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D

D/A. -- Abbreviation for digital-to-analog.

D/A (DIGITAL-TO-ANALOG) CONVERTER. -- An electronic device, often an integrated circuit, that converts a digital value to an analog voltage. D/A converters are used in many instruments to convert digital reading information into an analog signal for analog output.

DATA LOGGING -- A method of recording a process variable over a period of time. Used to review process performance.

DC (Direct current) -- An electrical current that flows in one direction.

DATA TRANSFER. -- Refers to the way data is transferred to and from memory, such as programmed I/O or DMA mode.

DEGREE -- The increments in a temperature scale, or the increments of rotation of a dial. The location of a reference point in electric or phase in a cycle, in mechanical or electrical cyclic scales. (One cycle is equal to 360 degrees).

DENSITY-- Mass per unit volume of a substance usually expressed in lbs/ft3 or grams/cm3.

Deutsche Industrial Norm (DIN) -- A set of technical, scientific and dimensional standards developed in Germany. Many DIN standards have worldwide recognition.

DEVIATION -- Any departure from a desired value or expected value or pattern. Sometimes referred to as delta.

DIFFERENTIAL INPUT ISOLATION. -- On a switching card, the isolation from signal high to low. Specified as resistance and capacitance.

DIFFERENTIAL INPUTS. -- An analog input with two input terminals, neither of which is grounded, whose value is the difference between the two terminals. See also SINGLE-ENDED INPUTS.

DIFFERENTIAL NON-LINEARITY. -- The maximum deviation of an actual quantized step width from the ideal quantized step width.

DIGITAL-TO-ANALOG-CONVERTER-- A device that converts a numerical input signal to a signal that is proportional to the input in some way.

DIN -- See Deutsche Industrial Norm.

DIGITAL I/O. -- Abbreviation for digital input/output.

DIGITAL LINES/PORTS/BITS/CHANNELS. -- In hardware, a digital line is physical hardware connection to a pin with a digital signal. A digital port is a physical grouping of digital lines. In software, a digital bit (1 or 0) is a logical representation of a digital line. A digital channel is a logical grouping of digital bits.

DIGITAL TRIGGER -- An event that occurs at a user-selected point on a digital input signal. The polarity and sensitivity of the digital trigger can often be programmed. See also TRIGGER, TRIGGER CONDITIONS, TRIGGER POLARITY, and TRIGGER SENSITIVITY.

DIRECT CURRENT -- The type of current normally supplied by batteries. It is called Direct Current (DC) because the flow of electrons (electricity) is always in the same directions; from the negative (-) pole of the power source to the positive (+) pole.

DISCRETE DEVICE. -- A class of electronic components that contain one active element, such as a transistor or diode. However, hybrids, optoelectronic devices, and intelligent discretes may contain more than one active element.

DISPLACEMENT CURRENT. -- The current that flows through a capacitor in response to the rate of voltage change across it: displacement current = C * dV/dt

DMM. -- An electronic instrument that measures voltage, current, resistance, or other electrical parameters by converting the analog signal to digital information and display. The typical five-function DMM measures DC volts, DC amps, AC volts, AC amps, and resistance.

DMA (DIRECT MEMORY ACCESS) CHANNELS.-- ISA bus PCs offer eight parallel channels for DMA mode data transfers. A number of these are reserved for exclusive use by the computer. The remainder are available for use by user-supplied I/O options, such as plug-in data acquisition cards. Also called DMA levels.

DRAM. -- Dynamic Random Access Memory. A semiconductor read/write memory chip, in which the presence or absence of a capacitive charge represents the state of a binary storage element (zero or one). The charge must be periodically refreshed.

DRIFT -- A change in reading or value that occurs over long periods. Changes in ambient temperature, component aging, contamination, humidity and line voltage may contribute to drift.

DRIVERS. -- Software that controls a specific hardware device such as a data acquisition board.

DSP. -- Abbreviation for Digital Signal Processing.

DUAL IN-LINE PACKAGE (DIP). -- A plastic or ceramic package with two rows of vertical leads.

DUTY RATIO. -- The ratio of pulse width to repetition period. Also known as Duty Cycle.

DYNAMIC DATA EXCHANGE (DDE). -- A Microsoft Windows standard mechanism for communication between programs. It allows your application to send and share data with other applications such as spreadsheets.

DYNAMIC LINK LIBRARY (DLL). -- A software module in Microsoft Windows containing executable code and data that can be called or used by Windows applications or other DLLs. DLL functions and data are loaded and linked at run time when they are referenced by a Windows application or other DLLs.

E

EARTH GROUND -- A metal rod, usually copper, that provides an electrical path to the earth, to prevent or reduce the risk of electric shock.

EAU -- Estimated annual usage is the amount of a specific part number a customer requires in a one year period.

EFFICIENCY -- The ratio of useful output energy (work) to input energy (output plus losses, or wasted energy).

EIA -- See Electronics Industries of America.

EIA/TIA -232, -422, -423 and -485 -- Data communications standards set by the Electronic Industries of America and Telecommunications Industry Association. Formerly referred to as RS (Recognized Standard).

EEPROM OR EPROM. -- Electrically-Erasable Programmable Read-Only Memory. Similar to PROM, but with the capability of selective erasure of information through special electrical stimulus. Information stored in EEPROM chips is retained when the power is turned off.

EISA BUS. -- Extended Industry Standard Architecture. A 32-bit wide, upwards-compatible extension of the 16-bit wide ISA bus. Sometimes called the XT bus.

