Glossary of Data Acquisition Terms
Data Translation®
A
Accelerometer – A device that senses acceleration. Typically, crystals that emit a voltage
proportional to the acceleration.
Accuracy – The combination of all error factors in a data acquisition device. This should indicate
the maximum deviation from an NIST standard for any reading. Some manufacturers do not
include all error factors in the specification that they call accuracy.
Acquisition Speed – See Sample Rate.
Active filter – An electronic filter that combines active circuit devices, usually amplifiers, with
passive circuit elements, such as resistors and capacitors. Active filters typically have
characteristics that more closely match ideal filters than do strictly passive filters.
A/D – See Analog-to-Digital Conversion.
Algorithm – A set of rules or detailed plan, with a finite number of steps, for solving a problem.
An algorithm can be used as a model for solving a problem, or for creating a computer program.
Alias frequency – A false lower frequency component that appears in analog data reconstructed
from original data acquired at an insufficient sampling rate.
Aliasing – The effect in which low frequencies, that did not exist, appear in data after it is
digitized by a data acquisition device. This is due to frequencies in the analog input that are more
than half the sample rate.
Alternating Current (AC) – Electric current whose flow alternates in direction. The number of
times the current changes direction in one second is called frequency.
Amplifier – The electronic device that amplifies an analog input signal. In a data acquisition
device the amplifier precedes the A/D converter and amplifies the signal to a level that the A/D
converter can accept. See Gain.
Amplitude – The size or magnitude of a signal.
Analog – A signal or number that can have any value between two extremes, such as
temperature. It can change continuously, unlike digital signals that change in discrete steps. See
Digital.
Analog Input – An analog signal that is measured by a data acquisition device. Analog inputs
refer to the number and type of analog input channels available on a data acquisition board.
Analog Output – An analog signal that is generated by a data acquisition device and sent out to
be measured or used by another device. Analog outputs refer to the number and type of analog
output channels available on a data acquisition board.
Analog-to-Digital Conversion (A/D) –The process of changing an analog signal into a digital
value that represents the magnitude of the signal at the moment of conversion.
Analog-to-Digital Conversion Time (A/D Conversion Time) – The length of time a data
acquisition board requires to convert an analog signal into a digital value.
Analog-to-Digital Converter (A/D Converter) – An electronic device, often an integrated circuit,
that converts an analog input signal into a directly proportional digital representation that can be
stored and processed in a computer.
Antialiasing Filter – A filter that removes frequencies from a signal that are higher than the cutoff frequency. When the sample rate is more than twice the cut-off frequency this filter prevents
aliasing, in which low frequencies, that did not exist, appear in data after it is digitized (converted
to digital) by a data acquisition device. This filter must be implemented in hardware in front of the
A/D converter.
Asynchronous – A communications protocol where information can be transmitted at an
arbitrary, unsynchronized point in time, without synchronization to a reference timer or clock.
Asynchronous Mode – See Polled Mode.
B
Base address – A memory or I/O address that serves as a point of reference. All other points are
located by offsetting (adding to or subtracting from) in relation to the base address.
Binary-Coded Decimal – A code for representing decimal digits in a binary format.
BIOS (or Built In Operating System) – The software that is built into a PC, rather than residing
on a disk drive. This is the most basic software that allows the PC to start operating (boot),
perform self diagnostics, and load more software from a disk or network. Ports such as the
parallel and serial port are usually accessed by software by using code in the BIOS.
Bipolar – A signal range that includes both positive and negative values. Bipolar inputs are
designed to accept voltage in the ±X Volts range, allowing positive or negative voltage inputs, for
example, ±5V.
Bit – One binary digit, either 0 or 1. There are eight bits to one Byte (or character); there are 1024
Bytes to one Kilobyte, and 1024 Kilobytes to one Megabyte.
Block – A portion of the data acquisition buffer that is processed by the application at one time. It
consists of one or more samples of input data when referring to an input buffer. It consists of one
or more samples of output data when referring to an output buffer. Blocks are used in interrupt or
synchronous data acquisition, not polled or asynchronous data acquisition.
Board Address – A code or address that is unique for each card plugged into a PC bus, such as
the PCI bus.
Bridge – An arrangement of sensors to measure small changes in large values where four equal
value resistors or sensors are arranged with two power inputs and two signal outputs.
