Glossary List
Accuracy. deviation of a measurement from a known standard
AC. alternating current -a periodic current whose average value
overtime is zero
A/D converter. analog to digital converter -measures electric potential
and provides results in digital words (see also multimeter and voltmeter)
Amplifier. electronic device that increases the voltage of a signal
Analog output. output signals that can vary on a continuum as compared
to outputs that vary in discrete steps
Analog signal. electrical signal that depends solely on magnitude
to express information (continuum of magnitudes)
Arbitrary waveform generator. a digital-to-analog converter that
can output a timed string of analog signals
Attenuator. electrical device that reduces the amplitude of a signal
without distorting it
Backplane. electrical device to which measurement or control hardware
(cards) plug into. Backplane is used to pass commands and data to cards plugged
into a cardcage -also found inside personal computers to hold various types of interface
cards
Bit. a unit of information expressed as 1 or 0 (high or low voltage)
-smallest component of a digital word
Bridge. electrical network used to measure a small change in resistance
-used with strain gages to get better resolution
Byte. group of bits (normally 8) -read or operated on as a unit
Capacitive coupling. interference induced from one wire to another
by an electrical field
Channel. a single path from a transducer to a measurement instrument
Clock. timing device in computers and electronic instrumentation
Closed loop control. a control system in which a quantity measured
is compared against a desired value -any deviation is fed back into the system to
reduce the difference between the measured and desired value
Common mode noise. electrical interference on both signal leads
of an analog measurement which change simultaneously relative to ground
Comparator. electronic circuit that performs a selection between
two analog signals
Completion resistor. high accuracy resistor used in data acquisition
to change a current measurement into a voltage measurement
Control hardware. electronic equipment used to direct the operation
of external devices in a data acquisition application
Counter. electronic equipment used to measure the occurrences,
frequency, period, or pulse width of electrical signals
Crosstalk. undesired energy appearing on one signal as a result
of signals on nearby paths
Current (I). flow of electrons in a material -measured in amperes
(amps)
DAC. data acquisition and control
Data Acquisition. use of electronic equipment to make and record
physical measurements
D/A converter. electronic equipment that converts a digital word
into an analog signal
DC. direct current -non-pulsating current or voltage with little
change in value over time
Dielectric absorption. phenomenon where electrical charges remain
in a material and dissipate slowly over time
Differential. measurement technique in which the sensed value is
the difference between two input terminals
Digital input. measurement in which one or more channels is monitored
to record the existence or lack of a voltage -assigned a value of 0 or 1
Digital output. digital signal (high or low voltage) on one or
more channels sent out by control hardware to turn on/off lights, fans, heaters,
or external devices that operate based on a digital word
Digital signal. electrical signal in which only the presence or
absence of a voltage is important, not its amplitude
Digital word. string of 8,16 or 32 bits (high/low voltages) that
a computer interprets as a symbol, letter, or number -groups of words can be sent
from the computer to the DAC hardware as commands or from the DAC hardware to the
computer as data
Error value. difference between measured value and setpoint in
closed-loop control
FET switch. field-effect transistor switch -a solid state device
that either allows or prohibits a signal from passing through it
Filter. electrical circuit that eliminates or reduces certain frequencies
of signals
Firmware. microcode usually stored in read-only memory used to
perform computer functions -used in instruments to interpret commands sent by computers
Four-wire ohms. resistance measurement made with four wires, two
from the current source to the device and two from the measurement hardware (multimeter)
to the device
Frequency. rate of occurrence of a string of pulses -measured by
a counter
Gate. electronic device that, depending on one or more inputs,
has the ability to permit or inhibit the passage of a signal -used in counters to
record pulses only when gate is “on”
Gated totalize. counter function used to measure pulses on one
channel only when the signal on another channel is on (high or low voltage)
Guard. a third signal line used to prevent unwanted electrical
currents from reaching the voltmeter -guard terminals are found on highly accurate
voltmeters
HP-IB. Hewlett-Packard Interface Bus (also known as GPIB) -implementation
of the IEEE-488 standard -mechanical and electrical protocol (parallel) for sending
data and commands between instruments and computers
Human interface. portion of a software program displayed on the
computer screen to display instructions or data
Hysteresis. difference in input signal values that effect the same
output value -(for example, a device is turned on when the signal is at 4 V and
off when the input value is 1 volt; hysteresis is 3 V) – used to prevent devices
from cycling on and off rapidly when input value is near the transition point
Integrating A/D. an A/D converter (voltmeter) that uses an integration
technique in order to reduce ac noise present on the input signal
Integration. technique used in some voltmeters to eliminate ac
noise by summing the voltage amplitude during a period of time
Interface. mechanical, electrical, and software elements between
electronic devices that allow them to communicate
I/O. Input/Output -the interface between a computer and a peripheral
or instrument
Isolation. electrical separation of one section of a system from
undesired influences of another
Isothermal block. terminal block specially designed to be at a
constant temperature. Used with thermocouples
Measured value. data obtained by the measurement hardware and compared
to the setpoint to determine an error value
Measurement hardware. electronic equipment used to make measurement
on physical phenomena (temperature, pressure, flow, etc.)