ELECTRICAL INTERFERENCE -- Electrical noise that can obscure desired information.

ELECTRICAL NOISE -- See noise.

ELECTROMAGNETIC COMPATIBILITY (EMC) -- The ability of equipment or a system to function as designed in its electromagnetic environment without introducing intolerable electromagnetic disturbances to that environment, or being affected by electromagnetic disturbances in it.

ELECTROMAGNETIC INTERFERENCE (EMI) -- Electrical and magnetic noise imposed on a system. There are many possible causes, such as switching AC power on inside the sine wave. EMI can interfere with the operation of controls and other devices.

ELECTROMAGNETIC RELAY -- See relay, electromechanical.

ELECTROMOTIVE FORCE (EMF) -- An increase in electrical potential energy measured in volts.

ELECTRONICS INDUSTRY OF AMERICA (EIA) -- An association in the US that establishes standards for electronics and data communications.

ELECTROSTATIC DISCHARGE (ESD) -- An electrical discharge, usually of high voltage and low current. For example, the shock that occurs when walking across a carpet.

EMC-- See electromagnetic compatibility.

EMF -- See electromotive force.

EMI -- See electromagnetic interference.

ELECTROMETER. -- A highly refined DC multimeter. In comparison with a digital multimeter, an electrometer is characterized by higher input resistance and greater sensitivity. It can also have functions not generally available on DMMs (e.g., measuring electrical charge, sourcing voltage).

ELECTRON. -- A negatively charged particle revolving round the nucleus of an atom.

ELECTRON, FREE. -- An electron not bound to the crystalline lattice, hence, free to conduct electricity.

EPROM -- Erasable, programmable, read-only memory inside the controller.

ERROR -- The difference between the correct or desired value and the actual measured value.

ESD-- See electrostatic discharge.

ETCHED FOIL -- The heater type that utilizes a resistance foil as the element with a print and etch method of manufacture.

EVENT -- A programmable ON/OFF output signal. Events can control peripheral equipment or processes, or act as input for another control or control loop.

EXOTHERMIC -- A process that releases heat.

EXTERNAL TRIGGER. -- An analog or digital hardware event from an external source that starts an operation. See also INTERNAL TRIGGER.

F

FAHRENHEIT -- The temperature scale that sets the freezing point of water at 32¼F and its boiling point at 212¼F at standard atmospheric pressure. The formula for conversion to Celsius is: ¼C = 5/9 (¼F - 32¼F).

FALL TIME. -- The time required for a signal to change from a large percentage (usually 90%) to a small percentage (usually 10%) of its peak-to-peak amplitude. See also RISE TIME.

FERMI LEVEL.-- The energy level in the semiconductor device at which the probability of finding an electron is 50%. In other words, it's like the water level in a glass. States below the Fermi level are full and those above it are empty.

FIELD EFFECT TRANSISTOR (FET). -- A transistor consisting of a gate, source, and drain. The voltage applied to the gate controls the conductivity of the channel between the source and drain. Examples of FETs are the MOSFET, JFET

FIFO. -- First-in/first-out memory buffer. The first data into the buffer is the first data out of the buffer.

FLASH MEMORY. -- It is a non volatile memory technique with fast access times; rewriteable many times and uses a block erase technique as opposed to EEPROM, which erases one bit at a time.

FLOATING. -- The condition where a common mode voltage exists between an earth ground and the instrument or circuit of interest. (Low of circuit is not at earth potential.)

FPS (feet per second) -- A measure of flow velocity.

FREEZING POINT -- The fixed temperature point at which a material changes from a liquid to a solid state. This is the same as the melting point for pure materials. For example, the freezing point of water is 32¼F or 0¼C.

FREQUENCY -- The number of cycles over a specified period of time usually measured in cycles per second. Also referred to as Hertz (Hz). The reciprocal is called the period.

FUSE -- A device that protects electric circuits by interrupting power in a circuit when an overload occurs.

G

GAIN. -- The factor by which an incoming signal is multiplied.

GIGA (G) -- A prefix that means 109 (one billion in the US). Note: The word billion refers to different numbers in Europe and the US. In the US a billion is one thousand million (1,000,000,000). In Germany, England, France and other countries, a billion is one hundred thousand million (100,000,000,000). That is a trillion in the US.

GPIB. -- Abbreviation for General Purpose Interface Bus. It is a standard for parallel interfaces.

GROUND -- An electrical line with the same electrical potential as the surrounding earth. Electrical systems are usually grounded to protect people and equipment from shocks due to malfunctions. Also called safety ground.

GROUNDED JUNCTION -- See junction, grounded.

GROUND LOOP -- A condition created when two or more paths for electricity are created in a ground line, or when one or more paths are created in a shield. Ground loops can create undesirable noise.

GROUND POTENTIAL -- The electrical potential of the earth. A circuit, terminal or chassis is said to be at ground potential when it is used as a reference point for other potential in the system.

GUARDING.-- A technique that reduces leakage errors and decreases response time. Consists of a guard conductor driven by a low-impedance source surrounding the lead of a high-impedance signal. The guard voltage is kept at or near the potential of the signal.

H

HARDWARE.-- The physical parts of a computer-controlled system, such as circuit boards, chassis, peripheral devices, cables, etc.

HAZARD. -- The potential for harmful effects.

HEAT SINK. -- A part used to absorb heat.