Buffer – That portion of RAM in the PC that is allocated to receive input data from a data
acquisition device. A buffer must be physical RAM, not virtual RAM that is swapped to disk.
Burst Acquisition – Acquisition of data by a data acquisition device at a high sample rate for a
short period of time. Burst mode is fast because the data is stored in a buffer on the data
acquisition device or in PC RAM without further processing.
Bus – Conductors used to interconnect individual circuitry in a computer. The conductors as a
whole are called a bus. All communication between the computer and the data acquisition board
occurs through the bus.
Bus Master – A type of bus interface architecture that requires minimal CPU involvement during
the transfer of data on the PCI Bus. A bus master can also use “burst mode” for faster data
transfer. Devices that are bus masters have priority over bus slaves when being serviced.
Bus Slave – A type of bus interface architecture that requires CPU involvement during the
transfer of data. Bus slaves have lower priority than bus masters when being serviced.
Byte – A term referring to eight related bits of information. Eight bits equals one byte.
C
Cache – High-speed processor memory that buffers commonly used instructions or data to
increase processing throughput.
Calibration – The process of adjusting the electronics of an analog input to improve its accuracy.
Sometimes this is done by turning potentiometers with a screwdriver. A more convenient way is
to make the adjustments in software.
Call – A software instruction used to pass control to a subroutine of a program. At the completion
of this subroutine, control is returned to the original program at the point of the “call” statement.
Often used for specialized routines such as “analog read” from a data acquisition system.
Central Processing Unit (CPU) – The central part of a computer system that performs
operations on data. In a personal computer the CPU is typically a single microprocessor
integrated circuit.
Channel – A single analog input or digital input on a data acquisition device to measure one
signal.
Chart – The display of a signal on a graph, either versus time or versus another signal.
Chart Recorder – A data acquisition device that draws a chart on paper. Many chart recorders
also have other functions.
Chromatograph – An instrument used in chemical analysis of gases and liquids.
Cold-Junction Compensation – A method of providing an artificial reference level and
compensation for ambient temperature variations in thermocouple circuits.
Cold Junction Sensor – A device that measures the temperature of the terminals to which a
thermocouple is connected. It provides a temperature reference that is needed
to measure temperature from a thermocouple.
Common Mode Input Voltage – The maximum voltage that can be applied to both sides of a
differential input.
Common Mode Rejection – The degree to which a differential analog input rejects a signal that
is applied equally to both the ‘+’ and ‘-‘ input. Ideally such a signal results in no measured
response.
Common-Mode Rejection Ratio (CMRR) – The ratio of a device’s differential voltage margin
(Adm) to its common mode gain (Acm). Expressed in decibels, CMRR is a measure of a device’s
ability to reject interference appearing as common mode signal. CMRR = 20 log (Adm/Acm).
Compiler – A particular type of high-level language used to preprocess a program to convert it to
a form that a processor can execute directly.
Contact Closure – The closing of a switch, often controlled by an electromagnetic or solid-state
relay.
Continuous Acquisition – Acquisition of data by a data acquisition device at a sample rate that
can continue indefinitely. Continuous mode allows continuous uninterrupted acquisition which is
useful for control applications or where a display is needed continuously.
Conversion Time – The time required, 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. This could include switching time, settling time, acquisition time, A/D conversion
time, and so on.
Counter – In software, a memory location used by a program for the purpose of counting certain
occurrences. In hardware, a circuit that can count pulses.
Counter/Timer Input – A signal input on a data acquisition device that accepts digital (on-off)
signals, but can count changes (or pulses) rapidly in hardware. A digital input, by contrast, is
limited by software in the speed at which it can count input pulses.
Crosstalk – Leakage of signals, usually via capacitance between circuits or channels of a multichannel system or device, such as a multiplexer, multiple op amp, or multiple DAC. Crosstalk is
usually determined by the impedance parameters of the physical circuit, and actual values are
frequency-dependent.
Current – The flow of electrons in an electronic circuit, expressed in units of Amperes.
Current Loop – A communications method that allows data to be transmitted over relatively long
distances and through relatively high-noise environments. With a current loop, the voltage levels
are converted to currents so that the signals are transmitted in the form of current instead of
voltage in a closed-loop circuit. Current loops are relatively insensitive to noise.
Current Sink – The amount of current the board can supply for digital output signals. With 10-12
mA or more of current sink capability, you can turn relays on and off. Digital I/O boards with less
and 10-12 mA of sink capability are designed for data transfer only, not hardware power relay
switching.
