plug-in boards that can be fully confgured in
software, without jumpers or switches on the boards.

A communications connection on a computer or a remote controller.

The technique used on a DAQ board to acquire a programmed number of samples after trigger conditions are met.
An electrical device of which the resistance can be
manually adjusted; used for adjustment of electrical
circuits and as a transducer for linear or rotary position.

Programmable Automation Controller
Also commonly referred to as a PAC, these are
stand-alone controllers that perform a function similar to a Programmable Logic Controller, but are
programmed in standard programming languages
(e.g. C or Visual Basic) rather that the Ladder Logic programming language used by most PLCs.

Programmable Logic Controller
Also commonly referred to as a PLC, these are
highly reliable special-purpose computer used in industrial monitoring and control applications. PLCs
typically have proprietary programming and networking protocols, and special-purpose digital and
analog I/O ports.

The technique used on a DAQ board to keep a continuous buffer flled with data, so that when the
trigger conditions are met, the sample includes the
data leading up to the trigger condition.

Programmed I/O
The standard method a CPU uses to access an I/O
device– each byte of data is read or written by the

Propagation Delay
The amount of time required for a signal to pass
through a circuit.

Proportional Control
A control action with an output that is to be proportional to the deviation of the controlled variable
from a desired set point.

The exact sequence of bits, characters and control
codes used to transfer data between computers and
peripherals through a communications channel, such
as the GPIB.

A modular instrumentation platform used for
building test equipment and automation systems
promoted by the PXI Systems Alliance (PXISA) and is
an a abbreviation that stands for PCI eXtensions for Instrumentation


Quadrature Encoder
A device used to measure rotation. The most popular type of encoder is the optical quadrature encoder,
which uses a rotating disk with alternating opaque
areas, a light source, and a photo detector. Each time
the photo detector crosses an brignt (or dark) as signal is generated. These signals are then turned into
rotational information by a quadrature encoder input

Quantization Error
The inherent uncertainty in digitizing an analog
value due to the fnite resolution of the conversion


Real Time
A property of an event or system in which data is
processed as it is acquired instead of being accumulated and processed at a later time.

Relative Accuracy
A measure in LSB of the accuracy of an ADC. It
includes all non-linearity and quantization errors. It
does not include offset and gain errors of the circuitry
feeding the ADC.

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The smallest signal increment that can be detected
by a measurement system. Resolution can be expressed in bits, in proportions, or in percent of full
scale. For example, a system has 12-bit resolution,
one part in 4,096 resolution, and 0.0244 percent of
full scale.

Resource Locking
A technique whereby a device is signaled not to use
its local memory while the memory is in use from the bus.

Ribbon Cable
A flat cable in which the conductors are side by side.
Resistance temperature detector. A metallic probe that measures temperature based upon its coefcient
of resistivity.


Sample-and-Hold. A circuit that acquires and stores an analog voltage on a capacitor for a short period of

Samples per second. Used to express the rate at which a data acquisition system samples an analog

Single-Ended. A term used to describe an analog input that is measured with respect to a common

DAQ board that calibrates its own A/D and D/A
circuits without external reference source.

A device that responds to a physical stimulus (heat,
light, sound, pressure, motion, flow, and so on), and
produces a corresponding electrical signal.

Signal-to-Noise Ratio. The ratio of the overall rms signal level to the rms noise level, expressed in dB.
Software Trigger
A programmed event that triggers an event such as
data acquisition.

Single-Pole Double Throw. A property of a switch in
which one terminal can be connected to one of two
other terminals.

Simultaneous Sampling and Hold. A property of a system in which each input or output channel is digitized or updated at the same instant.
Strain Gauge
A sensor whose resistance is a function of the applied force.
A set of software instructions executed by a single
line of code that may have input and/or output parameters.

Successive-Approximation ADC
An ADC that sequentially compares a series of
binary-weighted values with an analog input to produce an output digital word in n steps, where n is the
bit resolution of the ADC.

A property of a function that begins an operation
and returns only when the operation is complete.

System Noise
A measure of the amount of noise seen by an analog circuit or an ADC when the analog inputs are


A device on the General Purpose Interface Bus (GPIB) that sends data to the Listener on the GPIB.
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A set of standard protocols for communicating across a single network or interconnected set of
networks. The Internet Protocol (IP) for the low-level
service of taking data and packaging of components,
and Transmission Control Protocol (TCP) for high-reliability data transmissions.

Total harmonic distortion. The ratio of the total rms signal due to harmonic distortion to the overall rms
signal, in dB or percent.

Signal-to-THD Plus Noise. The ratio in decibels of
the overall rms signal to the rms signal of harmonic
distortion plus noise introduced.

A semiconductor sensor that exhibits a repeatable
change in electrical resistance as a function of temperature. Most thermistors exhibit a negative temperature coefcient.

A common type of temperature sensor used to convert thermal potential difference to electrical potential difference. More information on thermocouples
can be found at the NIST ITS-90 Thermocouple Database

Throughput Rate
The data, measured in bytes/s, for a given continuous operation.
A device that responds to a physical stimulus (heat,
light, sound, pressure, motion, flow, and so on), and
produces a corresponding electrical signal.

Transfer Rate
The rate, measured in bytes/s, at which data is moved from source to destination after software initialization and set up operations; the maximum rate
at which the hardware can operate.


Universal Asynchronous Receiver/Transmitter. Used
for translating data between parallel and serial interfaces by converting bytes of data to and from asynchronous start-stop bit streams.

A signal range that is always positive (for example, 0
to +10 V).

Uninterruptible Power Supply (or source). Designed to maintain electrical power to electronic devices in
the event of utility power failure.

Universal Serial Bus. Developed as a serial communications standard to replace Serial and Parallel


The symbol for Volt
Voltage AC (alternating current)
The unit of measure for electrical potential difference across a conductor.
The value of electrical potential difference across a
conductor expressed in volts.

Voltage-to-Frequency Converter
A device that converts analog input voltage into a
sequence of digital pulses at a frequency proportional to the analog input voltage.

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Wide Area Network. A wide area network is typically made up of two or more Local area Networks, or LANs
Wheatstone Bridge
A network of four resistors that allows a data acquisition system to measure very small changes in the resistance of sensors such as strain gages, load cells,
and similar measurement devices which change resistance in response to some external stimulus.

Wireles-Fidelity. Wi-Fi is a brand of high-frequency
wireless local area network (WLAN) licensed by the
Wi-Fi Alliance trade group, and is based on the IEEE
802.11 specifcations.


The horizontal axis on a graph


The horizontal axis perpendicular to the x-axis on a


The third axis in a three-dimentional coordinate graph perpendicular to the plane defned by the x
and y axes.

Zero-Overhead Looping
The ability of a high-performance processor to repeat instructions without requiring time to branch to
the beginning of the instructions.

Zero-Wait-State Memory
Memory fast enough that the processor does
not have to wait during any reads and writes to the memory.