Hertz (Hz) -- Frequency measured in cycles per second.

Hi-Pot Test --- A test that applies a high voltage to a conductor to assure the integrity of the surrounding insulation. See dielectric breakdown.

HOT JUNCTION. -- The junction of two dissimilar metals in a thermocouple circuit that is used to measure an unknown temperature. Also known as measurement junction.

I

ICE POINT -- The temperature at which pure water changes from a liquid to a solid (freezes). 32¼F (0¼C).

ID -- Inside diameter.

IEEE. -- Abbreviation for Institute of Electrical and Electronics Engineers.

IEEE-488. -- See GPIB.

IMPEDANCE -- The total opposition of a circuit to the flow of alternating current. It includes resistance and reactance, and is measured in ohms.

INDUCTIVE LOAD -- Generated by a wire coil such as a relay, solenoid, motor or contactor. In an Inductive Load when the thermostat opens, a high voltage will be generated until all the current that is stored in the coil is discharged across the contacts. This can result in severe contact degradation.

INERT -- A material or gas that does not react chemically or biologically; for example, military thermostats are backfilled with an inert gas to prevent oxidation.

INFRARED -- A region of the electromagnetic spectrum with wavelengths ranging from one to 1,000 microns. These wavelengths are most suited for radiant heating and infrared (non-contact) temperature sensing.

INPUT BIAS CURRENT. -- The current that flows at the input due to internal circuitry and bias voltage. Also, the current that must be supplied to the high input measuring terminal (with zero input signal and offset voltage) to reduce the output indication to zero.

INPUT -- Process variable information that is supplied to the instrument.

INPUT SCALING -- The ability to scale input readings (readings in percent of full scale) to the engineering units of the process variable.

INPUT TYPE -- The signal type that is connected to an input, such as thermocouple, RTD, linear or process.

INRUSH CURRENT -- Measurement of the initial current flow in Capacitive or Lamp applications. Inrush currents can be as much as 15 times the rated current flow of the application.

Instrument Society of America (ISA) -- An engineering society that defines and maintains standards for scientific and technical measuring devices.

INPUT IMPEDANCE. -- The shunt resistance and capacitance (or inductance) as measured at the input terminals, not including effects of input bias or offset currents.

INPUT ISOLATION. -- On a switching card, the isolation from signal high to low (or guard) for a two-pole circuit. Specified as resistance and capacitance.

INPUT OFFSET CURRENT.-- The difference between the two currents that must be supplied to the input measuring terminals of a differential instrument to reduce the output indication to zero (with zero input voltage and zero offset voltage).

INPUT OFFSET VOLTAGE. -- The voltage that must be applied directly between the input measuring terminals, with bias current supplied by a resistance path, to reduce the output indication to zero.

INPUT/OUTPUT (I/O). -- The process of transferring data to and from a computer-controlled system using its communication channels, operator interface devices, data acquisition devices, or control interfaces.

INSERTION LOSS. -- The attenuation of signals due to routing them through a switching card. Specified as a decibel value over a frequency range.

INSULATOR. -- A material that does not significantly conduct electrical current

INTEGRATING CONVERSION.-- An analog to digital conversion process where the output results in a digital representation of the integral of the input signal over a specified time interval.

INTERNAL TRIGGER. -- A software-generated event that starts an operation. See also EXTERNAL TRIGGER.

INTERRUPT. -- A signal to the CPU indicating that the board detected the occurrence of a specified condition or event.

INTERRUPT LEVEL.-- A specific priority that ensures that high priority interrupts get serviced before low priority interrupts.

ION. -- This is what an atom becomes when an electron is separated from the atom, leaving it with a net positive charge or, if an electron is added, leaving it with a net negative charge.

ISA BUS.-- Industry Standard Architecture. The 16-bit wide bus architecture used in most MS-DOS and Windows computers. Sometimes called the AT bus.

ISOLATED OUTPUTS.-- Output signals where a common reference is not connected to either input terminal.

J

JIT -- Just-In-Time is a manufacturing concept using work cells to reduce leadtimes and lower costs. Some people refer to Just-In-Time delivery which is another way to say on-time delivery.

Joint Industrial Standards (JIS) -- A Japanese agency thatm establishes and maintains standards for equipment and components. Also known as JISC (Japanese Industrial Standards Committee), its function is similar to Germany's Deutsche Industrial Norm (DIN).

JOULE -- A basic unit of heat energy, equal to the work done when a current of 1 ampere is passed through a resistance of one ohm for one second.

JUNCTION -- The point where two dissimilar metal conductors join to form a thermocouple.

• Cold Junction -- Connection point between thermocouple metals and the electronic instrument. See reference junction.
• Exposed Junction -- A type of thermocouple probe in which the hot, or measuring, junction protrudes beyond the sheath material and is fully exposed to the substance being measured. It usually gives the fastest response time. No electrical isolation provided.
• Grounded Junction -- Type of thermocouple probe in which the hot, or measuring junction, is an integral part of the sheath material. No electrical isolation is provided.
• Isolated Junction -- A form of thermocouple probe construction in which the measuring junction is fully enclosed in a protective sheath and electrically isolated from it. Commonly called an ungrounded junction.
• Measuring Junction -- The thermocouple junction that is affixed to or inserted into thematerial being measured. Also called hot junction.
• Reference Junction -- The junction in a thermocouple circuit held at a stable, known temperature (cold junction). Standard reference temperature is 32¼F (0¼C).
• Thermocouple Junction -- The point where the two dissimilar metal conductors join. In a typical thermocouple circuit, there is a measuring junction and a reference junction. See measuring junction and reference junction.