Cut-Off Frequency – The highest frequency in a signal that can pass through a filter (especially
an antialiasing filter) without being reduced in amplitude more than 30% (or 3 dB). Above the cutoff frequency the signal is rapidly attenuated.
D
Data Acquisition – The process of reading analog inputs and digital inputs and storing or
displaying them. Usually it refers to reading the inputs into a device that stores and processes the
data digitally, as in a PC.
Data Analysis – Processing of data by math, signal analysis, or other functions that change the
data so it can be analyzed.
Data Manipulation – See Data Analysis.
Data Reduction – The process of analyzing a large volume of data to extract and refine a
subset of data for some purpose, as in statistical summarizing of data.
Datalogger – A data acquisition device that can read analog or digital inputs and store them,
usually in RAM. Devices that are referred to by this name generally have slow sample rates, a
few samples per second or less, and operate on a built-in battery.
dB (Decibel) – See Decibel.
DDE (Dynamic Data Exchange) – A feature in Windows that allows data to be transferred in real
time between applications. Both applications must support this feature. Data transfer rates are
generally a few samples per second or less.
Deadband – The amount of hysteresis in a control. For example, if a thermostat turns a furnace
on when the temperature is 22° C, and turns it off when the temperature is 23° C, the thermostat
has a one degree deadband. If the thermostat turns the furnace off and on at the same
temperature, the thermostat has no deadband.
Decibel (dB) – A logarithmic measure of the ratio of two signal levels: dB=20 Log10 (V1/V2) =10
Log10 (P1/P2)
Device Driver – See Driver.
Differential Input – An analog input on a data acquisition device that has two inputs (‘+’ and ‘-‘)
that are separate from any other input, and separate from ground. This type of input is more
expensive but more accurate than a Single-Ended Input.
Differential Non-Linearity – See Linearity.
Digital – A signal or number that changes in discrete steps, such as a switch which is either off or
on, or a number representation in a computer which is represented by a
series of zeros and ones. It differs from an analog signal that can have any value between two
extremes. See Analog.
Digital Input – A digital signal that is measured by a data acquisition device.
Digital Meter (hardware) – A data acquisition device that has a built-in display (a meter) that
shows the value in numbers.
Digital Meter (software) – A software representation of a hardware digital meter. It displays a
value as a number.
Digital Output – A digital signal that is generated by a data acquisition device and sent out to be
sensed or used by another device.
Digital Signal Processing – See DSP.
Digital-to-Analog Conversion – The process of changing discrete data into a continuously
varying signal. Common uses are to present the output of a digital computer as a graphic display
or as a test stimulus.
Digital-to-Analog Converter (DAC) – A device that converts digital information into a
corresponding analog voltage or current.
DIP Switch – A set of switches contained in a dual inline package.
Direct Memory Access (DMA) – A method by which data can be transferred from the computer
memory to a device on the bus while the processor does something else. One of two methods of
data transfer, the other method being programmed I/O.
DMA – See Direct Memory Access.
Disk File Buffer – A portion of RAM in a PC where data is placed prior to being written to disk.
When the disk file buffer is full the contents are written to disk all at one time. If the PC loses
power before the disk file buffer is written to disk, the data is lost.
Drift – The change due to temperature in the measurement of an analog input by a data
acquisition device. It may refer to the change in accuracy, or the change in just one of
the factors that makes up accuracy, such as gain or offset.
Driver – The software that is used to control a specific hardware device such as a data
acquisition board or a printer.
DSP (Digital Signal Processing) – The processing of analog signals in the digital domain. Realworld signals, such as voltages, pressures, and temperatures, are converted to their digital
equivalents at discrete time intervals for processing by the CPU of a digital computer. The result
is an array of numerical values stored in memory, ready to be processed.
Dynamic Range –The ratio of the full scale range (FSR) of a data converter to the smallest
difference it can resolve. Generally expressed in dB, DR=20log2n. “n” is the resolution in bits.
E
Ethernet Port – The connection on a PC that connects to an Ethernet network.
ENOB (Effective Number of Bits) – A comprehensive figure of merit that specifies a data
acquisition board’s accuracy and performance.
Event Counter – A circuit used to count pulses that are related to the occurrences of a certain
condition. An event counter can typically be preset, reset, and can sum.