Menu-driven software. application software where the user tells
the software what to do by filling out screens or choosing selections from a menu
of choices Microprocessor. electronic device that controls the operation of a computer
Multimeter. electronic instrument that converts electrical (analog)
signals into digital words (data) -measures voltage, current, and resistance (see
also A/D converter and voltmeter)
Multiplexer. electrical device that can switch signals, one at
a time, to a measurement instrument -mechanical relays or solid state switches are
used to perform this function
Noise. electrical interference that causes a deviation in a signal
Noise rejection. the ability of measurement hardware to eliminate
the noise content of a signal in order to make an accurate measurement
Normal mode noise. voltage differentially between two wires that
appears to be part of the input signal
Offset. compensated ohms. Resistance measurement in which two readings
are taken -one of which is used to cancel the effect of unwanted voltages in the
external circuit
Ohms Law. basic electrical principle where voltage = current times
resistance (V=IR)
On/Off control. simplest form of control where the output signal
either turns on or off an external device
Open-loop control. control where the output does NOT depend on
the measurements being taken -output is predetermined by the user, stored in memory
or on disk, and output at the programmed time (see stimulus)
Plug-in card. electronic equipment that fits inside a computer
or instrument cardcage and is used to make measurements or control external devices
Period. time between pulses or identical points on a repetitive
waveform -measured by a counter
PID control. proportional-integral-differential type of closed-loop
control in which the output signal is a combination of the linear value, plus time
integral, plus time rate of change of deviation between the input signal and value
desired
Point. another name for a channel (see channel)
Power. electrical energy -voltage times current -measured in watts
Power line noise. electrical interference caused by radiations
from power cables or machinery in the vicinity of the signals to be measured
Programming software. application software to be generated by the
user for a specific purpose as opposed to menu-driven software that is already written
Pulse. digital signal that stays at a level (high or low) for only
a brief time -counters are used to measure the various parameters of these signals
Pulse output. electronic device that sends out digital signals
(pulses) – usually used to control stepper motors
Pulse width. length of time a digital signal is at a high (or low)
voltage level
RAM. random access memory -electronic devices used to store data
and programs in computers and instrumentation
Rectifier. an ac to dc voltage converter
Relay. electromechanical device for routing electrical signals
-used in multiplexers and switch cards
Repeatability. the closeness of agreement among repeated measurements
Resistance. opposition to current -measured in ohms
Resistor. electrical device where a voltage potential is developed
across it when a current flows through it
Resolution. smallest change that a measuring instrument can sense
in an input signal
RTD. resistive temperature device -a transducer that changes resistance
proportional to its temperature
Sample/Hold. measurement hardware that is used to capture events
on multiple channels -when triggered, hardware will hold value of input until a
measurement can be made
Scanning. making measurements on several channels one at a time,
one right after another -multiplexers are used in data acquisition to accomplish
this task
Scheduler. software that controls when certain tasks will be performed
– in data acquisition the scheduler plans when measurements will be taken and when
data will be stored
Sensor. device that converts a physical parameter such as temperature,
pressure, flow, strain, or position into an electrical signal -sometimes used synonymously
with transducer -a true sensor contains signal conditioning so its output is more
easily measured
Setpoint. the desired value from an external device -deviations
from the setpoint are used in closed-loop control to adjust output values
Shield. an extra layer of conductive material surrounding a wire
to prevent external electrical signals from interfering with the signal on the wire
Signal conditioning. to electrically amplify, reduce, eliminate,
or change a signal – used to alter signals (eliminate noise) before they are measured
in order to get more accurate measurements
Single-ended measurement. measurement technique where one wire
is attached to the device, earth ground serves as a return path -not recommended
for accurate measurements Software program. (set of instructions) that causes a
computer to perform a specific function
Stepper motor. electro-mechanical device that rotates (or moves)
proportional to pulses it receives from a stepper motor controller -DAC systems
sometimes contain stepper motor controllers -used for positioning equipment
Stimulus. electrical signals from data acquisition hardware used
to control external devices
Stress. force per unit area -cannot be measured directly (see strain)
Switch. electronic or electromechanical device that is used to
route electrical signals or power
Terminal block. a piece of hardware to which wires coming from
transducers connect to the data acquisition system
Thermistor. temperature transducer that changes resistance proportional
to its temperature
Thermocouple. a pair of dissimilar conductors that are connected
together at a point and produce a voltage proportional to temperature
Throughput. the total time it takes a measurement system to switch
and convert a signal into data as well as transmit, store, and display the results
Timestamp. a reading of the clock that coincides with a measurement
or group of measurements
Totalize. to measure the number of digital signals (pulses) performed
by a counter
Transducers. devices that convert physical parameters such as temperature,
pressure, flow, strain, or position into electrical signals such as voltage, current,
or resistance
Tree switch. an additional switch on a multiplexer whose purpose
is to reduce capacitive coupling between signal lines
Two-wire ohms. resistance measurement made with two wires to the
device -current is supplied by the multimeter
Up/Down Count. measurement to increment the count if the digital
signals occur on one channel and to decrement the count if they occur on another
channel -performed by some counters
Voltage (I). difference in electrical charge between two points
-measured in volts
Voltmeter. electronic instrument that measures electrical voltages
(see multimeter and A/D converter)