K

KELVIN CONTACTS. -- A means for testing or making measurements in electronic devices and circuits, particularly when low values are being measured. Two sets of leads are used at each test point, similar with respect to thickness, material and length; one set carries the test signal and the other connects with the measuring instrument. The effect of resistance in the leads is thus eliminated.

Kelvin (K) -- An absolute temperature scale. Zero Kelvin is absolute zero. No degree symbol (¼) is used with the Kelvin scale. (0¼C = 273.15K, 100¼C = 373.15K).

Kilo (k) -- A prefix meaning thousand.

Kilowatt (kW) -- Unit of electrical power equal to 1000 wattsor 3412 Btus per hour when the power factor equals 1.0.

Kilowatt Hour (kWh) -- Unit of electrical energy, or work, expended by one kilowatt in one hour. Also expressed as 1000watt hours.

L

LCZ METER.-- Inductance (L), capacitance (C), impedance (Z) meter. A general purpose instrument for measuring component L, C. and Z. Sometimes called LCR meter This instrument may be applied to C-V testing, but typically lacks features optimized for C-V. See C-V METER.

LEAKAGE CURRENT. -- Error current that can degrade sensitive measurements. Leakage current is any unwanted current that flows when test voltage is applied. The ideal leakage current is zero. Leakage currents can originate in instruments, cables, or the device being tested. Even high resistance paths between low current conductors and nearby voltage sources can generate significant leakage currents.

LEAKAGE RESISTANCE. -- The resistance calculated from the dependence of leakage current on applied voltage. leakage resistance = V / leakage current

LEAST SIGNIFICANT BIT (LSB). -- The lowest order bit in a digital quantity.

LED -- See light emitting diode.

LEG-- One connection in an electric circuit.

LIFE CYCLES -- The endurance rating of a device expressed in number of operations with the stated electrical load applied.

LIGHT EMITTING DIODE(LED) -- A solid state electronic device that glows when electric current passes through it.

LINEARITY. -- The maximum deviation from a straight line between instrument readings at zero and full range. It is expressed in ppm at a specific temperature.

LINEAR INPUT -- A process input that represents a straight-line function.

LINEARITY -- The deviations in response from an expected or theoretical straight line value for instruments and transducers. Also called linearity error.

LINEARIZATION INPUT -- See linearization, square root.

LINEARIZATION, SQUARE ROOT -- The extraction of a linear signal from a nonlinear signal corresponding to the measured flow from a flow transmitter. Also called square root extraction.

LIQUID CRYSTAL DISPLAY (LCD) -- A type of digital display made of a material that changes reflectance or transmittance when an electrical field is applied to it.

LOAD -- The electrical demand of a process, expressed in power (watts), current (amps) or resistance (ohms). The item or substance that is to be heated or cooled.

M

MAGNETIC COILS -- Applications involving Magnetic Coils such as solenoids, relays and contactors are inductive loads which do not usually produce high Inrush Currents. However, when this type of load is turned off, the magnetic field collapses resulting in arcing across open contacts. This can cause serious contact deterioration.

MAINFRAME. -- A self-contained instrument in a cabinet, which provides a measurement or connection capability without requiring other instruments in the circuit. Some mainframes may be designated as a "master" or "slave."

MAKE-BEFORE-BREAK. -- Connecting a new circuit before disconnecting the present circuit.

MASTER. -- A mainframe that has control of other mainframes (slaves) through an external connection. A slave unit adds capacity or functions to the master. The master/slave combination has one IEEE-488 bus address. See also SLAVE and MAINFRAME.

MAXIMUM ALLOWABLE INPUT.-- The maximum DC plus peak AC value (voltage or current) that can be applied between the high and low input measuring terminals without damaging the instrument.

MAXIMUM LOAD IMPEDANCE -- The largest load that the output device can operate. Usually specified in ohms.

MAXIMUM OPERATING TEMPERATURE -- The highest temperature at which a device can operate safely or with expected normal service life.

MAXIMUM POWER RATING -- The maximum operating power at which a device can operate safely or with expected normal operating life.

Mega (M) -- A prefix that means one 106 (one million in the US).

MELTING POINT -- The temperature at which a substance changes from a solid to liquid state. This is the same as the freezing point of pure materials

MICRON -- A unit describing wavelength. One micron is equivalent to 1 x 10-6 meters.

MICROVOLT (µV) -- One 10-6 of a volt (one millionth in the US). Mil One thousandth of an inch, or 0.001 inches in decimal form.

Milliampere (mA) -- One thousandth of an ampere.

Millivolt (mV) -- One 10-3 (thousandth) of a volt.

MINIMUM LOAD CURRENT -- The smallest load current required insuring proper operation of an output-switching device.

MINIMUM OUTPUT IMPEDANCE -- See offstate impedance.

MULTIPLEX -- Connecting one instrument to multiple devices under test or multiple instruments to one device under test.

N

N-TYPE. The conductivity type of a semiconductor material when the majority of carriers are electrons, hence negative. N-type silicon is doped with dopant atoms from column V of the periodic table, such asphosphorus.

NANOVOLTMETER. -- A sensitive DC voltmeter (typically one decade more sensitive than a digital multimeter) with a low thermal input connection.