External Trigger – Sampling can be triggered by a voltage pulse from an external source.
F
FFT (Fast Fourier Transform) – A mathematical transformation that converts time based data
into frequency based data. An FFT is performed on data to see the frequency content.
Filter – See Antialiasing Filter.
Filtering – Attenuates components of a signal that are undesired; reduces noise errors in a
signal.
FIFO (First-In, First-Out) – The first data value placed in the buffer (queue) is the first value
subsequently read.
Floating-Point Numbers – Numbers that contain decimal parts or are presented in scientific
notation (digits multiplied by a power of 10). Also known as “real” numbers. Integers are a subset
of reals containing whole numbers only.
Frequency – The rate of repetition of changes, measured in Hertz (Hz). One Hz is one cycle per
second.
Front End – The preprocessing of data before a program uses it. Could refer to signal
conditioning in a data acquisition system.
G
Gain – The factor by which a signal is amplified, sometimes expressed in dB. The same as
amplification. Also used instead of Range. Gain is the inverse of Range. See Amplifier.
Gain Stage – An electronic circuit used to amplify a signal.
Ground – An electrically neutral wire having the same potential as the surrounding earth.
Normally, a non-current carrying circuit intended for safety. A reference point from an electrical
system.
H
Harmonic Distortion (HD) – A form of distortion in analog circuits in which harmonics (signals
whose frequency is an integer multiple of the input signal) are generated. Measured in decibels
(dB), harmonic distortion is calculated as the ratio of a single harmonic to the level of the original
signal. Also related is total harmonic distortion (THD), the ratio of the sum of multiple harmonics
to the level of the original signal.
Hertz (Hz) – A unit of measurement for frequency. One Hertz is one cycle per second.
Hexadecimal – A numbering system to the base 16.
High Level Voltage – A voltage that is approximately one volt or more. A high level analog input
to a data acquisition device does not need much amplification and is not very susceptible to
noise.
High-Pass Filter – An operation that attenuates low frequency components of an analog signal.
Hysteresis – The lagging of an effect behind its cause; especially the phenomenon in which the
magnetic induction of a ferromagnetic material lags behind the changing magnetic field.
Hz (Hertz) – See Hertz.
I
IEEE488 or GPIB – A standard for transmitting data between instruments at speeds that can
exceed 100,000 samples per second. The cable has 24 wires. PCs do not have an IEEE488 port
built in. It must be added with a plug-in card.
Impedance – The total opposition to alternating current provided by a circuit.
Input Impedance – The physical impedance of a circuit to the voltage applied to it.
Input/Output (I/O) – The process of transferring data from or to a computer system including
communications channels, operator interface devices, or data acquisition and control channels.
Input Range – Selectable voltage ranges that a data acquisition board accepts as input. Can be
specified as a numeric value (±10 V) or bipolar/unipolar.
Instrumentation Amplifier (IA) – An amplifier circuit with both high-impedance differential inputs
and high common-mode rejection.
Integral Non-Linearity – See Linearity.
Integrating A/D Converter – An A/D conversion technique in which the analog input is integrated
over time. Different types of integrating A/D converters include dual slope, triple slope, and
charge balancing.
Interrupt – A computer signal indicating that the CPU should suspend its current task to service a
designated activity.
Interrupt Handler – The section of a program that performs the necessary operations to service
an interrupt when it occurs.
Interrupt Mode (or Synchronous Mode) – Data acquisition where the sample rate is paced by
an IRQ or other clock. The rate is generally quite precise in interrupt mode. See Polled Mode.
I/O (Input/Output) – See Input/Output.
I/O address – A method that allows the CPU to distinguish between the different boards in a
system. All boards must have different addresses.
IRQ (Interrupt Request) – The signal line on a the interface bus of a PC or on one of the ports of
a PC that sends an interrupt signal to interrupt the processor to have it execute different software
code for a short period of time. The IRQ line is usually used to pace data acquisition in interrupt
or synchronous mode.
ISA (Industry Standard Architecture) Bus – The connectors in a PC that accept plug-in cards
having 62 pins and often a second connector with 36 pins. This bus is slower than the PCI bus,
with a maximum transfer rate of 3-5 MB/s.
Isolation – Two circuits are isolated when there is no direct electrical connection between them.