National Bureau of Standards (NBS) -- Now called the National Institute of Standards Technology (NIST).

National Electrical Code (NEC) -- A set of specifications devised for the safe application and use of electric power and devices in the United States.

National Electrical Manufacturers Association (NEMA) -- A United States association that establishes specifications and ratings for electrical components and apparatuses. Conformance by manufacturers is voluntary.

National Institute of Standards and Technology (NIST) -- A United States government agency responsible for establishing scientific and technical standards. Formerly the National Bureau of Standards.

National Pipe Thread (NPT) -- The taper pipe thread standard used in North America.

NBS -- See National Bureau of Standards.

NEC -- See National Electrical Code.

Negative Temperature Coefficient -- A decrease in electrical resistance that occurs with a temperature increase. See thermistor.

NEMA -- See National Electrical Manufacturers Association.

NEMA 4X -- A NEMA specification for determining resistance to moisture infiltration and corrosion resistance. This rating certifies the controller as washable and corrosion resistant.

NIST -- See National Institute of Standards and Technology.

NOISE. -- An undesirable electrical signal from an external source such as an AC power line, motors, generators, transformers, fluorescent lights, CRT displays, computers, radio transmitters, and others.

NOISE SUPPRESSION -- The use of components to reduce electrical interference that is caused by making or breaking electrical contact, or by inductors.

NON-EQUILIBRIUM. -- The condition in a semiconductor device or material when there is still a tendency for its macroscopic properties to change with time. In other words, there still remains some settling to occur before equilibrium is achieved. Devices in which current always flows, like diodes and bipolar transistors, do not ever reach equilibrium.

NORMAL MODE REJECTION RATIO (NMRR). -- The ability of an instrument to reject interference(usually of line frequency) across its input terminals. Usually expressed in decibels at a frequency.

NORMAL MODE VOLTAGE. -- A voltage applied between the input high and input low terminals of an instrument.

O

OBJECT LINKING AND EMBEDDING (OLE). -- A Microsoft Windows standard mechanism for embedding one program within another. For example, an Excel spreadsheet can be pasted into a Visual Basic program. If a file is linked to an OLE control, the data stored in that file is displayed in the OLE control.

OD -- Outside diameter.

Ohm -- The unit of electric resistance. The resistance value through which one volt will maintain a current of one ampere.See Ohm's Law. Ohm's Law -- Current in a circuit is directly proportional to the voltage, and inversely proportional to resistance; stated as: E = IR Where: I = Amps, I = E/R, E = Volts, R = E/I, R= Ohms The complete Ohm's Law also includes the relationships of watts to amps, volts and ohms.

ON/OFF -- A method of control that turns the output full on until set point is reached, and then off until the process error exceeds the hysteresis.

OFFSET CURRENT. -- A current that comes from a switching card even though no user signals are applied. It comes mostly from the finite coil to contact impedance. It is also generated by triboelectric, piezoelectric, and electrochemical effects present on the card.

OHMIC CONTACT. -- A resistive contact between a semiconductor material and metal. This type of contact exhibits a straight line I-V characteristic with a small value of resistance and does not significantly rectify.

OPERATING SYSTEM. -- Base-level software that organizes the computer's resources and capabilities, runs application programs, interacts with users, and communicates with installed and peripheral devices. Popular operating systems include DOS, Windows, OS/2, and UNIX.

OVERLOAD PROTECTION. -- A circuit that protects the instrument against excessive current at the input terminals.

P

P-TYPE. -- The conductivity type of a semiconductor material when the majority of carriers are holes, hence positive. P-type silicon is doped with dopant atoms from column III of the periodic table, such as boron.

PANEL LOCK -- A feature that prevents operation of the front panel.

PARALLEL CIRCUIT -- A circuit configuration in which the same voltage is applied to all components, with current divided among the components according to their respective resistances or impedance's.

PARAMETER-- A value that determines the response of an electronic controller to given inputs.

PARTS PER MILLION (PPM). -- Represents the concentration of gases or vapor in air. For example, 1 ppm means that 1 unit of the gas is present for every 1 million units of air..

PC. -- Abbreviation for Personal Computer.

PCI. -- Abbreviation for Peripheral Component Interconnect. It is a standard for a local bus.

PCMCIA. -- Abbreviation for Personal Computer Memory Card International Association. This organization establishes standards for portable computers.

PEAK RESPONDING. -- A measurement where the displayed value is equal to the peak value of the input signal.

PER CHANNEL RATE. -- The sample rate for each channel of a scanning A/D system.

PN JUNCTION. -- The interface between n-type and p-type semiconductor material. This structure forms a pn junction diode.

POLARITY-- The electrical quality of having two opposite poles, one positive and one negative. Polarity determines the direction in which a current tends to flow.

POLE. -- A combination of mating relay contacts: normally open, normally closed, or both.

PROGRAMMED I/O. -- A standard method of accessing an I/O device - the CPU reads each byte of data from or writes each byte of data to the device.

PYROPHORIC. -- A chemical that will ignite spontaneously in air at a temperature less than or equal to 54.4¡C (130¡F).

Q

QUANTIZATION. -- A process where the continuous range of values of an input signal is divided into non-overlapping sub-ranges and, to each sub-range, a discrete value of the output is uniquely assigned.

R

RADIATION -- Radiant energy emitted in the form of waves or particles. See emissivity and infrared.

Radio Frequency Interference (RFI) Electromagnetic waves.between the frequencies of 10 KHz and 300 GHz that can affect susceptible systems by conduction through sensor or power input lines, and by radiation through space.

RAMP -- A programmed increase in the temperature of a set point system.

RANGE -- The area between two limits in which a quantity or value is measured. It is usually described in terms of lower and upper limits.

RATED ACCURACY. -- The limit that errors will not exceed when the instrument is used under specified operating conditions. It is expressed as a percentage (of input or output) plus a number of counts.

READING.-- The displayed number that is proportional to the measured magnitude of the input signal.

READING RATE. -- The rate at which the displayed number is updated.

RELIABILITY. -- The ability of a device to perform within the desired range over a measured period of time.

REPEATABILITY.-- The ability of an instrument to measure the same input to the same value over a short period of time and over a narrow temperature range.

RESISTANCE -- Opposition to the flow of electric current,measured in ohms. See ohms.

RESISTANCE TEMPERATURE -- Characteristic The characteristic change in a sensor's resistance when exposed to a change in temperature. See positive temperature coefficient and negative temperature coefficient.

Resistance Temperature Detector (RTD) -- A sensor that uses the resistance temperature characteristic to measure temperature. There are two basic types of RTDs: the wire RTD, which is usually made of platinum, and the thermistor, which is made of a semiconductor material. The wire RTD is a positive temperature coefficient sensor only, while the thermistor can have either a negative or positive temperature coefficient.

RESISTANCE INSERTION. -- A current measuring technique where a known resistor is connected in series with the circuit to be measured. The voltage drop across the resistor is proportional to the unknown current.

RESISTIVITY. -- The geometry independent parameter of a material that determines the relationship between the current through it and the current across it. Resistance of a cylinder of material is: resistance = resistivity * (length / area)

RESOLUTION. -- The smallest value of input (or output) signal,other than zero, that can be measured (or sourced) and displayed. Also called sensitivity or minimum resolvable quantity.

RESOLUTION (DATA ACQUISITION). -- The smallest signal increment that can be detected by a measurement system. It is usually specified in "bits."

RESPONSE TIME. -- For a measuring instrument, the time between application of a step input signal and the indication of its magnitude within a rated accuracy. For a sourcing instrument, the time between a programmed change and the availability of the value at its output terminals. Also known as Settling Time.

RISE TIME. -- The time required for a signal to change from a small percentage (usually 10%) to a large percentage (usually 90%) of its peak-to-peak amplitude. See also FALL TIME.

RMS RESPONDING. -- A measurement where the displayed value is equal to the root-mean-square (rms) of the input signal, for all input waveforms having components within the specified frequency range and crest factor limit.

S

SAE -- See Society of Automotive Engineers.

SAMPLE. -- A value that is read from or written to one channel.

SAMPLE RATE. -- The rate at which a continuous-time signal is sampled. It is frequently expressed as samples/sec (S/s), kilosamples/sec (kS/s), or Megasamples/sec (MS/s).

SCHOTTKY DIODE. -- A junction or barrier formed by the direct contact of semiconductor materials with a metal. This type of contact rectifies signals and may also exhibit some resistance.

SEEBECK COEFFICIENT -- The rate of change (derivative) of thermal EMF (voltage) with respect to temperature. Expressed as millivolts per degree.

SEEBECK EFFECT -- When a circuit is formed with a junction of two dissimilar metals and the junctions at each end are held at different temperatures, a current will flow in the circuit.

SEEBECK EMF -- The net thermal electromotive force (EMF) in a thermocouple under conditions of zero current.

SEMICONDUCTOR -- Any material that exhibits a degree of electrical conductivity that falls between that of conductors and dielectrics.

SERIAL COMMUNICATION -- A method of transmitting information between devices by sending all bits serially over a single communication channel.

• EIA/TIA-232 (formerly RS-232) -- An Electronics Industries of America (EIA)/Telecommunication Industry Association (TIA) standard for interface between data terminal equipment and data communications equipment for serial binary data interchange. This is usually for communications over a short distance (50 feet or less) and to a single device.
• EIA/TIA-485 (formerly RS-485) -- An Electronics Industries of America (EIA)/Telecommunication Industry Association (TIA) standard for electrical characteristics of generators and receivers for use in balanced digital multipoint systems. This is usually used to communicate with multiple devices over a common cable or where distances over 50 feet are required.

SERIES CIRCUIT -- A circuit configuration in which a single current path is arranged among all components.

SEMICONDUCTOR. -- An element such as silicon or germanium or a compound like GaAs that has an intermediate band gap. Unlike metals that freely conduct and insulators that do not conduct charge, semiconductors selectively conduct charge through the movement of holes and electrons.

SETTLING TIME. -- For a measuring instrument, the time between application of a step input signal and the indication of its magnitude within a rated accuracy. For a sourcing instrument, the time between a programmed change and the availability of the value at its output terminals. For a switching card, the time required for establishing relay connections and stabilizing user circuits. Also known as Response Time.

SETTLING TIME (DATA ACQUISITION) -- Time it takes for a voltage to settle and remain within a specified error band around the final value.

SHIELDING. -- A metal enclosure for the circuit being measured or a metal sleeve surrounding wire conductors (coax or triax cable) to lessen interference, interaction, or current leakage. The shield is usually grounded.

SHUNT-- In an electrical circuit, a low resistance connection between two points that forms an alternate path for some of the current. Dielectric materials lose resistance at temperatures above their operating range. This condition can cause shunting of the sensor's signal, causing an error in the reading.