Isolation Amplifier – An amplifier with electrically isolated inputs and outputs which allows it to
amplify a differential signal superimposed on a high common mode voltage.
Isolation Module – A small module that provides isolation and protection from high voltages
between a sensor and the analog input of a data acquisition device.
Isolation Voltage – The voltage that an isolated circuit can normally withstand. Isolation voltage
is specified from input to input or from any input to the amplifier output, or to the computer bus.
Isothermal – A process or area that is maintained at a constant temperature.
Isothermal Block – A piece of thermally conducting material that keeps all the screw terminals of
multiple analog inputs at the same temperature as the cold junction sensor. This is an important
part of an accurate multi-channel thermocouple input.
K
k – Kilo. In referring to computers, a “kilo” is 1024 or 210.
L
Latch – A term used to indicate that the state of a digital signal will remain stored until changed
by a CPU or external command signal.
Linear – A general term for a system that has an output directly proportional to the input signal.
Linearity – The deviation of the analog values from a straight line. Linearity error can be divided
into differential non-linearity and integral non-linearity. Differential non-linearity means any
deviation of the “step” from the ideal width. Integral non-linearity refers to deviations of the overall
shape of the conversion, not individual steps, over the full voltage range.
Low Level Voltage – A voltage that is much less than one volt. A low level analog input to a data
acquisition device needs to be amplified and is susceptible to noise.
Low-Pass Filter – An operation that attenuates high frequency components of an analog signal.
LVDT (Linear Variable Differential Transformer) – A device that measures position or
displacement with a moving iron core surrounded by two coils of wire. The unconditioned output
is an AC signal with an amplitude that is proportional to position.
M
Millisecond (msec) – One thousandth of a second or 0.001 s.
Modular – The use of building blocks (modules) in a computer or data acquisition system. A
modular device, for instance, is one that is built, tailored, and expanded by connecting various
mutually compatible components.
Modulation – The process of modifying the amplitude, frequency or phase of a carrier wave by a
modulating signal.
Monotonicity – The desirable characteristic of a digital-to-analog converter to produce a
continually increasing analog output for a correspondingly increased digital input code.
Multiplexer – The part of a data acquisition device that switches multiple input channels into a
single amplifier and A/D converter. Some data acquisition devices have no multiplexer. Instead
they have a separate amplifier and A/D converter for each channel. Also called mux.
N
NIST – National Institute of Standards and Technology, the organization in the United States that
maintains standards for measurements to which all instruments need to be calibrated.
Noise – Random electrical signals that are not part of the desired signal or sensor output. Noise
comes from power lines, radio transmissions, coupling from adjacent signal wires, and even from
the electronics in a data acquisition device.
Non-Destructive Testing – A method of inspecting materials and products without affecting their
subsequent properties and performance.
Non-Linearity – Linearity error. Linearity error can be divided into differential non-linearity and
integral non-linearity. Differential non-linearity means any deviation of the “step” from the ideal
width. Integral non-linearity refers to deviations of the overall shape of the conversion, not
individual steps, over the full voltage range. See Linearity.
Nonvolatile – A memory or data storage device that retains its information content when
electrical power is removed. Ordinary RAM is volatile whereas ROM, bubble memory, batterybacked-up CMOS RAM, floppy, and hard disks are nonvolatile.
Nyquist Sampling Theorem – If a continuous bandwidth limited signal contains no frequency
components higher than a specified frequency then the original signal can be recovered without
distortion if it is sampled at a rate of at least twice the specified frequency.
O
Offset –The deviation from zero that an analog input measures when a zero volt signal is applied.
In one sense it is an error factor in the analog input of the data acquisition device. In another
sense it is a measure of the amount of correction a sensor needs. For example, if a pressure
sensor outputs 2 volts at a pressure of zero, it has an offset of 2 volts.
Ohm – Standard unit of electrical resistance.
On-Line Processing – See Real Time Processing.
Output – Information leaving a device.
OPC (OLE for Process Control) – An open industry -standard device interface that provides
interoperability between disparate field devices, automation/control systems, and business
systems.
Oscilloscope – A laboratory instrument commonly used to display and analyze the waveform of
electronic signals. In effect, the device draws a graph of the instantaneous signal voltage as a
function of time.
P
Parallel Port – The connector on a PC that connects to printers and other devices. It is also
called an LPT port. It has 25 pins and can transmit data about 100,000 samples per second,
depending upon the type of parallel port.
Passive Filter – A filter circuit using only resistors, capacitors, and inductors (no active devices
such as integrated-circuit amplifiers).
PCI (Peripheral Component Interconnect) – A high-performance, 32-bit expansion bus
architecture offering a theoretical maximum transfer rate of 132 MB/s. It accepts plug-in cards
having 112 pins. This bus operates faster than the ISA bus.
PCMCIA (Personal Computer Memory Card International Association) – A high-performance
expansion bus architecture for notebook computers.
Peripheral – The input/output and data storage devices attached to a computer such as disk
drives, printers, keyboards, displays, data acquisition systems, etc.
PID (Proportional Integral Derivative Control) – A type of proportional control that overcomes
some of its disadvantages by including an integral and derivative calculation. The control signal
that is generated is usually an analog output, although it can be a pulse output with a variable
duty cycle or frequency.
PLL (Phase-Lock Loop) – An analog circuit that uses feedback to synchronize the phase of a
voltage-controlled oscillator with the phase of an incoming or reference signal.
Plug-In Card – Any card that plugs into a bus connector, such as ISA, PC Card, PCI, VME, or
VXI.
Pole – A characteristic that determines how well a filter rejects frequencies slightly above the cutoff frequency. A larger number of poles means higher frequencies are rejected better. It often
means that the amplitude of frequencies just below the cutoff frequency is affected as well.
Polled Mode (or Asynchronous Mode) – Data acquisition where the sample rate is determined
by the application software. The rate is generally not very precise in polled mode. Also see
Interrupt Mode.
Polling – A round-robin canvassing of data acquisition inputs synchronized in software to a clock
or external trigger.
Port – A communications connection on a computer or a remote controller.
Potentiometer – A device with a variable resistance that is adjusted by a knob or a screwdriver.
Programmable Gain – Applied to the incoming signal, gain acts as a multiplication factor on the
signal, increasing the number of ranges the board is designed to accept. For example, if the ±5V
range is selected and the fain is set to 10, signals in the ±0.5V (500mV) range are usable; with a
gain of 20, the range would be ±250 mV.
Programmed I/O (PIO) – A method of transferring data in which all data must pass through the
computer processor. One of two methods of data transfer, the other method being direct memory
access).
Proportional Control – Control where the control signal is an analog output (or sometimes a
pulse output with a variable duty cycle or frequency). The analog output is equal to the difference
between the input signal and a set point, multiplied by a proportional constant. See PID Control.
Protocol – The exact sequence of bits, characters, and control codes used to transfer data
between computers and peripherals through a communications channel.
Pseudosimultaneous Sampling – The process where each channel of a data acquisition device
is read rapidly, or nearly simultaneously, independent of the sample rate. There is a short delay
between reading each channel and a large delay before repeating the reading of each channel for
the next sample.
Pulse – A temporary change in voltage of any length.
Pulse Counting – The counting of the number of times a digital signal transitions from low to
high (or sometimes from high to low).
Q
Quadrature Encoder – A device that generates two pulse outputs that, when counted, represent
position. The two pulse outputs differ in phase in such a way that a special type of pulse counter
can determine the direction of motion.
Quantization – The process of transforming a continuous signal into one of finite steps or levels,
as in an A/D converter.
Quantizing Error – The inherent uncertainty in digitizing an analog value due to the finite
resolution of the conversion process. This error can be reduced only by increasing converter
resolution.
Queue – A temporary storage location or list of things to be done such as messages that are
awaiting transmission.
R
R/W – Read/Write.
RAM (Random Access Memory) – Computer memory that allows data to be
read or written at a particular location without having to pass sequentially through preceding
locations. Also the memory in a data acquisition device buffer. See Buffer.
Range – The voltage input span that can be measured by a device from the lowest to highest
voltage. The maximum allowable full-scale signal (input or output) that yields a specified
performance level. See Gain.
Rate Generator – A device that provides a TTL-level pulse output at a software programmable
frequency.
Read-Only Memory (ROM) – Computer memory in which data can be routinely read but written
to only using special means when the ROM is manufactured. ROM is used for storing data or
programs on a permanent basis.
Real-Time – Data acted upon immediately instead of being accumulated and processed at a later
time.
Real Time Processing – Performing of mathematical or signal analysis functions during data
acquisition. This allows data to be scaled, transformed, or reduced before it is stored or
displayed. It differs from off-line processing which is performed after data is acquired and stored
in memory or on a disk.