SIGNAL -- Any electrical transmittance that conveys information.

SILICON -- A tetravalend nonmetallic element.

Silicon Controlled Rectifier (SCR) -- A solid state device, or thyristor, with no moving parts, that is used in pairs to control AC voltages within one cycle. SCRs control voltage from a power source to the load by burst firing (also called zero-cross firing) or phase-angle firing. See burst fire.

SIGNAL/NOISE RATIO. -- The ratio of the maximum signal that can be measured to the level detected with no signal present (noise level). It is expressed in decibels.

SILICON.-- The most common elemental semiconductor. From silicon crystals that are grown and sliced into wafers.

SIMULTANEOUS SAMPLE-AND-HOLD.-- Operation in which the analog input channels are sampled at the same time and the values held until sequentially read by a scanning A/D system.

SINGLE-ENDED. -- The condition where the low terminal of a two-terminal instrument is connected to a specific reference point, such as power line common, earth ground, or circuit common.

SINGLE-ENDED INPUT (DATA ACQUISITION). -- An analog input with one input terminal whose value is measured with respect to a common ground. See also DIFFERENTIAL INPUTS.

SLAVE. A mainframe that is externally connected to a controlling mainframe (master). A slave unit adds capacity or functions to the master. See also MASTER and MAINFRAME.

SLEW RATE.-- The maximum charge rate of the signal sampling capacitor in the sample and hold circuit of an A/D converter. It is expressed in volts/microsecond.

SOFTWARE TRIGGER. -- A programmed event that starts an operation such as data acquisition.

SOLID STATE -- A device that can control current without.moving parts, heated filaments, or vacuum gaps; transistors, germanium diodes, and thermistors are all solid state devices.

Solid State Relay (SSR) -- See relay, solid state. Span The difference between the lower and upper limits of arange expressed in the same units as the range. See range.

SOURCE IMPEDANCE. -- The combination of resistance and reactance that a source presents to the input terminals of a measuring instrument.

STAIRCASE WAVEFORM. -- A waveform in which the voltage is incremented in uniform steps from the start voltage to the stop voltage.

STANDARD -- A set value or reference point from which measurements or calibrations are made.

SUBLIMATION -- The change of state in matter from solid to gas.

SURGE CURRENT -- A short duration rush of current that occurs when power is first applied to capacitive, inductive, or temperature dependent resistive loads, such as tungsten or silicon carbide heating elements. It also occurs when inductive loads are de-energized. Surge currents usually last no more than several cycles.

SUBROUTINE. -- A set of software instructions executed by a single line of code.

SUBSTRATE. -- The underlying material upon which a device circuit or epitaxial layer is fabricated. This term also applies to the MIS capacitor, which has only two terminals: the gate and substrate.

Systeme Internationale (SI) -- The system of standard metric units.

T

TCR -- Temperature Coefficient of Resistance is the average resistance change/degree over the range of 0 to 100¼C. By knowing the TCR a customer can convert a resistance (ohm) value to temperature.

TEMPERATURE, AMBIENT -- The temperature of the air or other medium that surrounds the components of a thermal system.

TEMPERATURE CALIBRATION POINT -- A temperature at which the output of a sensor is compared against a standard.

TEMPERATURE COEFFICIENT.-- A change in reading (or sourced value) with a change in temperature. It is expressed as a percentage of reading (or sourced value), plus a number of counts per degree change in temperature.

TEMPERATURE COEFFICIENT OF RESISTANCE. -- The amount of resistance change of a material per degree of temperature change.

THERMOCOUPLE (T/C) A temperature sensing device made by joining two dissimilar metals. This junction produces an electrical voltage in proportion to the difference in temperature between the hot junction (sensing junction) and the leadwire connection to the instrument (cold junction).

THERMOCOUPLE TYPE -- A particular combination of metallic elements and/or alloys that make up the conductors of a thermocouple, and defines their EMF output relative to absolute temperature. ANSI designated types include: B, E, J, K, N, R, S and T.

Non-ANSI types include: C, D and G (tungsten based thermocouples) and Pt 2.

THERMOELECTRIC VOLTAGE. -- Voltages resulting from temperature differences within a measuring circuit or when conductors of dissimilar materials are joined together. Also known as thermal EMF or thermal offset. See CONTACT POTENTIAL.

THIN FILM. -- A thin layer of material grown or deposited on a substrate.Typically, layer thickness is measured in tens to thousands of angstroms for insulators and microns for metals.

THRESHOLD VOLTAGE (VT). -- The gate voltage needed to turn on a MOS enhancement-mode device.

THROUGHPUT. -- The maximum rate at which a data conversion system can perform repetitive conversions within a specified accuracy. It is determined by summing the various times required for each part of the conversion system and then taking the inverse of this time. The throughput rate takes into account the total time required to process a signal and store the value in either on-board or system memory.

TIMEBASE ACCURACY. -- A measure of how closely the internal timebase of an instrument tracks a known time standard.

TIMING JITTER.-- The short-term variation of the time period between sample points.

TOTAL HARMONIC DISTORTION.-- The percentage of harmonic distortion present in an output signal over a specified frequency range.

TRANSCONDUCTANCE. -- The ratio of the incremental change in the output current of any amplifying circuit or device to the incremental change of input voltage causing it, when the output voltage is held constant.

TRANSFER RATE. -- The rate at which data is transferred to or from memory.