Relay – Electromechanical device that opens or closes contacts when a current is passed
through a coil.
Repeatability – The ability of a data acquisition analog input to give the same reading when the
same signal is applied under identical conditions. This is less rigorous than accuracy, since the
reading is not compared to an NIST standard.
Resistivity – The ratio of an electric field to current.
Resolution – The smallest significant number to which a measurement can be determined. For
example, a converter with 12-bit resolution can resolve one part in 4096.
Ribbon Cable – A flat cable in which the conductors are side by side rather than in a bundle.
ROM (Read-Only Memory) – See Read-Only Memory.
Routine – A self-contained program designed to accomplish a specific task.
RTD (Resistance Temperature Detector) – A temperature sensor that changes resistance with
temperature. Usually has a resistance of 100 Ohms at zero degrees C. An electrical circuit
element characterized by a positive coefficient of resistivity.
S
Sample and Hold – A circuit which acquires and stores an analog voltage on a capacitor for a
short period of time.
Sample Rate – The rate that an input is sampled, in samples per second. See Acquisition Speed.
Sampling – The process of measuring the value of a signal at discrete points in time.
Scaling – The linear adjustment of an analog input signal by a data acquisition device or by
software. Scaling is the application of the constants ‘m’ and ‘b’ in the equation y = mx + b, where
‘x’ is the measured input signal, ‘y’ is the resulting value, ‘m’ is the scale factor, and ‘b’ is the
offset.
Scan – Normal channel scanning in a data acquisition system involves stepping around and
reading each input channel in turn. The scan will return to the first channel once all the channels
have been sampled.
Screw Terminal Panel – A circuit board with screw terminals or that allows convenient
connection of field signals to a data acquisition system.
Seebeck Effect – The basic principle behind thermocouples. When a circuit is created by the
junctions of two dissimilar metals and the junctions are held at different temperatures, a current
caused by the difference in temperature between the two junctions will flow.
Sensitivity – A measure of the minimum change in an input signal that an instrument can detect.
Sensor – A device that responds to a physical stimulus (heat, light sound, pressure, motion, etc)
and produces a corresponding electrical output that can be measured by a data acquisition
device. Many sensors require signal conditioning.
Serial I/O – A form of data transmission, in which the bits of each character are sent one at a
time over the line.
Serial Port – The connector on a PC that connects to peripherals such as mice and many data
acquisition devices. It is also called a COM port or RS-232 port. The connector has either 9 or 25
pins. The serial port can transmit data up to about 1,000 samples per second. The maximum
speed depends upon the baud rate and the way the data is formatted by the data acquisition
device.
Settling Time – The time required, after application of a step input signal, for the output voltage
to settle and remain within a specified error band around the final value. The settling time of a
system includes that of all the components of the system.
Shielded Cable – A cable with foil or other sheathing around it to stop radio frequency
interference and magnetic fields from generating extraneous signals on the cable conductors.
Signal – The information handled by an electronic system, usually in the form of varying electron
flow.
Signal Conditioning – Electronics that converts a sensor signal into a linear voltage that can be
measured by a data acquisition device as a value representing a physical parameter. Signal
conditioning usually is the first part of the data acquisition device, followed by a multiplexer and
an amplifier. Certain signal conditioning, such as linearization, can be done in software at a lower
cost than in hardware.
Signal-to-Noise Ratio (SNR) – The ratio of total signal to noise expressed in decibels (dB). The
larger the number, the better. Also related is the signal-to-noise and distortion ratio (SINAD), the
ratio of the input signal to the sum of noise and harmonics.
Simultaneous Sample and Hold (SS&H) – A data acquisition system in which several sample
and hold circuits are used to sample a number of analog channels at the same instant. This
eliminates time skew between channels that can cause errors in some applications. One sample
and hold per analog channel is required.
Sine Wave – Waveform of a single frequency, indefinitely repeated in time.
Single-Ended Input – An analog input on a data acquisition device that has one input (‘+’) that is
separate from any other input, and one input that is common with all other inputs. This type of
input is lower cost but less accurate than a Differential Input.
Software Trigger – Indicates that the board allows software control of data acquisition triggering.
Spike – A transient disturbance of an electrical circuit due, for example, to load variations on the
ac power line.