TRANSISTOR. -- A semiconductor device in which a small control signal is used to control a larger current flow.

TRIGGER (INSTRUMENTATION). -- An external stimulus that initiates one or more instrument functions. Trigger stimuli include: an input signal, the front panel, an external trigger pulse, and IEEE-488 bus X, Talk, and GET triggers.

TRIGGER (DATA ACQUISITION). -- An event that starts or stops an operation. A trigger can be a specific analog, digital, or software condition. See also ANALOG TRIGGER and DIGITAL TRIGGER.

TRIGGER CONDITIONS. -- Refers to trigger sensitivity, polarity, etc.

TRIGGER HYSTERESIS. -- Applies only to analog triggers. It helps prevent noise from triggering an operation or event. For a positive-edge trigger, the analog signal must be above the specified voltage level by at least the amount of the hysteresis value before the trigger occurs. For a negative-edge trigger, the analog signal must be below the specified voltage level by at least the amount of the hysteresis value before the trigger can occur.

TRIGGER LATENCY. -- The fixed time offset between the trigger event and the first sample point.

TRIGGER MODE.-- Refers to when data acquisition begins and ends in relationship to the trigger. Trigger modes include normal-trigger, pre-trigger, about-trigger, post-trigger, trigger-to-trigger, and trigger-to-about-trigger.

TRIGGER POLARITY. -- For edge-sensitive triggers: trigger polarity defines whether the trigger occurs when the signal is rising (positive direction) or falling (negative direction). For level-sensitive triggers: trigger polarity defines whether the trigger occurs when the signal is above a level (positive) or below a level (negative).

TRIGGER SENSITIVITY. -- Refers to edge and/or level of a trigger. For analog triggers, trigger sensitivity defines whether the trigger occurs on a transition across a specified value (edge) or whether the trigger occurs when it is above or below a specified value (level). For digital triggers, trigger sensitivity defines whether the trigger occurs on a transition from one state to another state (edge) or whether the trigger occurs when it is at a specified value (level).

TTL. -- Abbreviation for transistor-transistor-logic. A popular logic circuit family that uses multiple-emitter transistors. A low signal state is defined as a signal 0.8V and below. A high signal state is defined as a signal +2.0V and above.

TWISTED PAIR -- Two insulated conductors that are twisted together. An effective method of duplexing and reducing electromagnetic interference (EMI).

TWO-TERMINAL RESISTANCE MEASUREMENT. -- A measurement where the same current flows through the unknown and the test leads.

U

UL¨ -- The registered trademark and abbreviation for the Underwriters Laboratories, Inc. An independent testing laboratory that establishes commercial and industrial standards, and tests and certifies products in the United States.

ULTRAVIOLET -- The portion of the electromagnetic spectrum that is just beyond the violet in the visible spectrum. Ultraviolet light can degrade many insulation materials.

UNDERSHOOT -- The amount by which a process variable falls below the set point before it stabilizes.

UNIPOLAR. An analog signal range that is always positive (through zero.)

V

VAC/VDC -- Volts alternating current, volts direct current.

VDE -- Abbreviation for Verband Deutscher Elektrotechniler, an independent German testing and certification institute concerned with the safety of electrical products. Authorizes use of the VDE Mark.

VISCOSITY -- The resistance of fluid to sheering forces (flow). High viscosity indicates a tendency for a fluid to flow or move slowly. The viscosity of fluids decreases as their temperatures increase. Heating gases will increase their absolute viscosity.

Volt (V) -- The unit of measure for electrical potential, voltage or electromotive force (EMF). See voltage.

Volt Amperes (VA) -- A measurement of apparent power. The product of voltage and current in a reactive circuit.V (voltage) áI (current) = VA. The term watt is used for real power.

Voltage (V) -- The difference in electrical potential between two points in a circuit. It's the push or pressure behind current flow through a circuit. One volt (V) is the difference in potential required to move one coulomb of charge between two points in a circuit, consuming one joule of energy. In other words, one volt (V) is equal to one ampere of current (I) flowing through one ohm of resistance (R), or V = IR.

VALENCE BAND. -- The energy levels of the electrons in he outermost shell of the atoms making up a solid material.

VAPOR. -- A gaseous form of a material that is normally a solid or liquid at room temperature and pressure.

VAPOR DENSITY. -- The density of a vapor compared to the density of an equal amount of air.

VARIABLE FREQUENCY. -- Frequency settable over a range. This term is applied to a capacitance meter that offers a choice of test signal frequencies from so many that it is essentially continuously adjustable.

VOLTAGE BURDEN. -- The voltage drop across the resistor for the resistance insertion technique of current measurement.

W

WARM-UP TIME.-- The time required after power is applied to an instrument to achieve rated accuracy at referenced conditions.

Watt (W) -- A measurement of real power. The product of voltage and current in a resistive circuit. V (voltage) áI (current) = P (power in watts).

WORD. -- The standard number of bits that a processor manipulates at one time. Microprocessors typically use 16-, or 32-bit words. (Or 2 bytes and 4 bytes respectively.)

WORK FUNCTION. -- The minimum energy required to remove an electron from the Fermi level of a material into field-free space. Work function is normally expressed in electron volts.

Y

YIELD. -- The ratio of good units obtained divided by the total units produced. It is the percent of wafers, dice, or packaged units conforming to specifications.

Z

ZERO OFFSET. -- The reading (desired or undesired) that occurs when the input terminals of a measuring instrument are shorted.