Square Wave – A wave that alternates between two fixed values, with very rapid rise and fall
times.
Stability – The ability of an instrument or sensor to maintain a consistent output when a constant
input is applied.
Strain Gauge – A device that measures the stretching or compression of a material by changing
its resistance. A strain gauge is usually bonded to the material and responds to the change in
length primarily in one direction. The most common strain gauge is 350 Ohms at rest. Other
common values are 120 Ohms and 1,000 Ohms. Strain gauges are always used in a bridge
configuration.
Strain Relief – A bracket or clamp used to secure a cable so that it does not become
disconnected accidentally or apply stress at the point of connection to the system.
Stream-to-Disk – The rapid transfer of data from a data acquisition device directly to a disk,
usually a hard disk, in a PC. This allows fast collection of data for an extended period of time,
limited by the capacity of the disk drive. Streaming to disk is slower than burst acquisition and
faster than continuous acquisition.
Successive-Approximation A/D Converter – An analog-to-digital conversion method 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 A/D converter. This process is
analogous to weighing an unknown quantity on a balance scale using a set of binary standard
weights.
Surge – A sudden change (usually an increase) in the voltage on a power line. A surge is similar
to a spike, but is longer lasting.
Synchronization – The coordination of the activities of several circuit elements together.
Synchronous Mode – See Interrupt Mode.
T
Terminal Panel – A circuit board with screw terminals or another connector system that allows
convenient connection of field signals to a data acquisition system.
Thermal – Coefficent of resistance. The change in resistance of a semiconductor per unit
change in temperature, over a specific range of temperature.
Thermistor – A device that changes resistance with temperature. They are nonlinear, but the
change is very large, making them more immune to noise.
Thermocouple – A temperature sensor created by joining two dissimilar metals. The junction
produces a small voltage as a function of the temperature.
Throughput – The number of results produced per unit time.
Throughput Rate – The maximum repetitive rate at which a data conversion system can operate
with a specified accuracy. It is determined by summing the various times required for each part of
the system and then by taking the inverse of this time.
Time Stamp – Information added to a message, record, or other unit of data indicating the time at
which it was processed by the system.
Total Harmonic Distortion (THD) – See Harmonic Distortion.
Transducer – A device that converts length, position, temperature, pressure, level, etc, into a
different energy form (i.e., voltage or current).
Transient – A short surge of voltage or current, often occurring before steady-state conditions
become established.
Trigger – A signal that starts data acquisition. It can be a digital signal that starts data acquisition
when it goes from low to high or from high to low. It can be an analog signal that starts data
acquisition at a particular level.
TTL (Transistor-Transistor Logic) – Referring to logic circuits consisting of two or more directly
interconnected transistors, to provide conditional switching capability.
TTL Level – A digital signal in which a low level is approximately zero volts (less than 0.8 volts),
and a high level is approximately five volts (more than 2.0 volts).
Twisted Pair – A cable that consists of individual wires wrapped around each other for carrying
data. Reduces pickup noise levels in signals.
U
Unipolar – When set to accept a unipolar signal, a channel detects and converts only positive
voltages in the 0-X Volts Range, for example, 0-10V.
USB (Universal Serial Bus) – A high-speed serial bus that allows plug-and-play capability and
use of devices external to the computer. Up to 127 devices can connect to a single port
simultaneously, using expansion hubs and cable assemblies that are widely available at low cost.
USB 2.0 increases the speed of peripheral to PC connection from 12 Mbps on USB 1.1 to 480
Mbps on USB 2.0.
V
Velocity – The rate of change of displacement.
Volatile memory – Memory that does not retain its contents when power is removed.
Volt – Standard unit of voltage.
Voltage – Quantitative expression of the potential difference in charge between two points in an
electrical field. The greater the voltage, the greater the flow of electrical current (that is, the
quantity of charge carriers that pass a fixed point per unit of time) through a conducting or
semiconducting medium for a given resistance to the flow.
Voltage-to-Frequency Converter (VFC) – A device which converts an analog input voltage into
a sequence of digital pulses with frequency proportional to the input voltage.
W
Waveform – A representation of how alternating current (AC) varies with time. The most familiar
AC waveform is the sine wave, which derives its name from the fact that the current or voltage
varies with the sine of the elapsed time. Other common AC waveforms are the square wave, the
ramp, the sawtooth wave, and the triangular wave.